buttock angle ?

[frank smith]

I notice a great variation in buttock angle of sail boats . It look as though there are trends that change with time. There is the obvious consideration of making a smooth transition from midships section to the stern . And this will change with the water plane shape and draft.

What are some of the things that effect
the buttock angle.

Thanks Frank

[TeddyDiver]

Fr number and Cb for instance.. Smaller buttock angles are typical for semiplanning and planning hulls and then also the other "numbers" related to speed of particular design change accordingly..

[frank smith]

Originally Posted by TeddyDiver

Fr number and Cb for instance.. Smaller buttock angles are typical for semiplanning and planning hulls and then also the other "numbers" related to speed of particular design change accordingly..

So a shallow draft hull with a high Cp will end up with a steep buttock angle ,
unless the aft sections are very wide ?

[daiquiri]

You also have to consider that, besides being determined by purely hydrodynamic or internal accomodation considerations, trends in sailboat hull shapes have been heavily influenced through years by various handicap rating systems and by designers' efforts to carve a maximum advantage out of them.

You can find a nice summary of how these rules have changed the shape of hulls from 19th century to present days in this site: http://www.sailtexas.com/handicaparticle.html

[frank smith]

daiquiri , thank for the link. My reason for the question is an interest in a shallow hull with high initial form stability , like the junk or barge type boat. In particular I am looking at the work of Bolger , Griffiths , Hunt, and also the presto type sharpie.

[jehardiman]

Originally Posted by frank smith

daiquiri , thank for the link. My reason for the question is an interest in a shallow hull with high initial form stability , like the junk or barge type boat. In particular I am looking at the work of Bolger , Griffiths , Hunt, and also the presto type sharpie.

Where are you looking to measure the buttock angle?....

A barge could be said to have low buttock angle or a high buttock angle depending on where you take the measurement.

As Daiquiri said, buttock angles have much more to do with the intended use of the hull than with the age of the hull. So a low capacity shallow water sharpie may have a 2-4 degree buttock angle at the aft waterline, where as a high capacity canal boat of the same length may have a 90 degree one at the aft waterline.