Tank testing the ART at the dock

I took a couple of videos at the dock to compare the motions of my boat without the antiroll tank (ART) and with the ART filled to about 75% of what I thought I would need.  Without that and the graphs that follow I would just be thinking "well, I think that it probably made a difference."  My wife is a true believer, so there is that.  I later modified the amount and viscosity to improve the ability of the ART to decay a roll.  

In an attempt to get an apples to apples video, I stepped on the gunnel 3 times in sync with my roll period.  Not perfectly scientific, but close enough.  I actually pressed a little harder in the second video to create the same roll angle from which to start measuring the decay.  That is because the ART starts decaying the roll even as I was trying to generate it.  Which would, of course, naturally effect the time required to decrease the roll.  To have "apples to apples" I needed to use more force.  So, I guess not really apples to apples in the end.  The ART starts working immediately, even before my measuring.

Here is the action without the ART.

About 8 rolls to the end of the video.  I wish I had longer videos, but Blogspot doesn't allow lengthy videos, so it wouldn't have helped here.  

Here is the action with the ART partially filled.  The same number of "boardings" to create approximately the same roll (which required a little more effort on my part) but the roll decays to approximately the same degree in 4 rolls after I instigate it.

Here is what the action looks like graphed with a g-force meter.  This is actually a graph of acceleration rather than inclination and was developed when using a bag instead of a tank, but I have found the same effect.  Acceleration is usually what makes the rolling unpleasant.  If my roll was slower, I might not be experimenting with an ART.  But my roll is "snappy" and can be uncomfortable and even dangerous.  

I wasn't careful enough when creating these graphs.  First, I didn't make sure that the recorder (my cell phone) was completely level.  That meant that zero g-force wasn't always aligned with zero on the graph.  Bummer.  Even more confusing was that the graph created by the program decided on its own what metric to use on the Y axis, and I didn't notice that.  The X axis is always in seconds, but the Y axis is different on the two graphs.  Bummer.

Here is the graph created without the tank.

 

As the sine waves are getting bigger (until right after the 30 second mark), they are not symmetrical because I was rocking the boat.  After that point is the natural smooth sine wave decay of the roll.  The first Y axis mark is .05G, but the actual g-force isn't what I was interested in (although I would guesstimate the max was .1G.)  From about 34 seconds to 61 seconds, the g-force disintegrates by 50%.  Or a 50% decrease in 27 seconds.

Here is the graph made with liquid added to the ART.

It took me awhile to get the boat rocking and the graph isn't perfectly centered on zero on the X axis.  Also, as stated above, the Y markers have now changed to 1 instead of .5 (so the beginning g-force is still about .1G).  I tried hard to get the same amount of roll going, but I actually got pooped out before I got the full 7 degrees I was hoping for.  The graph is still instructive.  I had stopped "exciting" the roll and was only interested in the decay time.  The amount of roll at 150 seconds decreased about 50% by the 163 second point.  With the tank filled, a 50% decrease took only 13 seconds.  

The ART stifled a similar roll to the same amount in half the time.  Plus, it was much harder to get the roll going in the first place.  It was one of those situations where it felt like somebody must be working against me.  Well it wasn't somebody, but some thing.  Free surface liquid first making a wave and then transferring side-to-side during larger induced rolls.  Amazing what a little water slopping around can do.

 

Video of Antiroll Tank in Action

Here is some video and photos of our recording inclinometer to show the effect of the roll tank installed in the prior blog.  The videos are short because they are size limited by Blogspot.  

We had already crossed the Straight of Juan de Fuca, but no video there.  The next chance to really take advantage of the antiroll tank were encounters with the BC Ferry and other large vessels.  Here is a little video of a BC Ferry passing us (they tend to travel at about 20kt to our 6.5kt).  When they pass, the wake slowly passes us, making for lots of time to build synchronous rolls.  We usually turn and go directly into the wake of large vessels in order to stop building violent rolls.  Not necessary with the antiroll tank. 

Here is what the approaching wake looked like.  A good ten 2' waves perfectly spaced to get us violently rocking.  Time to batten the hatches.

The antiroll tank handled it just fine.  We could feel the wake, of course, but the rolling never built above 4 degrees.  It would lean us to one side, but the tank reduced the "snap back" such that the effect of the next wake was again fairly mild.  It felt like the tank was constantly frustrating the ability of the wake to really get things rocking.

I later went solo around Cape Caution in order to pick up Beth, who flew home from Bella Bella for a week and was flying back into Refuge Cove.  Here is a little video of "Cape Caution" (which I actually never even saw that day, except on the RADAR).  Very foggy and some long swell on the beam coming in from the Pacific.  Nothing the tank couldn't handle.

Oddly enough, the roughest part of my trip this year was from Squirrel Cove in Desolation Sound, where I spent the night at anchor, across the 3 or 4 miles to Refuge Cove, where Beth was coming in on a float plane at noon.  If I'd had a choice, I probably wouldn't have crossed until later in the day after the tide changed.  This is +20 knot wind against the flood tide and right on my beam.  The ride wasn't pleasant, but also not too uncomfortable once I got used to the fact that the max rolling was all of about 10 degrees and seemed fairly mild mannered compared to similar conditions in the past.  

The inclinometer shows that the maximum roll to port was about 5 degrees more than any roll to starboard.  That is because the wind was coming across the deck from starboard, causing a constant list of about 5 degrees from just the wind, even had there been no waves.  It was howling pretty good.  So the actual wave induced rolling was about 10 degrees.  That is about 1/2 of what I would have expected from these conditions without the tank.

