damping ballast tanks?

[kbowen]

It seems that if a boat is water-ballasted, even if the tanks are totally full so that there is no "free surface," one might achieve some roll-damping by having some anti-slosh plates within the tanks. My college-era (long time ago) memory of oscillating systems also makes me think that it might work best if the damping wasn't total, ie, as the boat gets to the extreme point of a roll, maybe the water in the tanks is just barely getting up to rotational velocity in one direction, so that it also slows the snap back toward center.... yes / no? Does anyone know of designs that work like this?

[NoEyeDeer]

Question: if the tanks are totally full how is anything inside them going to slosh?

[kbowen]

Originally Posted by NoEyeDeer

Question: if the tanks are totally full how is anything inside them going to slosh?

You can spin a hard-boiled egg on the table but not a raw egg......

[tom28571]

Originally Posted by kbowen

You can spin a hard-boiled egg on the table but not a raw egg......

True enough but probably not of importance in the case of a ballast tank since rotation is not much involved.

[kbowen]

Originally Posted by tom28571

True enough but probably not of importance in the case of a ballast tank since rotation is not much involved.

I am imagining a centerboard sailboat of the heavy "shallop" type which historically would probably have had a variable combination of beach-stone, nets, and dead fish as ballast. To update this to a day-sailor, it would seem best to concentrate water-ballast amidships so that trim doesn't change as you fill or empty the tanks, which might make the tanks a mold-station or two long x the the midship section below the waterline. Am I thinking correctly? If so, this shape tank might have appreciable water-rotation when a boat rolls, yes / no?

[tom28571]

Originally Posted by kbowen

I am imagining a centerboard sailboat of the heavy "shallop" type which historically would probably have had a variable combination of beach-stone, nets, and dead fish as ballast. To update this to a day-sailor, it would seem best to concentrate water-ballast amidships so that trim doesn't change as you fill or empty the tanks, which might make the tanks a mold-station or two long x the the midship section below the waterline. Am I thinking correctly? If so, this shape tank might have appreciable water-rotation when a boat rolls, yes / no?

The question is, where are you thinking this reasoning will go? Draw the cross section of this water tank and think of where any damping reaction will come from. Even in the case of a cylinder, which is best for your damping action, the only transfer of energy or rotational retarding force takes place in the boundary layer friction near the wall of the cylinder. This is much less in the cross section of the hull below the waterline where rotation is constrained by the shape which is generally wide and thin. The damping action is not zero but not very great either.

[kbowen]

Originally Posted by tom28571

, the only transfer of energy or rotational retarding force takes place in the boundary layer friction near the wall of the cylinder. . The damping action is not zero but not very great either.

I am sorry I wasn't clear, I was wondering about baffles within the tank. If the tank was essentially a sponge, like the gas tanks on fighter aircraft, then the fluid is forced to move in unison with the craft, and the effect is as dead weight with it's predictable inertia, if, instead, it is a smooth cylindrical tank, the boat might roll Port and and back to Starboard before the ballast water rotates much at all, so the mass is there, but less rotational inertia. I am wondering what happens if the damping is somewhere between these two extremes?

After I wrote my original post, I noticed a thread in the "Stability" section of this Forum, about passive stabilizing tanks, where apparently water pours from one tank to another through adjustable valves, and I am starting to wonder if I am trying to reinvent the wheel :-)
Thank you for your patience and time btw..

[Tim B]

I think most people would agree that adding baffles to a pressed full (ie. no free surface) tank is not a particularly useful thing to do. With no free surface effect, it is quite reasonable to treat the tank as an object with mass. It's effect on motions due to internal flow will be negligable. (The MMI of the whole boat will change though, relative to an empty tank).

However, where the tank is not full, then you have a reduction in stability due to free surface. The metacentric height is reduced by the second-moment-of-area of the free surface in the tank, divided by the volume of fluid contained. In order to reduce this effect, baffles are added to the tank to reduce the effective free surface. If the baffles are large enough, then the effect is the nearly the same as having several separate tanks.

Passive roll tanks need some careful design work. If you get it wrong it will destabilise the vessel.

Hope this helps,

Tim B.

[TeddyDiver]

I think kbowen makes a good point and is right in the reasoning.. thou I don't believe it has much noticeable effect. Maybe with round a sectioned tanker but that kind of ship wouldn't roll much anyway.. IMHO

[kbowen]

Thanks to all for your thoughts and patience with an "old dog (me) trying to learn new tricks.."

[NoEyeDeer]

I can see your point about rotation, but one thing to take into account is that for best ballasting effect you will want the tank as low as possible, and this will be below the roll centre of the boat. Because of this, you would (my guess here) be unlikely to get much rotation of fluid within the tank. Also, using a round tank would raise the CG of the water ballast, making it less effective for its primary job.

The sort of anti-roll tanks you read about elsewhere actually rely on not being full. The free surface effect is a vital part of how they work. Problem is that for a sailing boat they are no good, because the water will just end up on the low side when sailing heeled.

[Tim B]

Not quite.

The "surface effect" will always destabilise the boat. This is actually the effect of a mass of water moving in a single tank.

The stabilisation effect is due to the water flowing from one tank to the other which moves significant mass out of phase with the roll-motion of the vessel. However, if you get it wrong, and the fluid moves in phase (or close to), then you will end up upside down rather quickly.

Tim B.

[NoEyeDeer]

My point was that unless there is free surface in damping tanks you wont get any flow between them. But yes, I agree that my previous post was somewhat badly worded.