How do you calculate the rudder size and position?


As stated in the title, i have this question(s): how do you calculate/find the distance between the fin and the rudder/the rudder position on a sailboat ?
How do you do it when you start a new design and how do you do it when you have a rudderless design? - i have a turkish plan for a pond freesailing yacht that has no rudder, only the fin; in all respects it looks just like an elongated Dumas Ace Sloop.
And how do you calculate the rudder area/size for the both abovementioned cases?


Hi Eduard,
rather simple !
The 1st Rule of Thumb suggest that the total appendices area, like Fin, Rudder and Bulb, should stay around 6/6.5% of the total sail area.
Generally the bulb is negletted therefore the 6% is composed by Rudder and Fin.
The 2nd Rule of Thumb suggest that the Rudder surface shall be 1/3 of the Fin surface.

Various sailing conditions like no wind (larger rudder) or strong wind (smaller rudder), may suggest variation of the Rudder surface to be find out once on the water.
The distance from the Fin shall be the maximum obtainable and in accordance with the Rules when applicables.


Hello Claudio,

Many thanks for the rudder sizing info.
About positioning, you say maximum obtainable or by the rules (class rules i presume). In absence of any class rules since i have in mind scratch designs outside the racing area, how do you quantify this maximum obtainable? Is there a coeficient? A formula?
I know that this distance dictates how maneuverable or stabile the boat is.
I have a small handbook on designing sailboats - life size ones - and there is given, among other things, a formula for calculating the distance between the fin and rudder based of course on the LWL. So i presumed that there must be as well a similar formula for model(scaled) sailboats but what are the coeficients ?
That is the big question - since i know that the life sized coeficients for the CLR-CE or for the mast height par example cannot be used on scale model sailboats.

      Many thanks,

Models Rules like the Class M do not allow to put the Rudder outside the Hull limits, while for the Footy Class it is possible within the specified limits.
There is no formula, except the one I told you, just linear dimensions.
I suggest you to read rules of some Classes for better understanding and visualise existing models drawings.

Well , if it is of any use, here are the life sized formulas/coeficients for the distance between the fin and the rudder (CG to CG) as stated in that book:

 - for full keel: approx. 25%LWL
 - for fin keel: approx. 48-46%LWL

 - for short boats like skiffs, cats: approx. 49%LWL

I guess i will just do what you say and start to measure on the plans of various classes and see if i can get to a resolution.

 Many thanks again Claudio,

By the way Claudio, regarding the no.2 rule that you mentioned, do you think that could be possible to have a rudder less than 1/3 of the fin surface for a sailboat with full keel or for a sailboat like a pilot cutter but with deeper keel (vintage pond yacht style)?

the values you gave would need a verification with respect to some typical models. For separated Rudder the variance is within 3% if Long Keel escluded where BTW the Rudder is part of the keel. I doubt the application to a model; ie, for 100cm model the rudder should stay within 1cm !!!
The size of the Rudder is contained in my first answer.

Since the rudder efficiency, (lift vs drag ratio), is directly dependant from the speed, it is obvious that more speed is available less rudder surface is needed.

(remember low wind-high wind).

For instance a Hobbiecat need a smaller rudder compared with a monohull boat of same lenght.

Vintage models and long keel boats requires generally a larger rudder compared with the real size one, to be effective.

Hope is OK for you !

Yes Sir, it is.
Thank you very much for your help Claudio.