My assessment is that the antiroll tank is definitely worth the time and effort.  I still have some experimentation to do with the amount and viscosity of the tank liquid,* but even with just a guesstimated amount of tap water, its utility was proven.  If I decide to integrate the tank into the flying bridge structure, which would entail some fiberglass work, it would still be only a $1,000 investment.  From what I've heard, it would be more effective than bilge keels (at $15,000).  Might be close to being as effective as active fins (at $40K).  Maybe not as effective as a gyro stabilizer (at +$60K).  And there are the advantages of no maintenance, nothing to snag crab lines, works at zero speed (at anchor), no need to haul out, no decrease in speed, no increase in fuel usage, no need to run a 240V generator (I've removed mine), etc.  And, as I said in a prior blog, there is also no profit margin for a seller/installer/designer.  The latter is likely the reason that antiroll tanks are rare.

I have read a couple of places where antiroll tanks are claimed to be dangerous in that they might affect the vessel's stability.  Well, yes, that is possible with any system, although probably less likely with a "gravity activated" system as opposed to an "electrically controlled" system.  Gravity is predictable and always "on."  Other antiroll systems have had their failures, but for some reason don't inspire the same level of criticism (and even fear).  Take for example the effects of an active fin system that gets a bit out of tune.  The fins started rotating the wrong direction, exacerbating the rolls rather than calming them.  Here is a picture of seas similar to what I was in, but their fins got out of sync and the system had to be shut down before it sank the vessel.  Gravity doesn't have glitches.

I know what that feels like.  Here is a picture of my inclinometer taken before I had the antiroll tank operational (going through Race Passage on Johnstone Straight).  This is about what the boat above was doing with their expensive active fins. I don't intend to do that again thanks to my DIY antiroll tank.

* I found the chemical I had used to slightly increase the viscosity of the water in my tank allowed a little growth to form.  It don't hurt the runnin' none, but it was kind of gross.  I have an idea for a sanitary next fill.

Antiroll Tank: First impressions

We have now been cruising with our test antiroll tank for over a month.  I have purposely taken the boat into conditions that I would normally try to avoid.  One of those instances was crossing the Straight of Juan de Fuca from Port Townsend to Sydney, B.C.  It can be calm, but even then, it is likely that there will be swells coming in from the Pacific.  Our crossing was sort of "medium" conditions for the Straight.  2-3' waves on a 6 second period.  Underneath was the swell, but it only made itself known when it coincided with the wind waves.  

We had a mini-disaster during the crossing, but it wasn't really related to the antiroll tank.  I had changed our hollow fiberglass mast to a tabernacle mast so that it could be folded downs when entering our newly acquired boat house.  The hinge was strong enough.  The lock down mechanisms were strong enough.  What wasn't strong enough was the thickness of the fiberglass mast (and I didn't properly reinforce).  

I had some questions about the PO installation of the Garmin radar dome on the antenna (with a custom stainless mount).  I wasn't sure if the original mast was intended to be merely decorative.  I had already reinforced the bottom bolts where the motion of the mast had begun to wear larger holes in the fiberglass.  When I removed the mast for cutting and adding the tabernacle, I was surprised at how light the Garmin dome was, and that lead me to not super-reinforce the newly installed hinge.

In purposely getting into "test conditions" for the antiroll tank, I put plenty of strain on the unstayed mast.  And so the hinge failed in the Straight.  The mast fell onto the solar panel, but not much damage.  Even though the rolling motion had been reduced, the strain was still too much for my tabernacle design (or lack of design).  The top of the mast was then sistered to the stub on the deck.  It was secured with duct tape.  Radar was still functional, but anchor light was not.  We seldom have other boats in the remote anchorages we chose (most don't even have names), but still we left on the aft cockpit light at night.  Sort of like leaving on the porch light.

That's our new Starlink antenna on the deck to the right of the mast.  Mindboggling that I'm typing this while at anchor in a foggy little cove SW of Bella Bella.  I said in the prior post about the tank's construction that I might be able to continue the posts using Starlink, and it definitely is possible.

But the antiroll tank was a success.  Our rolls didn't hardly register on my recording inclinometer.  Below is a photo of the tank installed.  It has "8 inch inspection ports" on each end, not so much for inspection or filling, but because I needed to access the interior in order to do the final epoxy fillets to construct the tank.  Right now, the ports are simply sitting in their openings.  I didn't have time to finalize the tank before we left.

You can also see the hinge on my tabernacle mast in the down position in the lower left corner.  That is the hinge that failed to handle the rocking motion in the Straight, despite the rocking motion being greatly reduced by the tank.  How much it was reduced will be the topic of the next post.

Antiroll tank on a small recreational trawler

My initial testing using an "antiroll bag" could only get me so far.  There is an amazing amount of flexing that goes on when the boat rolls and the available materials for a bag don't seem to be up to the challenge.  A rigid tank would not wear out (hopefully), but presented some additional calculations in order to synchronize with my boat's roll period.  Actually, not quite synchronize.  The idea is that the liquid in the tank will transfer from side to side slightly slower than the vessels roll period.  The liquid (or a wave) will reach the "down side" of the roll tank just after the vessel begins to right itself.  Thus each "snap back" is diminished, making building synchronous rolls difficult or even eliminating the problem.

The issue can be addressed mathematically by measuring the vessel's natural roll period, measuring the moment arm of the tank location, calculating the dynamic viscosity of the water in the tank, adjusting internal baffles to effectuate the proper timing of the fluid, etc.  For those of you who like to nerd out on mindboggling calculations, check out the grad school papers on the subject of antiroll tanks that are on Google.

