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 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.  28 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, a freezer, two kayaks, or 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?  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 one, likely because none exists.

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 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 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 $34 cost that is the biggest Con of an anti-roll bag to a naval architect, a commercial business, and a boat yard.  Where's the profit?  An ARB business couldn't even afford a booth at a boat show.

Next, I need to experiment with what a $34 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.

The Anti-Roll Bag ver. 1.0

 

Anti-Roll Tanks for boats have been around for over a century.  They basically consist of a tank that runs athwart ship filled with water.  When the vessel heels to port, a surface wave (or the entire volume of water) runs to port.  Various methods control the water's arrival until just after the boat has begun to rock back to starboard.  The water's additional weight transfer, and the water's momentum in hitting the port side of the tank, effectively neutralizes some of the righting force, i.e., it takes some of the "snap" out of the recovery and reduces the ability of the boat to begin synchronous "increasing" rolling (as when encountering the wake of another vessel).

There has been a considerable amount of research on "fine tuning" the tanks for a certain roll period, vessel hull design, etc.  Over the years, changes to the anti-roll tank’s placement, shape, internal baffles, etc. have been tried to increase the effectiveness. There are also tanks which are not “free surface” and have constrictions, pressurized air, water pumps, etc. to increase effectiveness. Of course, the more complex the system, usually called “active” systems, the more likely the system can fail. 

My idea was to experiment with a strong bias towards the KISS principle. I’m not looking for a system that would need to respond effectively to 30-degree rolls or stabilization in 10-foot seas. I’m just looking to calm my boat’s “excessive stability,” caused in part by its large beam to length ratio (11.5’ x 27’ at the waterline). It has a deeper than average single keel with ballast (46”) but even still the boat feels “snappy” during big rolls.

My flying bridge is about 8.5 feet across (and 10.5 feet above the waterline). I bought 3 yards of vinyl coated polyester (54” wide) and heat welded it into a bag, adding a fill port. That produced an anti-roll bag (ARB) that was about 24” across and 9’ long for about $100. I wanted the extra length because I figured that in a big roll, a lot of water would go into the end of the ARB and I didn’t want to constrict that motion. As you can see from the video below, it may still constrict some of the water despite the extra room at the end.

Wave speed is affected by several variables. One of the variables is the pressure above the liquid surface. By not having a “free surface” and using the tension of the bag, I think the wave is slowed a little. We are talking about a fraction of a second. My boat’s roll period is about 3.5 seconds. That makes port side up to port side down 1.75 seconds. If I can slow the side-to-side water transfer down to +1.8 seconds, mission accomplished. Filling the ARB semi-tight (about 20 gallons) with no air in it seemed to do the trick.  That is 167 lbs. (less than I weigh).  While I don't normally run back and forth on the flying bridge, I'm sure the structure can handle the weight shift.

Here is a short video of the bag in action on my flying bridge. This was taken while crossing the Straight of Juan de Fuca. We were experiencing occasional 10-degree rolls (one 11 degree) in a beam sea from swells coming in from the Pacific.  I was measuring using an inclinometer app on my phone.  The 10 degree rolls were always the result of building synchronous rolls, never just a 10 degree roll out of the blue.  We were always given a warning when it was going to happen.  You probably know the feeling.  It starts with 3 degrees one way, 6 the other, and you can then tell the next will be even bigger.

I then filled the ARB using my potable water. You can see that when I roll to starboard, the port side of the bag is almost empty. When rolling to port, the water would arrive just as the port started to lift (and the same on the other side). We no longer built synchronous rolls to 10 degrees. 6 was the max, the building was much less frequent, and the rolling subsided faster. A noticeably better ride. With no air inside the bag, the water moving back and forth could not be heard from the lower helm.

I should have put the camera on a tripod or steadied it better.  That would have given a better understanding of the relationship between  the boat's rocking motion and the arrival of the "wave" (more like a "slosh" on the larger rolls).  But the horizon can be seen just over the coaming on the left.  The boat tips to port, and just as it begins to lift, the bulk of the water arrives and dampens the snap back to starboard.  Not completely, as can be seen, but enough to make quite a difference for the safety and comfort of passengers.

I also tested the ARB in the marina.  I rocked the boat myself by standing on the dock and using my weight on-and-off to get the boat rocking.  Seven degrees was possible without attracting too much attention.  Seven degrees was really difficult when the bag was full, as the shifting water was fighting me from the beginning.  But what I was interested in was the increased "roll decay" caused by the ARB.  For this, I switched to an accelerometer app.  That would show the timing and decay of the rocking, rather than the roll angle.

