Center of rotation of powerboat hull

[rascalfair]

Does anyone know about where from bow to stern the center of rotation is on a full keel power cruiser or sailboat?

I've heard it's as far forward as 1/3 from the bow....my boat behaves differently. I'm interested in what the architects think, particularly.

Thanks in advance.....

[Raggi_Thor]

For a fin keeled sailboat it would be close to the center of the fin, maybe 1/3 from the leading edge. For a full keeled boat I beleive it is closer to the center of latareal plane, which may be further aft.

[rascalfair]

Thanks for the reply. I believe it is aft also, probably about 2/3 from the bow, but I'm interested in others opinions....

Rascalfair

[ChrisF]

Rascalfair, it sounds like you're asking about the center of pitching, the longitudinal location around which the bow goes up and the stern down when she meets a wave?

If you walk from the bow to the stern, she'll pitch around the theoretical "Center of Flotation", the geometric center of the waterplane, which is generally between 50 and 60% of the waterline length from the bow, depending on hull shape. But in practice, we mostly think in terms of the situation I used above: meeting an oncoming wave. As well as pitching the bow up, this heaves the whole boat straight up, so the actual spot that "stands still" is farther aft. The amount will vary from wave to wave, but your 2/3 aft is reasonable.

[rascalfair]

Actually, I'm thinking of lateral rotation, not pitch. If left adrift, no current but say 8-10k wind, around which point will she rotate. I'm guessing about 2/3 of the way aft....approximately at the helm station

[jehardiman]

rascalfair, What you are looking for is the instant center of yaw. That point is a function of wind forces on the exposed structure including apparent angle, hydrodynamic drag on the submerged structure relative to instant drift angle, center of mass, and added mass caused by rotation about the instant center. It is a very dynamic point that changes with the entering conditions (i.e. it is different if the boat is released from rest, or if the boat is already yawing, swaying, or surging). A fairly detailed calculation (think CFD panel model) is required to to accurately (+/- 5% LWL) calculate it but there are some techniques that will get you in the ballpark (+/- 15% LWL). Generally, the knowledge of where it is falls under the "seamanship" catagory.

[ChrisF]

Ah, I stand corrected.

Then it depends on the boat and the speed. What Raggi said for sailboats under way: most of the lateral resistance centered on the forward part of the fin, but pulled aft by contributions from the hull and rudder/skeg. Planing powerboats better have it pretty far aft (at speed) to remain controllable. One thing is safe to say: it'll be in a different place when she's adrift than under way.

[rascalfair]

Thanks.....that's exactly what I'd like...an estimate. My boat, whicn we're trying to model, is a 36' lobsterboat....Spencer Lincoln designed it...and I'm interested in about where the point is about which it will rotate at docking speed under docking conditions. I'm not looking for real accuracy..but an estimate. I think it's about 2/3 back, my partner thinks its about 1/3 from the stem.

What's your best off the cuff guess?

Thanks in advance.

[rascalfair]

Apparently there are three axis about which rotation occurs...but in the docking situation, roll and pitch are essentially zero...leaving only to calculate yaw.

Anybody know how to do that? The formula for that calculation? My boat is 36' lobsterboat....basically a full keel sailboat hull...big rudder and thruster. About which point does it rotate when steering to the dock?

Complicated.....moreso than I thought when I asked the question... Any help is appreciated.

This is related. Turn and or skid fins. I have seen them on everything up to WW II 80' PT boats. Are the the same? Same location. Same size? Do they help the 18 foot bow riders in turning or are they useless or even dangerous. No marina or boat mfg. say anything about them.