My Anti-roll Bag Experiment

[Delta Riverat]

I'm going to try the 12 inch by 10 foot diversion tube thing on the floor of the FB and see how it goes. That will be very easy to do and easy to get rid of if it doesn't pan out. If it works then I'll expend the effort to move it under the helm and build a "dam" for it in the nose.

But this will be added to the end of the list with haul out and bottom paint / through hull service / shaft packing replacement, LiFePO4 house upgrade, washdown pump replacement and getting a working TV installed coming first -

 

[SteveK]

I'm going to try the 12 inch by 10 foot diversion tube thing on the floor of the FB and see how it goes. That will be very easy to do and easy to get rid of if it doesn't pan out. If it works then I'll expend the effort to move it under the helm and build a "dam" for it in the nose.

But this will be added to the end of the list with haul out and bottom paint / through hull service / shaft packing replacement, LiFePO4 house upgrade, washdown pump replacement and getting a working TV installed coming first -

So Saturday?

 

[Xlantic]

To complicate the issue, my experience with the bag shows that it reduces the degree of the roll, dampens it, and probably reduces roll period (still haven't found a recording inclinometer app).

Marco, this iPhone app may do what you are looking for:

Physics Toolbox Sensor Suite

https://apps.apple.com/us/app/physics-toolbox-sensor-suite/id1128914250

 

[Marco Flamingo]

Marco, this iPhone app may do what you are looking for:

Physics Toolbox Sensor Suite

https://apps.apple.com/us/app/physics-toolbox-sensor-suite/id1128914250

Thanks. That toolbox (now downloaded) should be very helpful. It showed me that I may have been looking for the wrong tool. I thought a recording inclinometer would be helpful to show angle of roll over time (and allow examination of roll period). I would simply put my phone next to me at the lower helm and get the angle of roll over time. But that might not give the full picture from a safety/comfort point of view.

The Tool Kit has an accelerometer which, although it doesn't show roll angle, is probably the real issue. The acceleration/deceleration graph is going to be interesting to examine. Instead of placing the phone next to me at the lower helm, I would put it at the upper helm to get more motion on the recorded graph (in meters per second). As we know, a 10 degree roll at the lower helm isn't like a 10 degree roll at the upper helm even though the roll angle and period would be the same. At the upper station, there is a much stronger "lurch" when the boat rolls, and that lurch is right at the up/down (or down/up) roll point. Which made me wonder how much of the effectiveness of the anti-roll tank is in part calming that transition point.

The transition is a point at which rolling chocks and paravanes haven't yet really engaged. At the split second when the boat approaches, is at, and recovers from maximum roll, there isn't much water "rolling" under the boat for the chocks to catch and be effective. At the same point, there would be no up or down force on either paravane. Once the vessel begins to roll back, the chocks and paravanes become effective, and the faster the roll the more resistance to roll by those systems.

But the ART operates so differently that it made me wonder whether a stabilization system could increase comfort even without substantially decreasing either roll amplitude or period. My original thought about a recording inclinometer was that it would show roll angle over time. If that graph was "spikey" with the line being a zig-zag, it would mean the roll motion changed very quickly from going up to going down. Like bouncing a ping pong ball. If there were a way to slow the point of acceleration/deceleration, the graph would then look more like a sine wave. More like bouncing on a trampoline. What would the perceived stability feel like if the lurch was reduced? It's likely that reducing the lurch would also have the effect of reducing amplitude.

Hopefully, I can get familiar enough with my new toy to learn something.

 

[Hippocampus]

Have done Bermuda races. One was on a Chris White racing tri. Although conditions were moderate (rare squalls no storms or gales) ride was uncomfortable. Ended black and blue on all lateral surfaces of my body, arms and legs from mid calf on up. Problem was although we didn’t heel much the movements were fast with vey rapid acceleration and not rhythmic. Had trouble timing my moving about.

This has nothing to do with ultimate stability but everything to do with comfort. The tri was an amazing boat. Very fast and beautifully designed but optimized for that purpose. No one in the crew got motion sick but it wasn’t a comfortable ride. You see the same in the smaller Gunboats, Outremers and Catana to a lesser degree and other fast multis in my opinion. They float on the water more than a FD boat. They are more responsive to smaller fluctuations in the surface of the waterplane until they get larger. . You see more boat response to those small, short period fluctuations.

