waterwitch coe

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waterwitch coe

Post by rob_dangereux »

Hi all, i am changing the rig on my ww to gaff. to do this i may have to reposition the mast. my question is, does anyone know where the centre of effort of the HULL is? i can work out where the coe of the sails is and can work out the lead needed but need the hull coe. my boat is the triple keel type. i will be fitting a bowsprit and twin headsails and probably a small mizzen gunter rigged. hop someone knows. cheers rob
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Re: waterwitch coe

Post by lukedh »

Rob, sorry for the late reply. For what it is worth here are my thoughts.

The centre of lateral resistance, clr, isn't marked on the plans. There will be a small lead of the centre of effort, coe, of the sail plan over the clr. If you work out the coe of the existing rig then the new rig would have the same lead over the clr if the coe was in the same position.

Alternatively you could work out the centre of the lateral plane and take that as the best guess of the clr. Most likely that is what was done originally as it would be very difficult to work out the clr of any boat. You could trace the lateral plane from the plan onto some graph paper and count the squares as one way or try balancing a cut out of stiff paper or card on a knife edge. Either will give you a guestimate of where the clr might be.

If it is the bilge keels which trouble you then I suppose you could take the clr of the hull and the clr of the bilge keel worked separately, work out there relative area and find their combined centres as you would for sails. But that presumes that the forces add up in such a neat way which they won't due to things like the curvature of the hull, movement and sea state. They won't for sails either, as they interfere with one another and in any case it is dynamic and the force moves depending on many factors such as the direction of the wind and sail set and trim.

By all means try and work it out but perhaps using the existing lead might make a good starting point. So design your rig and sail plan. Then work out the coe and position that over the coe of the existing plan to get your mast position. Or, probably harder, design a rig that uses the existing mast position.

Fair winds, you need them.
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Fiddler's Green
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Re: waterwitch coe

Post by Fiddler's Green »

Just seen this post and wondering if you have a set of drawings? they are available at cost to all members inc post £5.00 UK, from them you can work all this out! just a thought.. I know it is not actually marked for CLE, but basic school geometry allows you to do it and the card template of the underwater shape from the drawings balanced on a knife blade is the easiest way to give the CLR.

Not the easiest thing to do in many respects as the bilge keels do effect the CLR when boat heeled. the CLR when heeled calls for a very trick card template!

contact us at enquiries@eventides.org.uk to ask about the drawings.

Proud owner and builder of 'Fiddler's Green'
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