You will be glad to learn that there is another way of doing this.  Commonly called "tank testing" by those wanting to sound professional, it is essentially trial and error using hunches.  Keep in mind that the Wright brothers were bicycle mechanics, not rocket scientists (or even aviation engineers, as that moniker hadn't been a thing before manned flight).  I'm guessing that Wilbur and Orville couldn't write a graduate thesis on the mechanical transfer of kinetic energy into centripetal force resulting in velocity directed by a secondary rotational mechanism using gravitational friction (grad speak without the Greek letters) to explain bicycles.  Also keep in mind the immortal words: "It won't never get off the ground, Orville."  We can ignore equations, explanations, and diagrams for the moment and concentrate on results.

Scale models of antiroll tanks are easily built and studied in a wave tank, but for full scale testing, it is a little more complicated.  Fortunately, my marina is near a ferry dock and a Washington State ferry leaves about every other hour (my state-funded "wave generator").  The perfect tank test site.  My trials with the antiroll bag gave me some idea of what would be needed.  But it seemed that the best experimentation would require something that allowed ongoing "tuning" of the tank.

The requirements are two-fold.  First, there is the timing of the transfer of the liquid from side to side. The second issue if the amount of force needed to negate the vessel's natural tendency to roll back beyond its upright position.  Keep in mind that this antiroll "force" is a little different than other antiroll systems.  It doesn't impose a drag on the vessel's speed or tendency to roll (as with dragging plates in the water, fins that rotate to lessen a roll, or "rolling chocks").  In fact, it works the same at anchor.  The transfer of weight in the tank merely reduces a return from a roll, thereby reducing the ability of the vessel to build synchronous rolls.  There's nothing that needs routine maintenance, wiring, rigging, hydraulics, etc.  Nothing to snag fishing nets, kelp, flotsam.  No need to haul out to install.

Most boaters have already felt this type of roll reduction when transiting a confused seaway.  At certain points the boat starts rocking and one thinks "here comes a big roll" only to have an unexpected wave "slap" against the hull and negate the coming roll.  Thank you, errant wave.  Well, the effectiveness of an antiroll tank is based on that principal.  In this case, every roll receives its corresponding "slap back" wave.  Except the wave is overhead and there is no slapping noise from my tank.

Like all antiroll systems, one cannot expect total elimination of the vessel's motion.  I have seen a few systems that cater to land lubbers, costing lots of dollars and diesel, that claim to steady the boat like a dirt home.  Basically, a dirt boat.  I'm not convinced that is necessary.  I hope to do some kind of a "dollars per degree damping" analysis when I get my system tuned.  So far, I have less than $300 into my tank, so comparing it to a mid-level $30,000 active hydraulic fin system should be fun.  I've heard that the +$50,000 240V gyro systems (with their own dedicated generator and required annual haulout for maintenance) are even more efficient.  Well, one would hope so.

But on with the experiment.  Having tried a few things, here is what I've come up with.  

Yes, it looks like a coffin.  I got some weird looks when transferring it to the marina on the roof of my car.  The curvature has a significant effect on the "wave" inside of the tank.  As we all have been told, waves slow down in shallower water.  I needed a way to slow down the wave in the tank, as the natural transfer of a water wave is generally too fast for an antiroll tank.  Naw, you will see the reason in a later blog.  But the curve is the bottom of the tank, i.e., the tank is upside down in the pictures.  It was just easiest to build that way.

The construction is epoxy fillets, no fasteners, similar to a stitch-and-glue dinghy that I built years ago.  I had some old West System epoxy, but not enough to do the entire project.  So I had to buy another gallon of West System ($200).  That's why my project was $300.  Not shown in the picture is my super secret baffle system that makes the transfer of the special antiroll fluid perfectly timed.

Actually, although there is a baffle of sorts, the timing of the fluid transfer relies on something else.  I didn't get enough time to "tank test" my project using the Washington State ferry system, but I did get some understanding on how to "tune" the system without the use of baffles.  Adding or removing baffles from the tank would have been very difficult, so I chose to alter the viscosity of the fluid used inside the tank.

Here are the basics.  The "wave speed" in the tank is partially controlled by the viscosity of the fluid.  The higher the viscosity, the slower the wave.  A wave in acetone moves faster than a wave in fresh water.  A wave in vinegar moves slower than a wave in fresh water.  And a wave in saltwater also moves slower than a wave in fresh water.  Now, you may have never noticed this because we are talking about very small differences, but we are also talking about small differences in the timing in an antiroll tank.  

Just to make things fun, there are two types of viscosity.  There is absolute viscosity (like we were talking about above).  That is basically how fast will a liquid drains through a hole.  Water will be one of the faster liquids, 50 weight oil will be slower.  Simple enough.  But the second type of viscosity is called kinematic viscosity.  It is based on the density of the liquid.  Sort of like how much "punch" does the moving liquid have.  That is a function of mass.  Think of it this way.  Getting hit by a water balloon compared to getting hit by a molasses balloon.  Set aside the stickiness, the molasses balloon packs a bigger punch when it hits you.  It has more mass.  That results is a bigger smack from the same sized balloon.

Water has been the traditional fluid in antiroll tanks.  Not much has been said about whether to use fresh or saltwater.  Understandable because the absolute and kinematic viscosity isn't that great between the two (although saltwater wins on both counts.)  But a greater absolute viscosity doesn't always win.  Take the example of 50 weight oil above.  Thicker than water, i.e., higher absolute viscosity.  But lighter in density, in fact it floats on water.  It loses in the kinematic viscosity battle.  

