Leeboard dimensioning

Dear all,

I have a doubt about leeboard dimensioning.
The general rule that I found for keels, is to have a surface of 4% of the sail area.
But, with a centerboard, the surface decreases as heeling increases, on the contrary, with leeboards, the submerged area increases when the boat is heeled.
Can I keep the 4% rule (this is what I plan to do), or should I use a different method, for example calculating the submerged area with a 10 degree heeled vessel?

Thank you in advance.

DVV

 

One consideration is that leeboards are surface piercing, which in theory makes them a lot less efficient than centreboards or keels.

 

An advantage of a lee board is you can easily vary its size and shape so you can experiment.
With a centre board you are stuck with what will fit in the casing.

 

4% is more of an observation than a rule. A rule would look like -
For tacking upwind in righting limited conditions:
-leeboard lift~80% sail force
-leeboard angle of attack needs to be in an efficient range 1 to 5 deg
-hull direction through the water should be within 2 deg of symmetry

If you think about this for a second you will realize there is no percentage relationship between sail and leeboard, because that would mean all sailboats have the same waterspeed/windspeed. The force on foils is proportional to the velocity of the fluid squared -so fast, easily driven hulls will have small area, high aspect foils and slow boats will need larger foils and longer cords to get the Reynolds number up. For keel boats that are stuck in displacement mode, keel proportions to sail area will increase as boat size decreases.

For sizing your leeboard -forget 4%, find a similar boat (that goes to windward with the same waterspeed/windspeed). Note the area and the aspect ratio -if one end terminates orthogonally at the hull it has full lift right to that end. Your leeboard looses lift at both ends. The top is in the waves and stream of water around the hull so it is not likely to give any lift in the right direction. until the leeboard gets about half a cord clear of the hull it's lift is impaired and it's drag is increased.

 

Whilst that's true, on dinghies at least the limiting factor for area is not the optimum for going upwind at speed, but what is necessary to get the boat smoothly out of tacks without stalling in a high load/low speed situation. That doesn't in any way reduce the validity of the empirical method of taking a look at similar craft and seeing what works: indeed it rather reinforces it.

 

Thanks for this, I think I will look at similar design.

 

You are welcome.
What ggg says is important, but of course all sailboats have an issue when water speed drops toward zero. If you are racing -thus very impatient, maneuvering might dictate design -in particular aspect ratio. For leisure, with more patience and lower aspect ratios you shouldn't have problems.

 

Dunno. When racing, one is intrinsically trying to go fast so you are hiking and trimming well, and concentrating on tacking well etc. In addition, one rarely tries to slow down so there is normally a reasonable amount of flow over the foils.

Cruising, one is often chillin', and the boat is going slower and therefore needs more lateral area. You may often sail slower to stay dry, or lose concentration and therefore slow.

It was interesting when I was lent a minute baby Laser style boat. Its foils were far smaller than those of an Opti or Sabot style of boat. It was OK when sailed on the 400 acre lake, but when cruising on our pond it was very hard to sail at times. Being intrinsically slow, it lacked sufficient speed to creep out of the reeds that surround the dock; it made excessive leeway and didn't want to turn at very low speeds. At "pottering" speeds on a narrow waterway it was actually a quite hard boat to sail because of the small foils. That experience really made me re-assess the size of the leeboards I plan to make for a sailing cruising canoe.

 

Sounds like a fine demonstration of linear scaling being wrong. When that canoe hits the water be sure and post a pic -love that kind of adventure craft.