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 inorder to syncronize with my boat's roll period.  Actually, not quite syncronize.  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 just after the vessel begins to right it,self.  Thus each "snap back" is diminished, making building syncronous 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 that are on Google.

You will be glad to learn that there is another way of doing this.  Commonly called "bench 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 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 centripital 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 explanations for the moment and concentrate on results.

Scale models of antiroll tanks are easily built and studied in a wave tank (bench testing), 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 bench 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 tendancy 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 tendancy to roll (as with dragging plates in the water or with fins that rotate to lessen a roll).  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 syncronomous rolls.

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."  Except 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.  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.  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 on the roof of my car to the marina.  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 baloon compared to getting hit by a molassas baloon.  Set aside the stickiness, the molassas ballon packs a bigger punch.  It has more mass.  That results is a bigger smack from the same sized baloon.

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.  Unfortunatly, 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.