In an antiroll tank, both types of viscosity matter.  For a small roll, the surface wave crosses from one side to another at a certain speed.  On larger roll, the entire volume in the tank may transfer from side to side.  And it isn't just the weight, but also the "smack" that determines the roll cancelling effect.  Well, that and the moment arm, which I haven't gotten into here.

Turns out that there are fluids that can be adjusted with noncorrosive additives to create the right absolute and kinetic viscosity without using baffles or specially shaped tanks.  That's where I'm at right now.  Adjusting the amount of fluid and the kinematic and absolute viscosity of the fluid in my tank.  Unfortunately, I had already planned to cruise to northern B.C. this summer.  The tank is with me, but I don't have what I need to fine tune the tank.  It works, but I don't know if it is at full potential.  Since I now have Starlink aboard, I may be able to update this blog while "on the road," so to speak.  

* Starlink was quite amazing and I was able to make cell phone calls from remote areas, get better weather updates, and even write a blog about the continued use of the antiroll tank.

Anti-Roll Bag ver. 2.2.

I took a break in my Anti-Roll Bag research and spent several months cruising in northern B.C.  I did take both my homemade and my store bought anti-roll bags.  As I mentioned in the prior blog, ARB ver 2.1, my store bought bag was lightly made and immediately sprung leaks.  Also, it was longer than the width of my flying bridge and therefor was folded upwards at both ends.  I was wondering if this might effect the timing of the water passing back and forth.  I would say yes, in a bad way.  The end of the bag would "inflate" with water, but then drain back out slowly because of the crease at the end of the bag.  It lost some of it "umph" or slosh effect.  Here is a picture (empty) showing how the bag folded at each end.

The $35 store bought bag with about 1 foot extra at each end.

Just as bad was that the bag end folds and unfolds thousands of times a day as the boat rocks.  The material couldn't handle it and developed leaks.  So I went back to my other DIY bag made of stronger material.  But it developed the same issues.  Little leaks and obvious wear points where the bag folded and unfolded thousands of times.  Plus, the force of the "inflation" slowly opened the end seams.  By the time we reached Race Passage on Johnstone Straight, it had just a bit of water in it and was doing nothing.

Here is the bag shortly after a roll to port.  Basically all 150# of water is resisting the roll back to starboard and damping the motion.  I was hoping that the seam being up against the side wall would reduce the stress.  It probably did, but unfortunately, the force of water opened the end seams after days of this.  The material was also starting to delaminate at the creased areas.  You can see that under the bag end and around the coaming is wet from the small leaks.

For a video of the bag in action, look at ARB 1.0.  Unfortunately, the camera was handheld and I didn't include the horizon line.  Still, you can see that the bulk of the water reaches the port side just as the port side is lifting.  Even though a transfer of only about 150# of water, it significantly reduced the roll.  Part of that is the "slosh effect," i.e., the liquid mass has accelerated (for 8 feet across the deck) and has momentum.  The "moment arm" from here to the boat's center of gravity is about 9 feet, thus allowing 150# of water to have more effect than one would imagine.  The boat wants to lurch back, the water says "Not so fast, Buddy."

For those of you who are unfamiliar with this area of B.C., it is notorious for rough water when the wind runs against the current.  We ran it going north on such a day.  Just as we were getting ready to cross to the west to exit by Current Passage, I was hailed by a cruise ship (still out of sight) and asked if I could stay east on the Kelsey Bay side.  The ship was travelling at about 25 knots, so we slowed and let it pass.  But that +20 minutes changed the wave train considerably.  We crossed over to Earl Ledge and rode a fairly smooth laminar flow at about 3.5 knots.  Until the end.  Then the wave train started.  Steep 6 to 8 foot standing waves that were curling over at the top because of the wind.  Even when slowed down, we were still crashing through.  

I decided to move over close to Hardwicke Island so that we could bail out and retreat a little to anchor in Forward Harbor for the night.  But in order to get out of the mess, I had to take several waves on the beam and miss a powerful eddy.  We had prepared for this and everything was stowed properly.  Still, a couple things went flying.  And I got read the riot act by my wife.  What is important to me is to not frighten her and effect her desire to go cruising.  Also important to survive, of course, which we did.  Here is a photo of my recording inclinometer.  It would have been a good time for testing an anti-roll system, however, I don't expect to repeat this.

That's right.  Maximum was 23 degrees to port and 38 degrees to starboard.  Enough of that.

The ARB was proof of concept, but it looks like an anti-roll tank is required for several reasons.  Although the bag slowed the internal wave/water transfer a little without the use of baffles, a bag has other interference issues.  And vinyl coated polyester (or probably any flexible bag material) simply can't handle being creased and re-creased thousands of times under pressure.  Looks like it is time to build an actual tank.  

I've looked for something off-the-shelf that might be modified, but having no luck it will likely need to be completely fabricated.  The interior of my flying bridge cowling is just under 8 feet side to side so plywood is probably the material for a test tank.  I'm thinking that it would probably be about 18-24 inches wide.  As you can see from the picture of the bag above, although a tank would only need contain 3-4 inches of water, the tank (or at least the ends of the tank) would likely need to be 10-15 inches tall to contain all the water that would be transferred in a larger roll.

To be continued . . . 

Anti-Roll Bag ver. 2.1

 Here is what my off-the-shelf Anti-Roll Bag (ARB) version 2.1 looks like.

Folded up.

Unfolded in front of the seat boxes on the flying bridge.

It was intended for use as a "water diversion tube" and cost $34, including shipping.  When full, it is supposed to be a 12" tube that is 12' long.  Laying partially filled on the deck, it is about 20 inches wide.  The problem is that the construction material is about the same as a child's inflatable pool.  It leaked right from the start and I had to take it home to patch it.  While not a long-term solution for an ARB, it was valuable for experimenting.

