Hull lines, curvature in kits and homebuilds

[wietzehuitema]

Hello everyone,

I talked to some homebuilders here in the Netherlands and I understand that some kit boats are way under weight when finished a bare hull. Thinking of building one my self the old fashioned upside down way in the coming years gives me courage to build one just up to weight…

I’m busy CAD designing the transom, stations and centreboard case that I want to have CNC machined and then build the rest from hand. In CAD I’m looking at the bottom curvature after I decided on the chines and sheerline. In an old UK article on the OKDIA they compared three designs (O’Donnell, Hoare and Skinner) and on curvature they say:

Only the Hoare hull exploits the curvature rule to a marked degree, and that only at station 3. Some curvature is necessary when building in either wood or GRP to help provide stiffness in the bottom panels, but none of the builders we are considering seems to have attempted to use curvature for anything other than stiffness in the after part of the hull. One or two continental builders have but not, it seems, to any great effect in terms of boat speed. The O’Donnell and Skinner moulds use approximately 30-40% of the permitted curvature at all the transom and station 1, approximately 50% at stations 2 and 3. The Hoare mould was approximately 50% at the transom and station 2, approximately 75% at station 1, and is close to maximum at station 3.

I’m wondering what the effect will be of curvature. More curvature is more wet surface, but also more volume thus lifting the hull and reducing wet surface. More curvature would also give less stability perhaps. I looked at different OK dinghies and it seems that when looking at the transom some are relatively less curved, while others have maximum curvature. Also modern designs (standberg) seem to have less volume in the bow because of carbon masts and they say there is less need for volume under the mast… I found some numbers on the web (australia, uk), but they are mostly from older designs (although lots of modern boats are also older designs off course)…

Does anyone know what the real effects are?
What’s the best for planing downwind? And upwind?
Does anyone know about the curvature of modern kits? where are they minimal curved and where maximum or mean?
A couple of millimeters gives a relatively big difference in cross section…
Any tips?

Regards,

Wietze

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