Unfortunately, I didn't notice two things about the application.  First, it matters somewhat as to whether the phone was level.  For the first test, I had the phone at the helm.  I then filled the tank and put the phone on the side deck, where it wasn't level.  That moved the "zero point" off center.  Second, I didn't notice that the application had an automatic set on the Y axis (the vertical scale).  Note that the first test (ARB empty) shows .5 as the first metric and the second text (ARB full) shows 1 as the first metric.  But basically, the max acceleration is .1 G.  And again, what I was interested in was the rate of decay.

Here, it took me over 30 seconds to get the boat rocking to the point I wanted.  You can see that the sine waves prior to about 32 seconds (X axis) have an asymmetric peak.  That is a result of me jumping back and forth off the boat to increase the rolling to 7 degrees.  Once I stopped, letting the rocking decay naturally, it took about 30 seconds for the G force to halve.

Here with the ARB filled, it took me 150 seconds to get my boat rocking to approximately the same .1 G force (at about 6 degrees) and only 20 seconds for it to dwindle to almost nothing.  As can be seen, ARB stabilization is just as effective at anchor.

I’ve read quite a few studies on how to slow down the wave speed in a regular tank. Baffles and complex plumbing seem to be common. One study had diagrams of 6 different shaped tanks with variable baffles. It seems to me that one of the problems is simply the liquid that is always used. Water.  If one used maple syrup, the wave would naturally be slower (but there would be other issues, of course). 

It might be possible to adjust the wave's motion by adding rock salt to regular seawater.  It increases the density (as shown in the picture above from the Dead Sea, causing the newspaper reader to float abnormally high in the water.)  You can also see the effect of the increased viscosity in the picture.  See the little wave just below the newspaper reader?  It isn't "splashing" over like a normal wave.  It is "slopping" over.  That is the increased viscosity that might slow a wave by the needed fraction of a second.

In looking at what liquid could have the right "sluggish" viscosity, I came across what is called “thick water.” It is water to which food-grade calcium chloride is added, possibly to the extent of making it goopy to the point of way too viscous.  But there might be a water to calcium chloride ratio that would slow the wave down without baffles, etc. As a plus, calcium chloride doesn’t increase the liquid volume, meaning that adding 2 pounds of calcium chloride to a gallon of water (8.34 pounds) results in the gallon of solution weighing 10.34 pounds. Thus, slowing the motion while increasing the liquid’s effective mass. We shall see.

Here are some further experiments.

DIY Muffler for Diesel Heater

When I installed a diesel heater on my boat, I was concerned about the exhaust noise.  I had tried the muffler that came with the heater, but it was a fail for two reasons.  1) It was spot welded around the edges and therefore was not possible to use aboard.  It would leak exhaust fumes inside.  2) It barely worked.  There are two versions of the little stainless muffler that comes with these diesel heaters, although they look identical on the outside.  One is a straight-through design and the other has a little bend.  They are a "glass pack" style of muffler with the exhaust pipe, having perforations, going though a canister filled with spun fiberglass.  But they are tiny and barely take the edge off of the whining exhaust note.

I decided to build my own.  The flexible stainless steel exhaust pipe that came with my heater was about 23mm.  That slipped over standard copper 3/4" pipe in my scrap pile.  I also had a few feet of 1.5" copper pipe.  I bought a couple end caps for the 1.5" pipe and went about building my muffler.  I drilled a bunch of little holes (1/8") in the center section of the 3/4" copper pipe.  I then drilled holes in the ends of the 1.5" end caps to fit the O.D. of the 3/4" pipe.  I soldered one end cap on the 1.5" pipe, slipped the 3/4" pipe through, filled the 1.5" pipe with spun fiberglass (from my attic insulation), put it all together and soldered it up (high-temp solder).

Here it is next to the stainless muffler that came with my diesel heater.

On the right is a 3/4" elbow and then a little length of the flexible stainless exhaust tubing that came with the heater. It is sealed with a hose clamp and high temp silicon.  I also attached a little of the stainless exhaust tubing on the other end (which then attaches to the through-hull) to make alignment easy.

Since my fabricated muffler is 4 times longer than the stainless muffler, I was expecting an improvement.  But it was even better than expected.  A neighbor in the marina asked how my diesel heater project was going and I told him that it was running as we were talking.  But it can't be heard.  I decided to make a video/sound recording.  To actually hear it, I had to climb on the boat next to me and hold my phone down by the exhaust outlet.  Even then, I had to turn up the microphone sensitivity.  Unfortunately, every time I recorded, some gulls started screaming a few docks away.  But it is possible to hear the heater if one were in a dinghy passing by a few feet away.

Here is another recording that I made at a really quiet anchorage in northwest British Columbia.  I stuck the phone out the window right above the diesel heater exhaust.  Dang it, again the background noise covered up the sound of the heater exhaust.