Rick,
A few questions. I have been playing around with the design again, based on Rob's WBC design, and info from your analysis. There are a couple changes that would be significant and I was wondering if you had a general feel for whether they were worth pursuing. I am at work now, so can't upload the layouts for another 12 hours, but just to give you a heads up:
1) I increased the beam width to 2' (610mm) on the lw hull. The hull sides are still vertical, since it seemed the simplest (cheapest) hull shape to make, and I need the vertical orientation for mounting the leeboard, but the bows could be flared, I suppose.
2) biggest change is that I am trying a schooner rig with each mast having a 34' luff, 9' boom and 80% roach, for a sail area of 245 ft^2 each and with an aspect ratio of 4.7. The sail area remains the same, and the aspect ratio is better, but I have no idea if that would help or hurt the bow down problem. The masts are each 9' back from the bows (21' seperation). This is pretty far apart, but allows the boat to fold on the water to 7.5' beam, which will be helpful for trailering.
3) The rudders are reduced in size (2' high by 1' wide) and also moved further apart (27' between the leading edges). The rudder shaft is supported 1' above the waterline.
4) The main leeway prevention is from a pivoting leeboard which is 1.5 wide and draws 5.5' when vertical, but can be rotated to any angle. It is mounted right on the centerline, so I am assuming that it will be canted, so the actual draft will probably be 5' max.
5) A side effect of all this is that the crossbeams are now 24' apart on center and the ww hull is extended from 26' to 28' to allow 1.5' of bow forward of the beam.
I don't know if these are too many changes to check with your current model. I am mostly interested if the schooner rig helps or hurts.
Thanks again for all this support. I am really grateful!!
- Gardner
On Mon, May 23, 2011 at 7:44 AM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
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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