When a sailboat when moves through the water driven by the wind, the forces on the sails are balanced by the water pushing in the opposite direction against the keel, the centerboard, and any other part of the boat that is underwater. If the shape and positions of these underwater pieces are not well thought out then the sailboat will be difficult to steer or will slide sideways (make leeway) more than is necessary when sailing towards the wind. Other possibilities are that the boat will try to turn away from the wind if you let go of the tiller (called 'lee helm') - an undesirable trait because things can get out of control if this happens rather than turning up into the wind and coming to a halt.
I already mentioned balancing the keel position against that of the sails in an earlier post, but the overall profile of the centerboard and keel and the position and weight of the ballast also need to be worked out. How thick should the centerboard be to handle the strain when sailing to windward in a 30 knot breeze? How deep and wide should it be? What shape should it be - a foil like an airplane wing perhaps or is a flat plate ok? Should it be straight up and down or at an angle? These questions have been answered by researchers and of course much of this research has been around improving performance in racing boats.
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| An attempt at keel, ballast, and centerboard |
The centerboard design I illustrated in an earlier post was based on my initial research and was different from Welsford's Penguin, which has a straight board. My design has the advantage of fitting better into the boat and leaving a lower trunk and more space near the companionway. The problem is that if I want to shape a centerboard that changes its width (cord) from top to bottom, the profile shape has to change continuously. This is a problem, because I intend to use a NACA foil section (like an airplane wing) to make the board efficient as possible. If I then try to make a board out of glued up strips of wood, I will want to use a router or some sort of other power tool to shape this. This will tricky because one would need a half dozen templates and interpolate in between them to do the shaping. It would be far easier to make a simple jig like this person has that follows one profile.
The other downside to my design is that because it tapers, it has a bit less area than a straight board. I am already proposing to use less than the recommended keel area versus sail area (the rule of thumb being about 3.5% or say 10 sq feet). So I have redesigned the board to be 2 feet wide all the way along and chosen a NACA 0010 section, which has a maximum thckness of 10% of its cord or width. Hence the board will be 24 inches wide by 2.4 inches thick at its thickest. The board will give me 8 sq ft of area and the keel and skeg will provide plenty of additional underwater profile.
Here is the latest attempt at a centerboard and keel.
Building the model and doing these drawings has helped me think through construction and hopefully avoid doing something that will be costly or difficult with the full sized boat. Drawing out the profile of the skeg and ballast section allowed me to work out the ballast placement and knowing the centerboard width was necessary to determine the size of slot that must be cast in the ballast and allowed for in the wood that supports the ballast. This slot affects ballast calculations too.
Balancing the location of the ballast fore and aft of the center of lateral resistance (CLR) is only a guess and in a small boat the weight and position of crew and outboard motor make it desirable to have a way to trim this and to place a bit more weight forward. Crew are usually either in the cockpit or main cabin, which are both behind the CLR. An outboard and gas tank add 100 lbs or so tacked onto the very aft end of the boat.
The ballast weight I have aimed for is a bit more than Penguin's because this boat is larger. If it is too little, I intend to use ingots of lead inside to adjust this.
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