Because it is several feet longer than my home-made bag (ARB ver. 1.0), I was curious if the water would, when passing side-to-side, inflate the excess tube at each end.  By "inflate," I don't mean with air.  All of the air has been bled out.  But the extra length would allow a more effective total transfer of the water and possibly a slight slowing of the release when the boat rocked back, as was discussed in ARB ver. 1.  At the end of ARB ver. 1 is a video that shows the effect of a flat seam at the both ends of that bag.  That restricts the amount and timing of the water.  The amount and timing of the water back and forth is the key to maximizing effect, i.e., comfort.

I partially filled the tube with about 3 inches of water.  It can't be filled much more than that and work properly.  At minimal rolls, the bag needs to be loose enough to take advantage of the "free surface" effect, although it is not quite "free" here.  There also needs to be enough "head space" in the tube to allow all of the water to transfer to the ends during a larger roll.  The tube could likely hold 70 gallons in theory (590 pounds).  I don't need or want that much shifting weight.

I put approximately 20 gallons (180 pounds) of water in the bag, which limits both the amount of roll attenuation and any possible deleterious effect to vessel stability.  The bag has the effective nature of a 180# person running back and forth on your flying bridge.  Some have expressed concern that this would collapse their flying bridge or overturn their boat.  The solution?  Do not board any vessel for which this would be a possible problem!!

Actually, I've found that the biggest problem with any roll reduction system is testing it, i.e., getting real numbers.  People who have added some type of roll attenuation systems (sometimes erroneously called "stability systems") are often pleased, despite having spent upwards of $40,000.  Yes, they report that their vessel is more comfortable.  Okay, but the question arises, at least for me, how much of that improvement is because of roll reduction and how much is confirmation bias after having dropped a pile of money?  What empirical testing was done before the modification?  Without that, and further testing after modification, how would one know whether simply filling the galley lockers with canned goods would have accomplished a similar roll reduction?  Or 500 pounds ($1,200) of lead in the bilge?  Or just spend the money to buy a lifetime supply of Dramamine for $40 (also available for dogs) and forget about your boat rocking?

What is needed is both a way to actually measure the roll reduction (if any) and then quantifying the effectiveness of a roll reduction system (maybe as dollars per degree of roll reduction??).¹  I'm not sure if there would be any agreement on the latter, although it seems like common sense to examine the cost/benefit issue.  If $40K stabilizer fins reduce a roll by 90% and $200 reduces the roll by 50%, which is the better system?  It could be that some will pay any price for the same feeling of stability as their "dirt home,"  i.e., they will pay any amount to have a "dirt boat."  Solid as a rock (which in a boat usually means you are hard aground.)  Seriously, I don't know if actual numbers comparing cost with effectiveness will change any opinions and would only cause arguments.  It seems that roll reduction has something in common with politics.

To further complicate things, roll reduction consists of two measurements, amplitude and acceleration.  A 10 degree roll can seem fairly minimal if the "snap back" is slowed down because of an increased roll period.  And it will depend on each individual's "sea legs."  ("Step right up and get your sea legs: $40,000 for a pair!")  But how does one test a system to get an accurate before and after picture?  One traditional way is to pre-guesstimate by doing complex calculations when the vessel is designed.  The other is to hire a naval architect after the fact and hope for the best.  As noted by the naval architect in the prior blog (ARB ver. 2.0), a naval architect can be expensive and still get the estimate all wrong.  

I think that what might be better for most recreational boaters is an old-school "tank test."  I'll just put my boat in a giant tank, generate a big wave of known repeatable amplitude, and see how the hull reacts given different adjustments to a roll reduction system.  Fortunately, I happen to have a huge tank and a giant wave generator available to me.  The Washington State ferry Walla Walla runs close by my marina several times a day.  At a length of 440' and a beam of 81', travelling at its standard 17.5 knots, the Walla Walla generates quite a wave.  It runs in a charted ferry lane in the calm Port Orchard Canal and its course, speed, and distance from me show up when I monitor its AIS broadcast.  That can be my tank with a wave generator producing a standard amplitude wake which I can monitor. 

As it turns out, the smaller Kitsap County "fast foot ferry," a catamaran running between Seattle and Bremerton, puts out a nastier wake for my boat when I'm running in the same direction.  These catamarans were designed to be "low wake" ferries (really?), but that is the wave generator I used for most of my experiments.  The sacrifices I make in the name of science.

Running my boat on repeated parallel courses at a standard speed gives me the required benchmark to accurately determine the effectiveness of changes to my ARB roll reduction system (or anybody's system, for that matter).  I can get the ferry SOG and COG from AIS.  For instance, when the ferry is running in the channel at its normal 271 degrees true at 17.5 knots (30 knots for the fast foot ferry), I can run parallel at 271 degrees, 5 knots, and exactly 300 yards away.  Having had the ferry pass me many times in the past, I know that within a minute or two this will give any roll reduction system a good workout.  And because of the ferry schedule, I can repeat the exact thing more than 12 times a day if I choose, or a couple times a day for a week, or try it again next month with a different system.  

It turns out that having the ferries approach from behind gives me the greater rolling.  If I'm going in the opposite direction, I would effectively be crossing the wake at a large angle.  When the ferry (or any large wake making vessel) sneaks up behind me, that is when I encounter a generated wave that most closely harmonizes with my boat's natural roll period (approximately 3.55 seconds).  At one point, I was lucky enough to get both the big Washington ferry and the little Kitsap ferry wakes at the same time.  I recorded a 12.5 degree roll (without the ARB).  We call those a "bell ringer" because my boat rocks enough to ring our bell.  I'm not going to do that on purpose again.  Although for big rolls, check out my post on Anti-Roll Bag ver. 2.2.

