Adventures in Centerboard Design 2 – Virtues of the Daggerboard

Original Daggerboard Plan


(to start of project)

The Melonseeds of the 1880’s had daggerboards, as have most built since. Above is the original Chappelle plan drawing shown in black lines. A couple of things are striking about the original. First is the unusual crescent shape. It’s unusual enough that I had never seen one done like that before, and the reason for this shape is both interesting and ingenious.

These boats were designed around the needs of a single hunter, working alone, without a motor. He needed a dry place to sleep when the wind died miles from home, and when a single hunting trip might take several days. The optimal location for the daggerboard, if it were straight, would require it to be located back in the cockpit area. But by making the board crescent shaped, the case could be moved forward out of the cockpit while still maintaining ideal balance of the boat. This gave just enough room in the cockpit for an average sized man to lay down flat for a good night’s sleep. In fact, Nathanial Bishop, in his Centennial Republic sneakbox, slept almost every night in the boat for three months, comforably. Pretty smart, those old boat builders.

The other interesting thing is that the boat handles so well, is so wonderfully balanced, with this configuration. Every Melonseed I’ve ever sailed using the daggerboard design is light and responsive, even in a heavy blow. It’s remarkable how well they handle, so much so that this characteristic alone may account for why these boats are still being built today. Last summer when I met Tony Thatcher for a sail in the boat he built, for instance, we sailed along in 15 mph gusts as comfortably as if it were a summer zephyr. Here’s a brief video clip from that trip:


Youtube Link



So, a real challenge is to try to “improve” on this design, by adding the convenience of a centerboard, without destroying the unique handling qualities.

In the picture below, the red vertical line is the Center of Effort of both the original sail plan and the Crawford sail (they’re almost identical though Crawford’s sail is quite a bit larger). The blue vertical line is the CoE of the Barto sail plan, which is even larger than the Crawford rig. The red star is the Center of Lateral Resistance (CLR) of the original daggerboard. Barto’s daggerboard shape is the same, but he moves it’s location back a few inches to maintain the same relative distance between the line and the star. The CoE and CLR are just fancy terms for those two bulls pushing on a sailboat from opposite directions (see previous post).




In theory, a boat will sail fastest when the star and the line are lined up perfectly. Any deviation requires corrective action from the rudder to keep the boat going in a straight line, and using a rudder is like dragging your foot to steer a bicycle – it slows you down. However, in practice, having the star and the line perfectly aligned doesn’t work out so well. Having the star slightly ahead of the line turns out to work better, mostly for safety reasons. For one, if you happen to fall overboard, the wind will make the boat turn to face it like a weather vane, bringing the boat to a stop so you can climb back in. But it goes beyond that. Having a little pressure on the tiller helps the person steering feel what’s happening with the boat. But even with a little pressure on the tiller, there’s a lag between what the helmsman feels and his (and the boat’s) reaction to it. That lag can be a surprisingly critical half second.

When a boat is hit by a sudden gust of wind a lot of things can go wrong – enough things that it’s the most likely moment that a boat will capsize. If the line is behind the star, as it is above, then, when a gust hits, the boat will automatically turn up into the wind slightly, reducing the force of the gust before the helmsman can even react. This effect is called Weather Helm, and it’s a good thing. If, however, the star is behind the line, that’s called Lee Helm, and that’s generally a bad thing. For one, if you fall overboard the boat will turn downwind and sail all the way to China without you. For another, when a gust hits, instead of turning into the wind and so taking some of the force out of the sail, it turns downwind, opening up even more sail to the full power of the gust. In unstable air, Lee Helm can be very hard to control – a moment of inattention can be disastrous. So, most boat designers and builders over the centuries have learned to purposely set their boats slightly out of balance to build in a little Weather Helm.

Too much of a good thing isn’t necessarily a good thing, however. If the amount of Weather Helm is too strong, it puts a lot of stress on both the boat and the person steering it, as he or she has to use more and more pressure on the rudder to keep the boat going in a straight line. The shape of the hull adds to the effect; more so if the boat has a very broad, beamy shape. The harder the wind blows, and the farther a boat heels over, the more pressure is required to keep it in line. In an odd inverse equation, the harder the wind blows, the less speed the boat makes for the amount of effort involved. This is one reason racing boats keep a crew of moveable human ballast on board – to use weight to keep the boat from heeling over where more rudder is required to stay on course. Cape Cod Catboats, for instance, sail very badly when heeled over, and in that situation, like many boats, actually sail faster by reducing sail area so they tip back upright. (I’m simplifying a lot of things here, to keep this from getting too technical.)


Anyway, if you’re still with me at this point, these are the concepts I’ve been dealing with while trying to design a centerboard for this boat. Generally speaking, any centerboard design will have to maintain something close to the original proportions – the size of the board and the distance between the star and the line – if it can be expected to handle as well.

Next comes how other people have approached the same problems, and the slightly different solutions they came up with.
melonseed skiff, mellonseed skiff, melon seed, mellon seed   

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