rudder surface

how do you determine the surface area of a rudder ?
is it a % of sail area ?
and with double rudders ? do you use the projected area of a single rudder at say, 20° of heel ?

 

JLP,

I have been asking a similar question, but no answers yet. During my search on the WWW, i found this website, hope it helps!
http://home.clara.net/gmatkin/therules.htm

GT

 

The surface of the rudder is a percentage of the underwatership. Each designer develops his own formula according to his own experience. Percentages can vary from 12 to 18 or more or less than those numbers. The shape and draft of the keel plays also a specific role in the calculation. The doulble rudder has been developed for having better efficiency at high angle of heel, also preventing broaching: in short it is an accumulation of factors but a factor of great imporance for sailing yachts and in a lesser degree for motoryachts. The Dutch designer Van de Stad was among the first to introduce the spaderudder just after the war.

 

I am told by the engineers here that for a moderate sort of sailing boat 10% of the non-heeled underwater projected lateral area is a good figure to work on. As Mr Dartois says the area may need tweaking if the boat has any extreme characteristics.

Cheers
Jim

 

Many of the older full keel designs hade a ratio of rudder area to lateral immersed (upright ) area of around six percent. Modern design practice suggests around 10% as a good effective area , this can be reduced if there is a leading skeg. Each design will need considering on its merit and performance charateristics, Double rudders are a bit more compicated and depending on the dellenbaugh angle of the design may be not much smaller in area, and with a tendacy for the weather rudder to vent.