So if your device is sufficiently out of phase it well might improve comfort. However I think your boat is a SD hull which is less responsive than a ultralight or racing multi. Perhaps more than a FD but still mostly floats in the water. Guest time will tell if it’s worth the effort.

 

[boating rich]

Wonderful discussion having just read the new version of Cruising Under Power. Thanks for all of the thought discussions.

The discussion has trended towards rolling during a passage, but what are your thoughts of using it at anchor? Seem like this is a great path since I could fill a 20 gallon flexible tank and then empty it when it is not needed.

We tend to time our offshore passages to days with low roll and acceleration with predict wind, but as we wait we can’t control the motion in anchorages.

 

[Marco Flamingo]

The ART works the same whether moving or at anchor. Not so with some of the common systems. My "rolling at the dock" test (or rolling at anchor) gives a good representation of the effectiveness of the ART system. Not true with rolling chocks or paravanes as they behave differently whether stopped or underway.

Rolling chocks or bilge keels will be effected by the boundary layer on the hull. That is the layer of water that travels along with the vessel (or at a slightly slower speed than the vessel). It is created by friction and the more friction, the larger the boundary layer. It can be millimeters thick at the bow and centimeters thick at the stern. Any slight obstruction, such as the leading edge of a bilge keel or a rough surface, can cause turbulence that greatly increases the thickness of the boundary layer. Same with trapping air bubbles under the bilge keels in rough weather.

Think of it as a layer of jelly spread over the bottom of the vessel. As it gets thicker, the effectiveness of the keels sticking through that layer is reduced. At the dock or at anchor, bilge keels will have their maximum effect. At speed, depending on speed, sea state, bottom conditions, and forward hydrodynamics of the keel, the effectiveness will be reduced. For instance, if the bilge keel is not in perfect alignment with the water passing by, it will create turbulence that will reduce its effectiveness during a roll.

Paravanes are the opposite at anchor. Only one paravane is pulling at a time. Both bilge keels would pull at anchor and have maybe 15x the surface area (although a smaller moment arm). The paravane needs to be moving (and dragging) in order to really be effective. Travelling at 5 knots, there is some pull. When rocking (and converting feet per second to knots) the paravane is pulled up at about 5 knots. Thus if travelling at 5 knots, the tug on the line is 10 knots and they become noticeably more effective underway.

The ART doesn't rely on friction exterior to the vessel so its effect is the same at anchor or moving. My guess is that if you designed a flexible tank that was effective at anchor you wouldn't bother emptying it when underway and then filling it at the next layover.

 

[Marco Flamingo]

I finally got a chance to spend some time at the boat with the accelerometer app on my phone. To get some baseline numbers, I put the phone in various positions and rocked the boat to about 6 degrees. Unfortunately, I haven't figured out how to run the inclinometer app at the same time as the accelerometer app. I need some grad student helpers. The zero line doesn't run through the center of the graphs perfectly because the phone wasn't always level.

One interesting measurement was placing the phone on the cabin floor. That's about as close to the center of gravity as I can get. I also placed it on the lower helm and on the flying bridge deck. The first picture shows the maximum X motion (in red) as a G force of .029. The graph shows the rate of decay. I stopped rocking at 30 seconds and wanted to get 30 seconds of graph to compute the roll period.

The second picture (from the flying bridge) shows the same rocking resulted in a G force of .23. Ten times more G force from the cabin floor to the flying bridge deck. That amount of rocking (about 6 degrees) is easily handled when standing at the lower helm by employing one's sea legs. But when sitting at the upper helm and experiencing 10 times the G force, one would have to have sea buns, and it still wouldn't be comfortable long term.

After filling my anti-roll bag, I intended to record 30 seconds of rolling to see if the bag changed the roll period. Turns out it was difficult to get the boat rocking enough to get a 30 second graph (third picture.) The app automatically chooses the G force axis on the graph, and I was having such a hard time getting a 6 degree roll that it automatically changed from the 1 G metric used on pics 1 and 2 to a .5 G metric. And then I couldn't get a decent 30 second recording because of increased rate of decay. I'm going to have to convince my wife to download these apps and help me rock the boat.

The final screen shot is me forgetting that I had the app running and walking around a little while the boat was rocking. You can see the jagged effect it made on the sine waves. The final spike is me tapping the phone to stop the recording. That's really sensitive.