Centreboards

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Rupert
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Centreboards

Post by Rupert » Fri Dec 19, 2008 5:08 pm

http://bmba.proboards85.com/index.cgi?b ... &thread=87

A fascinating link to Colin Hall (British Moth sailor) discussing the (very) technical aspects of centreboards on the Moth forum
Rupert

JimC
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Re: Centreboards

Post by JimC » Sat Dec 20, 2008 8:40 pm

Rupert wrote:http://bmba.proboards85.com/index.cgi?b ... &thread=87
A fascinating link to Colin Hall (British Moth sailor) discussing the (very) technical aspects of centreboards on the Moth forum
I'd challenge some of his conclusions though...

Weather helm especially is a very complex phenomenum and he seems to be treating it over simplistically. I have y opwn theories, synthesized from what I've understood (or misunderstood!) from various cleverer people over the years. I believe that moving the rig back and forwards relative to the hull initially just changes the balance between the side load on the daggerboard and the rudder. It used to be considered that a boat was set up right if you could take the rudder away and it balanced just as well without as with. That would actually mean that, if we assume his 3 degrees leeway, the tiller would have to be offset by three degrees to leeward so that the rudder was tracking exactly evenly in the water. Personally I fins that sort of thing angle impossible to judge from the side of the boat!

These days its sometimes considered that bearing in mind you are having to drag the rudder blade through the water with all its wetted surface anyway, it may as well also be providing some side force. So the rudder blade runs with a certain amount of side force on, which in turn, unless its a balanced rudder, means the tiller is pulling on your hand with a load equivalent to the side force on the blade, divided by the machanical advantage on the tiller lever. Given his 80kg total lift, and say 20% on the rudder, and 10:1 mechanical advantage on the tiller, thats 1.6kg on the tiller... That force isn't weather helm, its just side load...

Weather helm, on the other hand, (at least by my definition) is where the boat is attempting to turn and has to be kept straight by having the tiller offset to windward so much that the rudder is operating at a greater angle of attack than the daggerboard. To tell the truth I now don't think I've *ever* experienced this true weather helm on a boat that's being sailed *bolt upright* If its heeled of course, in whiach case the hull shape itself is trying to steer the boat up to windwward, then a steering input to correct is immediately required...

I also find empirically that the critical factor for board size, shape and performance is *not* going along in a straight line, but in getting up to speed again out of tacks, where speed is low, so lift from the daggerboard low, but by contrast the sideways force from the sails is at its greatest...

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Ancient Geek
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Re: Centreboards

Post by Ancient Geek » Sun Dec 21, 2008 12:05 am

It is potentialy even more complex than that you can add and subtract "lift" by trimming the foresail and or the mainsail indeed those of who sail classes without variable geometry do just that. Harder leeches = more lift = weather helm soft leeches wider sheetings angle = the opposite.
I've been sailing most of my 60 and a wee bit years and I find too many variables just far too "deeving" (Get ones head out of the boat and sail!) having said that a good well profiled and faired centrboard makes a real difference and the thesis that water is harder than air true. I had a crew in the early 60's a first class Physics Cambridge Graduate who went on the be an MIT professor who roughly suggested that for cross sections one should use those for supersonic air and profiles those of the Boeing 707! He went on to say that since the sailing yacht was neither weopon of war or a commercial item (In terms of efficiency.) the funds to properly research this or be dogmatic would never be available.
Simples.

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