Simple rules for resistance v beam

[CT 249]

Does anyone know any simple rule of thumb for calculating the approximate relationship of variations in waterline beam, compared to drag? I'm interested in the displacement (edited) and semi-planing regimes mainly.

[grob]

Is this the kind of thing you are after

http://www.hydrocompinc.com/knowledg...aningSpeed.pdf

Gareth

[CT 249]

Whoops, thanks for that but I should have said I'm interested in the DISPLACEMENT and semi-planing regimes.....Sorry for getting the post totally wrong!

That's an interesting formula but development in dinghies and skiffs is of course moving away from the type of hull that planes as soon as possible ("Ice" Int 14, scow Moth etc) towards one that is better at high displacement/transition speeds (Bieker 14/49er/narrow Moth/Woof 12' Skiff etc)

[Guillermo]

As far as I understand, keeping the same waterline length, speed, displacement, prismatic coeficient and midship area section (so increasing beam implies diminishing draught), then wave resistance varies as the square of beam (Havelock, U.S. Experimental Model Basin)

[MikeJohns]

Chris

As far as I understand and following on from Guillermo.....

For the displacement hulls the wave making resistance is related to the rate and shape of the curve of areas(coa). You need to look at the heeled coa too.

The wetted surface drag component is linearly related to area.

Gets more complex as you start to consider her RM (what sail she can stand up to) against her heeled resistance charts, how skewed her heeled waterline gets.. what rudder riesistance is introduced .....etc

Hope this helps.