how much rocker in a sailboat hull?

[Petros]

I have notices that what was always considered a fast sailboat hull design has changed over the years. In the past, perhaps 100 years ago, the fastest sailing boats had a fair amount rocker in the keel line. Than about 50 years ago it apears designers put less rocker in them, and they were winning more races with the lighter hulls with less rocker.

Now it appears the fastest of the modern monohull sailboats, both large and small, have very little rocker. They appear to have an almost flat bottom from just before the keel to the transome.

What does the rocker do for speed, and when would you want it, and when would you not want it? IN the smaller boats I have sailed and paddled it seemed to me that boats with more rocker turned tighter.

Do all the fastest hull design now have very little rocker in the keel?

how would having little rocker in recreational sailboat affect the way it behaves?

Just gathering opinions.

Thanks.

[ancient kayaker]

I imagine heavy rocker would be bad news if you wnated a planing hull. Even in a displacement hull, if it resulted in the stem and stern being well out of the water that would reduce LWL and therefore hull speed.

In my own area of competence (canoes and kayaks) a bit of rocker is an aid to stability so is often seen on boats intended for beginners, but again straight is the thing for speed.

I suspect the heavy rocker of years gone by had a great deal to do with a perception that it improved sea-keeping properties.

[Munter]

If you don't want to have submerged bow knuckles and transoms then you need to have rocker to get sufficient displacement low down. Boats have become lighter allowing the amount of rocker to be reduced to a tiny fraction of what it used to be needed to match the displacement.

Low rocker would not be fast if it weren't combined with low displacement.

[gonzo]

On light winds a hull with some rocker has less resistance(less wetted surface) than a similar one with no rocker.