About the canting dagger, seem to me it is a problem that it increases the heeling moment (boat will overturn in less wind or sail area has to be reduced) ... finding a way to fix it far to the lw side (of the lw hull) so the lifting moment fight heeling forces. Like DSS lateral foil or curved lateral foil ?
---In harryproa@yahoogroups.com.au, <rickwill@...> wrote :
As you point out a centrally located dagger board does not need to turn. In fact it could be set up to cant so it provides lift. There would be a brace to adjust the cant angle so the drum or pivot shaft would be much smaller as it is not required to take any moment. The shaft yoke would normally be set horizontally with a shear pin to fail on impact.
I have done a design for a small R2AK pedal/sail boat that has canting dagger boards. Over 8kts the L/D of the dagger boards is better than the displacement L/D. That means the cambered boards can be canted to reduce overall drag off the wind. The hinge plate has a fuse to fail on impact without damaging the blade. Rob has played around with a single canted board on their kite powered proa and demonstrated lifting.
If the dagger board cants it would be cambered to give best VMG when fully down and then canted up to the best angle to improve off wind speed.
The 18m proa was originally fitted with 900 deep rudders. But they succumb twice in various groundings. They have been replaced with 600 deep rudders that are strong enough to support the boat weight. With further development of rudder ideas I expect the drum and shaft tube as Campaen has would be even neater for small rudders but still heavier than the through hull rudders. Rudders 600 deep will have much smaller moments than ones 2m deep. Also the shaft tube off the side of the hull really needs to be faired to reduce drag when driving into waves.
If rudders are under the hull then they need a visible indicator. There is great benefit in easily assessing the angle of the rudder blades during a shunt so exposed rudders have this advantage.
Overall I expect two medium size rudders midway between beams and ends would be heavier than two small rudders even further out and a large canting dagger board in the middle. Although I have some reservations that a braced central dagger board would fit neatly into a folding arrangement.
On 27/04/2016, at 6:49 AM, "lucsimard@... [harryproa]" <harryproa@yahoogroups.com.au> wrote:
Thanks again for the super nice explanation.
The size and weight still impress me ... but am I wrong in guessing that the 2 rotating and swinging big rudder/dagger would not be heavier than 2 smaller rudders and a daggerboard since all 3 need to be swinging for grounding anyway and the 2 small rudders are also rotating ?
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Posted by: lucsimard@ymail.com
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