Video of Antiroll Tank in Action

 Here is some video and photos of our recording inclinometer to show the effect of the roll tank.  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 was 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 10 2' waves 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 was like the tank was 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 saw that day.  Very foggy and some long swell 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, 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 about 10 degrees and seemed fairly mild mannered.  

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 just from just the wind, even if there were no waves.  It was howling pretty good.  So the actual wave induced rolling was about 10 degrees.  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, it would still be only a $1,000 investment.  From what I've heard, it 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 generator (I've removed mine), etc.  And, as I said in a prior blog, 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 call."  Other antiroll systems have had their issues, 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 the 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 active fins. I don't intend to do that again thanks to my antiroll tank.

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.  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 kinetic 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 kinetic 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 kinetic 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 kinetic 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.