Size of Stabilizers

Stabilizers that you drag through the water have been mentioned numerous times as a way to control roll. But I have not seen anything the mentions how large they need to be without creating excessive drag. Are there any formulas to determine the size. Let's propose a 38' boat of say 20,000 lbs displacement. I would think that this is a case where you want them as small as possible. And how do you determine the size of the wires, poles and all the other paraphernalia that you you need to deploy them.
Chuck

 

Chuck,

The above is the stabilizer page from the Kolstrand catalog. There are general sizing recommendations there, but experience tends to vary with these. Some are happier with smaller paravanes, less drag, less stiff, more speed, others want max stiffness (bigger size paravane) and could care less about drag. Some folks carry various sizes. The fishermen (careful with dollars) on this coast have built their own version of these with plywood and a chunk of lead. This is also easier on wooden boat's topsides which can be cut up with thin steel.

There are some general hints on engineering the rigging part of the system in Beebe's original Voyaging Under Power, now long out of print. Generally his take was to work to the breaking strength of the paravane pennant.....sometimes this is rope, or wire, or chain......

The other method is to take a walk on a fisherman's dock with a measuring tape and see what they did.........

 

Thanks Tad. I always appreciate the information that you provide.
Chuck

 

What you ask is not a trivial queston at all, because it involves unsteady hydrodynamic forces and ship responses.

But if you limit yourself to off-the-shelf products, like this one by Vetus (pages 147-148): http://viewer.zmags.com/publication/fd2b8fee#/fd2b8fee/147
it apparently becomes a trivial problem. Oddly, this product is not available in the US version of Vetus' catalog.

There is a formula at the page 148 for calculating the stabilazer's fin area in function of boat's dimensions:
Fin Area = 3.5 (B x D)/(T^2 + V^2)
Where B = waterline beam, D = displacement in metric tons, V = speed in knots, T = roll period in seconds. Fin area is square meters.

No reference is given about it's origin, but (if you're interested) with few mathematical passages it can be traced back to a combination of the so-called waveslope capacity formula and the Weiss formula for the ship's natural roll period.

The coefficient 3.5 is empirical and arbitrary (depends on the roll response one wants to obtain, and on the geometry of the stabilizer), and we don't know why did Vetus chose that number. But, as usual when we want to keep things simple, we are asked to have faith that it suits every hullform...

 

Ooops, I thought we were talking about fin stabilizers here.

 

Daiquiri, I believe he was talking about another kind of stabilizer, the paravane.
Tad, paravanes are almost always terminally attached with chain. If you fish, they have to be because rope or cable hums cutting through the water. Well, I take that back - there are some species that are not bothered by it (tunas included).

 

mark,

Yes, we often see rope (stretchy nylon) to the waterline and chain below but I've seen plenty of boats (especially yachts) running wire which is seen as lower drag. The noise (hum off the wire) is sometimes annoying, and on other boats un-noticeable. The nylon is a shock absorber as well as damping the wire noise some.......

 

Exactly. I wouldn't know how to size them but I will say undersized is better than oversized! I have seen them do substantial damage to supporting gear!