LEEBOARDS
by Barend Migchelsen Migchelsen@aol.com  
http://members.aol.com/_ht_a/migchelsen/myhomepage/

by Barend
Migchelsen Migchelsen@aol.com  
http://members.aol.com/_ht_a/migchelsen/myhomepage/

The following is an excerpt from a 6 page booklet written by Barend. For your copy of the complete booklet, Send $5.00 US ($7.50 cdn.) which includes postage & handling to the address at right:

Barend Migchelsen
1515 Hamilton Place
Dorval  QC
H9S 1H3


The Dutch did not invent leeboards. They saw them being used in the Far East during their discovery voyages in the early 1500s.

Their merit is that they gave the boards the curved shape of a light aircraft wing with the thicker edge fore, becoming thinner toward aft. The convex side of the board is against the hull. The concave side is on the outside. The lift from the leeboard makes the boat point slightly higher on a tack.

It is very easy to explain with what we know today about aerodynamics. However, they discovered that 300 years ago without knowing anything about these principles.

For small boats it is not necessary to make the outside concave.

Look at the board as if they are longitudinal cuts from a streamlined centerboard. The right half is then mounted on the portside of the hull and the left half at starboard.

 

The streamlined convex sides are placed against the hull. The straight (cut) side is on the outside of the leeside.

Leeboards never became popular on the North American continent, mainly because their working is not properly understood.

A plank against the leeside of a hull does not make a leeboard.

Figure 9-5 is a dangerous no-no. It will give you the least when it is needed the most!

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Fig. 9 - 5
A dangerous no-no

 

What every self-respecting, teenage boy can explain to you about the front wheels of a car, applies identically to leeboards: For optimum performance the boards must have camber, caster and toe-in.

They are up to 20 percent more effective than a centerboard. It allows for shorter boards.

They never jam. They are easier to maintain and to repair in case of damage. Just take them off. Their main advantages are that they eliminate the need for a centerboard box, which in 90 percent of the cases is the cause of leaks, and leave a roomy, uncluttered cockpit.

The disadvantages are two extra ropes to handle when tacking, and you have to be (more) careful when docking.

They also require an extra plank above the sheer. The plank must tumble home to provide the camber for the boards. See figure 9-6A.

Leeboard Camber

The camber is determined by the inclination of the tumbled-home plank above the gunwales. You could call the angle of the plank a negative flare angle. In figure 9-6A it is 9.46 degrees. Tangent of the camber angle is 1/6 = 1.666667.

Maximum effect and efficiency are reached when the hull is sailed with the gunwale on the waterline, or settled on the gunwale. Then, the board is vertically down. The flat-bottom sails as a V-bottom.

In this position the leeboard is more effective than a centerboard. The lateral resistance of the centerboard is equal to the vertical arm of the illustrated centerboard vector, which is equal to the length of the centerboard x cos heeling angle.

The more heeling, the less resistance when it is needed the most.

It explains why leeboards are 15 to 20 percent shorter than centerboards.

Leeboard Caster

The hole to hang the leeboard is off center for a smooth sliding action. The caster angle is not too critical. For a small boat, it is not necessary to hollow the outside of the board. The convex inside will still give a lifting action toward windward. See figure 9-6B.

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Fig. 9 - 6 Leeboard camber and caster

Leeboard Toe-In

The toe-in depends on the shape of the hull. It varies from 0 - 2 as a minimum, to from

0 - 4 as a maximum. In both cases, the maximum value is reached when the board is fully down. The toe-in decreases when the board is pulled up. At the 45 angle of the board, the toe-in is 0. It is not much. But it is the same way with the front wheels of a car. See the table:

 

LB position Vertical down (90) 66-2/3 45
1.  Toe-in Max. 4 2 0
2.  Toe-in Max. 3 1-1/2 0
3.  Toe-in Max. 2 1 0
 

Leeboard Hanging

Heat the ends of a " OD galvanized pipe. Bend the ends to the same angle as the tumbled-home plank. See figure 9-7A. The pipe is then attached under the mast bench with wooden blocks. Slide the blocks on the pipe before the heat is applied. Thread the ends of the pipe to screw on the standard caps. Make the hole in the leeboard big enough to allow a bit of free play. The pressure of the water holds the board tightly against the hull. In the half-Breadth view, figure 9-7B, the gunwale is not drawn for clarity.

A metal plate on both sides of the leeboard serves as a washer. Cover plates of the galvanized, round electric wiring boxes are perfect for this purpose. The hole is already punch in, and they don’t cost much.

The optional leeboard bumper guard block is at least 1" (actual ") thick hardwood. The block is attached at the same height as the pipe.

The leeboard hoist is attached with a stopper knot through a small hole at the front bottom side of the board as is shown in figure 9-6B. An U-link with the pin through the hole works also. The photographs show different ways to lead the hoist aft into the cockpit.

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Fig. 9 - 7 Leeboard hanging

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figure9-8.jpg (18095 bytes)The boat in the picture in an original, 14-ft., Frisian Scow.  Friesland is the a northern province of The Netherlands.  It was built by a 4th generation builder, and brought to Canada by me.  She is built with 1" African mahogany on white oak frames. The bottom is TOLA wood. I was told that this is a very tough, oily wood, grown in Africa. The gaff is a "natural" grown oak branch.   The grain follows the curve of the gaff. You bring the gaff first, to have the sail made. She is rigged according to the Dutch folklorist rules.  The Skybroom, made from black wool is in top of the mast. (That is another long story.) The red weathervane indicates it is a pleasure craft.  Working boats carry a blue vane.  As a matter of curtesy red always gives way to blue.  In Canada, the provincial flag is carried on the bow sprit, the lands flag on the rudder, preferable on a bent stick so that the tip is just above the water. In Holland the flag fore would have been a "GEUS", six red-white-blue triangles.  A warship carries a DOUBLE-GEUS, 12 red-white-blue triangles.  

The top of the mast is black, the hardware on the top, white.  The ornament on the barndoor-type rudder is called a "PRINCE" It is also found on the vertical part of the mast bench. When the boat became too heavy for me to handle alone, the Dutch Consul in Ottawa bought her.  Six month later, he was transferred to Moscow.  She, probably, sails now on the Volga.

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