To measure the effectiveness, if any, of my roll reduction system, I use my inclinometer phone application.  I needed a standard roll test without any roll reduction system in operation to see just how much my boat rocks from that wake.  My experience has been that a 10 degree roll from the ferry wake is "normal."  Note that a 10 degree roll measured accurately with an inclinometer is what most boaters claim to be a 20 degree roll.  If one is not looking at an inclinometer, it just isn't possible to claim accuracy and, of course, the guess is always extremely high.  I introduce my inclinometer in Anti-Roll Bag ver. 2.2.

I also have an accelerometer phone app.  I have found that the "sensation" of a gentler roll is more easily felt than trying to analyze the sine wave recordings on the phone.  The resolution just isn't sufficient to really see what's going on.  Measurements to date indicate that the ARB has a significant effect on roll degree and, therefore, acceleration.   And what appears to be the biggest benefit is the reduction in a synchronous roll.  It's that old feeling, especially with a wake, where it starts small with back, Forth, BACK, FORTH . . . and then you know what's coming.  With the ARB (as likely with other systems), there is much less tendency to build.  Therefore, it is less likely that "the big one" will ever come.

The few detailed studies that I have found on anti-roll tanks (which have field-tested measurements) show that there tends to be certain amplitudes that are really reduced.  It sort of makes sense being that we are essentially "tuning" to offset frequencies and neither my roll period or the water transfer in the ARB remains exactly the same for different amplitudes.  The studies showed that, for instance, a tank may have sort of a "standard reduction" of 40%, but at several certain rolls the reduction can be 60%.  So the sine wave reduction pattern is not likely to be mathematically perfect. Kind of odd to think about, but one ends up with a 50% reduction overall.  All of the studies I have read were primarily concerned with reductions in amplitude with little addressed as to acceleration. It might be more complex to examine what an ARB does (and where it does it) to the vessel's roll acceleration.

With accurate measurements using my Washington State Ferry "tank and wave generator" and the accurate monitoring of my vessel's speed, direction, and distance off, I can generate accurate and quantifiable roll results, something which I have never heard any recreational boater mention when talking about their new "stability system."  

For the first numbers, I was surprised.  I had rolled my boat at the marina and timed 10 complete rolls.  It is actually quite complicated to do this without the boat sooner or later tugging on a mooring line.  But I got some clean "rocks" and came up with the 3.55 seconds per period.  Those tests usually started by building to a 7 degree roll, then I let go and measured.  With the ARB filled up, I had a very difficult time getting the boat to rock 6 degrees.  I was winded by that time and just let go and recorded the rocking.  Unfortunately, the rocking died down so that I couldn't get an accurate 10 rolls.  And what I did get surprised me.  The roll period averaged to 3.45 seconds.  That's not much of a change, but it is a faster roll period!  Just the opposite of what I was hoping to accomplish!  

But the benefit of the ARB can definitely be felt on board, so what's happening?  It appears that a reduction in roll angle/acceleration is more "sea kindly" than a reduction in roll period.  And one way to reduce acceleration is to reduce the arc, i.e., the distance traveled in a given period of time.  Getting the boat to rock was more difficult with the ARB filled, and the ability to stay rocking was greatly reduced, so much so that the additional 1/10 of one second in the roll period was immaterial.

Y

X

Above is a typical damping sequence.  The X and Y axis are not indexed, but we can see what's going on.  We will call the first roll (X₁) a "1," and that's basically the max we can get at the dock.  The boat then flops back to the other side to about an .8 (X₂), then back to .6, back to .5, back to .4, etc.  Not much damping going on and, looking at the entire graph, we could probably measure out to 10 complete rolls.  We then divide the time by 10 and get our seconds per roll measurement.  For my boat, that averaged 3.55 seconds.

Here is another damped cosine graph with greater damping.  The first roll at X₁ is a 1 again.  The second roll, X₂, is only a .6, followed by X₃ (a .36), then .24, .18, .11 and basically gone.  We can't measure a sequence of 10 rolls here and it's unlikely we could rock the boat enough.  Maybe 5 complete rolls is all we can get.  When the time is divided by 5, my boat averaged 3.44 seconds with the ARB filled (increased damping).  That is a 10th of a second faster.  But ultimately much more comfortable.

I think that some of the improved comfort when field testing was because of the "other side" of the graph that isn't taken into consideration here.  How does a boat generally get to a roll of "1"?  It is usually because of synchronous rolling.  Basically, what I was trying to do by running parallel to a ferry wake.  A graph for the building of a roll would look similar to a reflection of the above graphs because the same amount of damping effect would take place at the start of a synchronous roll sequence as well.  First port, then further to starboard, then even further back to port.  We've all felt that.  

But look at the difference increased damping would make in graph #2.  The first beginning roll would be choked more by the increased damping, making it unlikely that we ever get to a "1" based on the same wake height and frequency.  We would begin with a smaller roll because the boat is rocking less, a synchronous roll would then not build as fast (therefore as great), and the rolling stops sooner.  What more could we ask for for a $34 roll attenuation system?

Which brings me back to my $34 ARB.  It worked, but it leaked.  Bad.  I took it home and patched 4 leaks and still didn't get them all.  So I went with a slightly different system.  To be continued at Anti-Roll Bag ver. 2.2.

