Filling the Antiroll Tank

I installed the tank and put in the fluid.

I increased the viscosity by adding HPMC, as I said in the last installment.  I also increased the density of the water by adding calcium chloride.  Calcium chloride is a salt (like sodium chloride), but not as corrosive.  I actually added it for more than just increasing density.  Since density is basically just weight in my application, I could do approximately the same by adding more water.  But I was concerned about whether HPMC was biodegradable, i.e., whether it would rot or turn sour, etc., when sitting in the ART.

I couldn't find anything that really addressed the issue, but since HPMC is a type of cellulose, and cellulose is basically wood, and wood can decompose, I thought I'd make the environment a little less hospitable to whatever bugs and bacteria might live on HPMC.  Calcium chloride seemed to fit the bill.  It also increases the viscosity of the liquid in the ART, thus slowing the weight transfer of the water without using baffles, which tend to not only slow but also diffuse the effect of the internal wave.  Adding calcium chloride really heats up the water (which is why it is so effective when used to melt snow).  Because heat reduces viscosity (as noted in a prior post), I'll really have to wait a day to see the "room temp" viscosity.

I should say "internal waves" (plural) as what is going on inside the tank is not a single wave going back and forth.  I found a study where an ART was built with a glass side so that the authors could take pictures of the wave as the tank teeter-tottered.  Video would be nice, but stills were also interesting.  The study concentrated on commercial vessels (most of the tank dimensions given in the study were in the order of 25 meters wide) with water depths in the tank of 2+ meters.  Clearly thousands of pounds of moving water.  The study did mention the approximate requirements of a "recreational vessel," but the example was of a vessel of over 100 feet.  Not really applicable to my boat, but still interesting.

Here is the tank with just a little water at a 3 and 10 degree roll.  Too shallow for a surface wave, the effect is simply the entire amount runs side to side.  Without the video, it is a little difficult to deduce what is going on.  In a properly tuned ART, the water would arrive just after the "vessel" begins its recovery, i.e., its roll back to the other side.  There would be the added weight hindering the return roll as well as the water's momentum suppressing the roll.  The water begins arriving just after the left side of the tank starts to rise and stays there while the vessel tips the opposite direction.  No surface wave, just slosh.

Here is the tank with more water in it.  

 This is at small roll angles (.5 and 1 degree) with the tank half full.  There is a wave on the right and an approaching wave near the center.  Those will intersect and form a nodal point, all of which has to be timed such that it reduces the vessel's roll.  Not a lot of weight transfer or moment, but then it only needs to offset small rolls.

Here is the effect at larger angles (3 and 10 degree) with the tank still half full.

Here we can see some of the limitations of too much water in the tank.  On a large roll, there isn't enough room at each end to accommodate the water.  Also, we have gone from a wave to a slosh, with some of the attributes of a tidal bore.  The water that is stacked up to the right will be held in place to a certain extent by the water continuing to run left to right.  

The study mentions these two modes that a free surface tank exhibits.  The first is "free surface," where a wave passes back and forth.  This is usually in smaller rolls, say of less than 3 degrees.  And it isn't always a single wave.  It is more like a major wave and some minor waves.  Once the bulk of the fluid is moving side-to-side, it is a slosh, not a smooth surface wave.  At higher roll angles, the entire amount of fluid is transferring, leaving essentially a dry tidal flat at the high end.  Thus, ART effectiveness is a factor of how deep the fluid is in the tank and how much room there is at each end of the tank to accommodate the slosh.

My design has two features that I haven't seen in any of the writings.  First is a modified viscosity/density, i.e., not plain water.  Slightly higher viscosity results in slightly slower wave and slosh speeds.  Second is a curved bottom on the tank.  The initial free surface wave is slowed as it crosses the center "shallow" area.  And the tank ends empty at a higher roll angle because of the curved bottom.  Based on the depth of fluid I'm using, a flat tank would completely empty one side to the other at a 7 degree angle, while my curved bottom empties at a +10 angle.  How much difference does that make?  Calling all grad students!

I am purposely using a smaller (shallower) amount of water.  That means that the ART sort of taps out at around +10 degrees with no more water to send to the other side.  The sloshing water does have more momentum at higher roll angles, based on a longer "drop" during the heel.  So a larger roll still produces a larger offset, but not a great deal more.  My theory is that the ART will have dampened the rolls that might lead to a 30 degree roll, so no need to actually snuff out a 30 degree roll.  I will be able to show that in the following videos, where I induced my boat to roll with the new ART in place.

First, check out my video in a previous blog where I rolled my boat without the ART.  Three steps aboard on the gunnel is the test and still rocking pretty good 8 rolls later.

https://stuffsax.blogspot.com/2025/09/tank-testing-art-at-dock.html

Here is the same three steps with the ART in the flying bridge with 20 gallons of fluid.  It doesn't rock quite as much to start because the ART is already at work, i.e., it is countering the roll right from the start.  But what is really noticeable is the damping effect after I have induced the roll.  

In two or three rolls the boat is back to the tiny little motions that it has "at rest" when in the boathouse.  Time to take it out and test it with the Washington State ferry wakes.