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 (10.5’ x 29’ at the waterline). It has a deeper than average single keel (46”) but even still the boat is “snappy.”
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 3.4 seconds. That makes port side up to port side down 1.7 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 166 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. 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 less frequent, and the rolls 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." 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 shuts down 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.
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 get it too rock. 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 and only 20 seconds for it to dwindle to almost nothing. As can be seen, ARB stabilization is just as effective at anchor.