¹   Naval architects are very fond of abbreviations and acronyms for various formulas.  LOA for length overall, S/L for speed/length, etc.  I propose a new one for roll reduction systems (aka stabilization): Dollars per degree damping (abbreviated $/°d).  Capt. Beebe, in his book Voyaging Under Power, recommends that an ocean cruising powerboat should have both active fins (+$50K) and paravanes (+$20K).  But wait, there's more.  Operating both systems will increase fuel consumption and neither will be very effective at anchor.  I'll leave it up to the NAs to figure out how to calculate the actual ongoing $/°d of a +$70K stabilization system.  For that kind of money, the system better also keep my boat washed and my beer cold.

Anti-Roll Bag ver. 2.0

 Although my first experiment with the anti-roll bag (ARB ver. 1.0) was successful, I wanted to try a different bag. To accompany a blog about a brand new bag, you may want to open Papa's Got a Brand New Bag in a separate window for a musical accompaniment.    

I wanted to try some kind of a bag that was an "off-the-shelf" product.  I also wanted to get some actual numbers so that I could compare one version to another, both as to cost and effectiveness.  The cost for my first DIY bag, ARB ver. 1.0, was about $200.  The cost numbers are easy to calculate for version 2.0.  I found a "water diversion tube" on Amazon for $34 (free shipping).  These are intended to be filled with water and placed across on opening to a building in order to stop water intrusion.  In my case, the tube was 12 feet long (probably to protect a garage door opening.)  

At 12 feet long, the new bag is longer than what fits across my flying bridge.  Both ends of the tube have to be pointed up in the air, which makes them empty.  But that is not a bad thing.  First, all the air is bled out of the bag.  When my boat tips, the water rushes to one side and sort of fills up what was the empty space at the end.  Thus, unlike my first version, more water can "slosh" up into the empty tube.  This changes the timing a little and provides a little more bang for the buck than my original flat bag.  In the video of ARB Ver. 1.0, the end of the bag is constricting and doesn't allow the slosh to run up the side.  I thought I would see if that makes a difference in stability.

Here is an article by a naval architect that briefly compares the use of an anti-roll tank to other forms of vessel roll attenuation:

"Passive Anti-Roll Tanks . . .

Space requirements are very difficult for small pleasure vessels (say below 60 feet). Possible undesirable effects on stability, depending on the vessel (large free surface effect). Very unlikely as a retro-fit. Possibly noisy. Relatively complex to design correctly (therefore relatively expensive to design). Relatively inexpensive to build. Relatively simple in use."

Here are the claims broken down numerically:

Con

1.  Space requirements are very difficult for small pleasure vessels (say below 60 feet). 

2.  Possible undesirable effects on stability.

3.  Very unlikely as a retro-fit. 

4.  Possibly noisy. 

5.  Relatively complex to design correctly.

6.  Relatively expensive to design. 

Pro

1.  Relatively inexpensive to build.

2.  Relatively simple in use.

I'll use this NA's Pro and Con assessment of an anti-roll tank in analyzing my ARB.  I've already passed the first hurdle (Con #1, i.e., "space requirements are very difficult for small pleasure vessels say below 60 feet)."  I'm 30 feet LOA and space isn't an issue.  The "space" issue is simply one of personal preference.  Which would you rather have on your flying bridge, 1) a freezer, two kayaks, and a propane wiener roaster, or 2) a roll attenuation system.*  As we will see, a roll attenuation system can be by far the cheapest to install, maintain, and operate.

Con #3 ("very unlikely as a retro-fit") also hasn't been a problem.  I guess it depends on if one is willing to sacrifice some space for comfort, so this is simply a version of Con #1.  I don't know of a roll attenuation system that does not require any space, so this isn't really a Con for an anti-roll tank, it is a Con for every system.  In fact, I can't think of any roll attenuation systems that require less space than an ARB.  I'm not sure why this is included as a Con only for this particular system.

Con #2 ("possible undesirable effects on stability") is also true for all systems.  Take, for instance, the new concept of hydraulic operated fins on the outside of the hull.  I say "new" because anti-roll tanks of various types have been around for over 150 years.  And, yes, there have been a few incidents of undesirable stability (can't find them on Google, but can imagine it happening).  But when hydraulic operated fins have been around for 150 years, there will be the same or maybe more incidents.  Why?  Hydraulic gizmos on boats (just like electric gizmos) fail given time.  If the fins freeze in the wrong position, or get 180 degrees out of sync, there will be an incident.**  So Con #2 also applies to all systems.

Con #4 (the "possibly noisy" claim) is likely based on someone's imaginary perception of what a partially filled tank of water would sound like (in a thin steel tank?)  First, when a powerboat is in operation the engine noise generally covers up the sound of liquid moving in a remote tank, at least in boats under 60 feet.  To the operator, the "noise" would be like that generated by the bow wake or water tanks.  Have you ever heard anyone say that their boat is really noisy because of the splashing water in their water tank?  Or their bow wave is really noisy?  Based on the experience of millions of boaters, I would change "possibly noisy" to "implausibly noisy."  Certainly not a noisy as running a generator to operate a gyro stabilizer system.  (The NA ignores the noise issue for the gyro system).  And the majority of those dragging paravanes also note that there is a humming from the cable that it transferred throughout the boat.  The "cure" seems to be having a section of chain connected to the paravane, thereby reducing harmonics by increasing drag and fuel usage.  (The NA also ignores the noise and snag issues for the paravane system).  

