Gardner
I have looked more closely at the hull drag around your target 15kts. When I adjusted the regression factors to give better approximations around 15kts the speed prediction lifts to 15.7kts in 15kts true wind.
Some things to consider:
1. The speed potential is above 20kts. The trim of the main hull when at 20kts will be about 7" bow down and getting deeper with speed.
2. There is slight bow down trim throughout the speed range. The smallest is 40mm bow down at 12kts. This means that the flat bottom of the ww hull is not going to produce lift. Even when lightly loaded it will still have a relatively high drag. Having rocker in the ends would give it some planing surface to get it lifting rather than maintaining a lot of wetted surface. It would also counter the bow down tendency of the lw hull.
3. If you were game enough to press to the point of lifting the ww hull then the loads become quite large. The mast sees a force of 10.2kN (about 1 tonne force). The trailing rudder has a side force of 5780N (over 1/2 tonne). (I think the bow of the lw hull digging in will make this a scary exercise as you will be doing around 25 kts)
4. When I was redoing the hull regressions I also looked at possibility of reducing hull drag. For a lw hull displacing 1050kg the lowest drag hull for 15kts would be 14.7m long. The drag would be 681N. Imposing a length constraint of 11.5m results in a hull with drag of 700N. At 15kts your windward hull has a drag of 740N. So not far off the minimum.
5. The above analysis is with non-ventilating rudders. With ventilating rudders the speed drops from best of 15.7kts to 15.3kts in 15kts of wind. The rudders may not ventilate when everything is in the groove as the cambered shape will be at negative AoA once moving at speed. If you have to go to positive AoA then I expect they will be more likely to ventilate due to leading edge flow separation. As you can see the actual speed variation is not much but the rudder control would vary.
6. Using a non-ventilating centreboard the best speed gets to 15.9kts. However it has to be sized right and trimmed to get the best results. If it is too big the front rudder ends up working against it. The main advantage is the reduction in rudder loads down to about 25% of the load without the centreboard.
Now that I have the model set up it is quite easy to test different things.
There is likely to be merit in playing with the lw hull shape to improve trim. The sort of things to try are flared sides in the hull, rocker in the ends and flat lifting surfaces in the bows. The latter might not show up in my displacement mode analysis but at higher speed you will get dynamic lift.
Rick Willoughby
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