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.