I should speak a little bit about "sloshing" because an anti-roll bag differs from a tank. The "slosh effect" is a real thing and can be a problem, though not as big of a problem as is often stated.  Here is some info if you want to nerd out about the slosh effect.  It should be noted that "slosh" is both a noun and a verb.  We can have the verb (i.e., the slosh effect) without the noun (i.e., the slosh sound).  How?  The slosh sound is generated by the interaction of the liquid with the atmosphere.  If there is no interaction, there is no sound.  Think about trying to hold on to a big water balloon as it squirms around in your hand.  The water moves back and forth inside, but there is no slosh sound because the water balloon contains no air.   That is one advantage of an antiroll bag over an antiroll tank.  Of course, if one can soundproof an engine compartment one can soundproof an antiroll tank or bag.

On the Pro side in the above article, the NA notes that an antiroll tank is "relatively inexpensive to build and relatively simple to use."  As already noted, my ARB ver. 2.0 cost $34 and it is definitely "simple to use" (it required filling up with a hose.)  Filling it probably took 5 minutes and cost nothing at my marina.  There is no hydraulic system to activate, gyro to engage, generator to fire up, or paravanes to drop overboard.  Not even a button to push.  And the article calls that relatively simple?  What could be simpler?  Certainly no other roll attenuation system.

Which brings me to another plus that isn't included in the NA's critique.  Maintenance.  My $34 bag had a little leak even when new (grrrr).  It needed to be topped off every couple of weeks.  I have tried to fix the leak, but I expect that the bag will wear out over time from motion in rough seas.  I'll guesstimate that it will need to be replaced every other year.  So the annual maintenance cost for this ARB stabilization system is equal to one-half the installation cost.  $17 per year and hopefully the next one won't leak.  Check out the maintenance schedule, cost, and down time for a gyro system.  Hint: it's more than 5 minutes and $17/year.

And the NA forgot Pro #4, the ARB works at anchor (without running a generator).  Is there another roll attenuation system that works as effectively at anchor as underway?  The NA forgot to include that Pro, likely because no other system comes close.

But according to the NA we still have two remaining minuses for the use of antiroll tanks.  The article claims:

5.  Relatively complex to design correctly.

6.  Relatively expensive to design.

These two Cons are really just one claim.  In fact, all of the negative claims against the use of antiroll tanks for roll attenuation are just Con #5 restated.  Yes, an antiroll tank system can be "relatively complex to design correctly."  That's the only downside and term "correct design" subsumes all of the other Cons. 

"Correct design" requires the intelligent use of some space (Con #1).  If not designed correctly, it could have "undesirable effects on stability" (Con #2). Skilled designing could be required for a retrofit (Con #3).  An improperly designed system "could be noisy" (Con #4).  Designs created by an expensive naval architect could be expensive (Con #6).  Even more expensive if designed incorrectly and needing redesign (duh).  Of course, all of these difficulties also pertain to every other type of roll attenuation system available.  And all of the other systems are going to cost more than $34.  And all other types do not allow for inexpensive incremental experimentation in tuning the system as allowed with an ARB.  It turns out that "correct design" of an ARB can be a DIY project for the small recreational vessel.  A gigantic Pro only available for the ARB.

For instance, the common installation process for some forms of recreational vessel roll attenuation systems begin with 1) "haul out your boat" and 2) "cut a hole in your hull."  Sounds costly and makes me more nervous than a bag of water on my flying bridge deck.  If you don't want to haul out and drill holes, then "install a $50,000 gyroscope and the required $15,000 240V generator."  That's more than I paid for my boat.  Or "fabricate 20 foot masts operated by electronic winches" and then "drag stuff through the water" to make it work.  Really?  Is there is someplace that doesn't have low bridges, crab pots, abandoned gill nets, submerged rocks and floating snags?  What fun is that?

I recently saw all of the popular roll attenuation systems at the Seattle Boat Show (excepting the ARB, of course).  The booths reminded me of "Psst, hey buddy, I have a bridge to sell you."  Commercial systems have to be really, really expensive.  How could a business make money selling a $34 vessel stabilization system?  It is actually the low cost that is the biggest Con of an anti-roll bag for a naval architect, a commercial business, and a boat yard.  Where's the profit?  An anti-roll tank business couldn't even afford a booth at a boat show.

Next, I need to experiment with what a cheap roll attenuation system can do.  Specifically, what must it need to accomplish to be worth the time and expense of "installation?"  We could look at it mathematically.  If the installation of a $52,000 hydraulic fin roll attenuation system reduces an 11 degree roll to a one decree roll (a delta of 10 degrees), what must a $34 ARB system do for an 11 degree roll?  If I've done the math correctly, a $34 stabilization system should reduce an 11 degree roll to a 10.9925 degree roll based on a simple cost/benefit analysis. Any additional roll attenuation is a win for the ARB system.  I should have some test results in Anti-Roll Bag ver. 2.1.  Anti-Roll Bag ver. 2.1.

  *  Roll attenuation systems are often called stabilization systems.  I'll try to stay away from that misnomer.  The stabilization of a vessel generally remains the same and a roll attenuation system merely makes things more comfortable by damping the vessel's roll.  Still, "stabilization system" is so commonly used that I'll likely use the term inadvertently.

**  Since posting this, I did find a YouTube video of a 60' boat where the stabilizing fins got 180 degrees out of sinc because of some computer glitch.  It is violently flopping around in 5 foot seas.  The fin system was turned off before any down flooding or damage, but it looked nasty.  An active system that can greatly reduce a 20 degree roll can also greatly increase a 20 degree roll.  I'll link to the video if I find it again (or leave a note if you've found it).  The NA seemed to have missed this Con for the electrical stabilization systems.  A blown fuse can kill you.  And he was concerned about a sloshing sound.