Maximum Sail Surface

What should be the maximum acceptable sail surface that can be used on a model ?
What is the relationship between Sail area , Fin area , Bulb mass and Hull wetted surface ?
Formulas , diagrams …

Sorry to everybody if my question is too complicate.

research the various ratios. eg SA/D
examine other similar designs and explore their ratios as a guide for your own design.
Don’t reinvent the wheel,follow the time honoured principles of making gadual changes to what already works.

Brett is not that I search, I’m big enought , but rather on how the “others” have defined the Sail Area of their design. I do not beliewe is matter of severals trials.
For instance how has been decided the 0.5161m² used for the M class
It should exist a basic rule or a rule of thumb, where the SA shall be in relation with fin lenght/bulb weight and displacement in order to comply with : CE=CLR- probably a ratio between the wet surface an the sail area ? with eventually a correction factor as function of the bulb mass ?

Hi Claudio

OK, I’ll bite.

Your question isn’t too complicated. It is just too big. When I read “What is the relationship between Sail area , Fin area , Bulb mass and Hull wetted surface?” I think this is almost the same question as “What are the principles of yacht design?” … (smile)

Here is a copy of a similar question answered by Larry Robinson:

The information contained in Lester’s reference should be noted that it is for displacement hulls with “circular” (of sorts) underwater cross sections.

It does not always work for boats with slabside hard chines, or for multihulls.

In the case of two well known and prominent hull designs - Hobie 14/16 catamarans or Prindle 15/16/18 catamarans with asymetric hulls and no dagger/center boards, the flat outside hulls and rudders take on the responsibility for lateral resistance. Even in daggerboard cats and trimarans, the rudders play a bigger part of lateral resistance than one would expect.

For slab-sided hard chines such as the Lightning, the hard chines and somewhat flat sides below the water surface also lend themselves to additional lateral resistance over and above what is provided in the form of a centerboard. It also seems the centerboard on some older boats perform more of a function of balancing the helm as well as providing lateral resistance. Lester suggests that the hull of the boat itsself plays a part in lateral resistance. Even on small dinghies such as the Laser with very low freeboard, the hard chines add to the equation.

Back to cats for a moment, the “old theory” of boards that were short with a long chord have given way to very deep, high aspect ratio boards that have a very short chord length. For example - my old 18 Square had boards 18 inches in chord length andthat had about 22 inches of draft when fully down (aided by a rudder as well). The new thinking is boards that draw nearly 32 - 38 inches of draft, but only have a chord length of slightly more than 5 inches (and again assisted by the rudders).

I think - as can be seen by some monohull classes - the initial design might have “under-calculated” the needed sail area - as later production see the introduction of “turbo” versions - usually taller masts, more sail area, and possibly even deeper keels. Since some are pretty well known boats, it seems even the professionals can get it wrong sometimes.


It is also common to see an r/c class sail with less than maximum sail area depending on wind strength leading one to conclude the maximum sial area might have been designed for specific wind conditions - not as an overall design for the boat. If they weren’t looking at wind strengths for sail area design, we wouldn’t be buying/building multiple rig sizes such as A, B or C rigs (or more) for some classes. But in light winds, being underpowered wouldn’t be acceptable either.

Brett & Lester,

here instead is the page estracted from well knone book of P. Gutelle.
Unfortunately, the second diagramme stops at LWL of 4 m.
It is also written in the same book that this diagram apply for short keel while for long keel, the ratio should be reduced by 10-15%.
It is also suggested that for small dinghies, the ratio may be higher because of the crew control. Pitty ballast are not considered.
I have found several empiric relationship reading my books, but these diagrams are the ones that attracted most my attention.
Probably my question presented in this way may be more clear.
Can be noticed that the curve tends to go up as the LWL is reduced.
As an exemple : one of mine M has a total Wet Area, including appendages, of 0.263m? if multiplied by 2.7(see extreme left of diagram) = 0.710m? that is the real conditions for the Rig A.

Any comments that can be interpreted for our models ?


Plot these curves for all the differing r/c classes,this may yeild some clues

Is not a statistical value I’m searching but rather a simple formula, as such that, once any Hull design is completed, it will define the “reasonable” Sail Surface to use as a starting point, knowing that this choice shall comply with a wind range.

Here below is another book estract that put into relation the Water Plan Area with the Sail Area x Coefficient that include the Displacement parameter , instead of only the Hull Wetted Area.

In spite of the fact that in my opinion there is an error in the formula where a Water Plane is not simply LWL x BWL.
Applying these two formula with the real Water Plan, the Sail Area of the M is very very close.

Actually, I discovered with the previous post, that the Sail Area is about 2.7 - 2.8 times the Hull Wet Area for low wind conditions.

Further , remaining on the subject, I learn that, the Fin Area is laying between 3 and 6 % of the Sail Area and the Rudder is between 1.5 and 3 % .

I will appreciate your comments.


Last & not Least

I have found two index indicating with good approximation what should be the Sail Area for a given boat :

Index 1 = AS / V ²/³ (m² / m3)

for cruising boat = 16-18
racing boat = 20-22
fast racing = > 22
multihull = > 28

Index 2 = As / Aw (m² / m²)

An index below < 2 suggest that the Sail Area may not produce sufficient power with low wind

instead good sailing performance are expected with : index > 2.5


interesting site :