Gardner
You are correct that the loads are not anywhere near as high from the ww side. That said any looseness in the pin will allow it to flog any time it unloads - all the time when anchored. I will be interested in the detail you have for the pin and bump/wear pad for the side of the hull.
As far as your original rig goes I assumed the centre of pressure to be amidship. Where it actually is will depend to some degree on the sail trim. Most of what I have looked at with closely spaced sails shows the lift coefficient to be higher on the leading sail when looking for the maximum drive. Even your schooner rig will have a lot of interaction between the sails. I have not yet looked at it but I will see. The CoE may not be as far aft as you expect simply looking at the sail area centres. The individual elements need to be about 10 chord lengths apart to reduce interaction to negligible level and treat them individually. The multi-element feature of JavaFoil allows the combination to be analysed. I keep adjusting the relative angle between the sails until I get the maximum overall lift for the entire rig for what I consider typical sail shapes. In operation you can play with the sail shapes and relative angles of both booms. The only sail trimming I do in the VPP is to reduce the lift coefficient if the rig is generating enough force to cause ww hull to lift.
With the centre of effort amidship there is still a turning moment due to the drag on the ww hull. So there is benefit in being able to move the lift force on the board aft, however, as the boat is pressed, the unloading of the ww hull reduces the turning moment. Note that I also assume the lateral shift of the CoE due to roll to be negligible for such a wide beam and shallow draft.
My first thought is having the board vertical will work OK most of the time. Also I did not mean that you should alter the hull side shape. What I was referring to is making the top of the board cambered to leeward so the flow down the side of the hull stays attached. Reverse the camber below the waterline to generate the lift to windward. As speed goes up it is possible the board will suck a hole beside the hull to lose the end plate effect but if the flow remains attached it will not ventilate. To avoid flow separation and pressure peaks the curves have to be gentle and no gap between the board and side of the hull. This could be a good reason for your curved chine so any downward flow component does not separate around the chine. The difference in drag for a given lift between a ventilating board and end plated board is 4 times. The non ventilating, non end plated board is twice as good as one ventilating and half as good as one with an effective end plate.
Rick
On 25/05/2011, at 11:23 AM, Gardner Pomper wrote:
I was hoping that the ww forces were the only major ones and by making a large (12" diameter) pin, the flogging could be eliminated. Are there other forces underway, or just at low speeds before the lift takes over?
I also was under the impression that it was important for the leeboard to be angled back, away from the direction of travel to move the COE aft. If so, it would make shaping the hull difficult. If it can be vertical, the hull could be shaped as you say, but it would still need to be able to kick up in case it hit something.- GardnerOn Tue, May 24, 2011 at 6:33 PM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
Gardner
The forces on the board are not always forcing it ww. If it has any looseness it will flog against the hull. So it is flogging as well as scraping.With some good detail around the transition from the hub part to the foil part you may be able to get the same span efficiency as is possible with a centreboard through the hull. To achieve this the hub needs to be faired into the sides of the boat so there is no water flow separation along the side of the hull as it passes around the board. Essentially the camber in the board reverses as it goes from the side of the hull to the under hull potion. It only really matters when the board is vertical but you do not want steps that add drag when the board is partially retracted.RickOn 25/05/2011, at 2:57 AM, Gardner Pomper wrote:Rick,I will have to wait till this evening to d/l the linesplan. I will be very interested.In terms of the sail plan, I think I only mentioned in passing that the masts are teh same height (40' long, 34' luff) with a boom 9', so each mast has a sail area of 245 ft^2, so the total sail area is essentially identical to the single mast, but the AR of each sail is 4.7 now, instead of the total AR of 2.x for the sloop rig.It would be great if I could reduce the size of the leeboard. I am all in favor of any reduction in force.What don't you like about the leeboard up against the hull? One of the nice things about a proa is that the leeboard always has its force in the same direction, so it seems particularly appropriate. If your concern is just a scraping issue, I could certainly add rub-rails.- GardnerOn Tue, May 24, 2011 at 7:24 AM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
Gardner
I have uploaded a file showing the linesplan I am using for the wider hull. There is also a curve showing the trim at full displacement with CoE at 6m.As you can see this hull has much better trim at higher speed. Around your speed of interest it will trim level or a bit bow up.It is time consuming to iterate these shapes to look at trim so I have only looked at this one shape. I think the initial bow down is due to the flat sides. I expect there will be a trough aft of the curve from the bow to the sides that will move further aft as the speed increases. However at first it causes the bow to sink. I cannot say if the slight rocker in the ends actually improves the trim in displacement mode. At higher speed it will certainly help. With full displacement on this hull the KMl is 50m compared with 40m with the narrower hull. So improvement essentially from larger waterplane.I have not done drag at identical loads with the original lw but the difference looks to be small.I don't think you have mentioned the proposed height of the new masts and the sail area.I expect your board will have much greater area than needed. It may be possible to reduce the depth. This would reduce the bending in the board. I like the pivot idea but I do not like the idea of the board working against the hull.RickI will probably not do much more till the
On 24/05/2011, at 11:38 AM, Gardner Pomper wrote:Rick,
I don't have any quantitative data. I think I picked it up from reading an old designer (might have been one of the hereschoffs) talk about shallow draft boats and say that so long as you kept the chine rounded at least with a 4" radius, the boat would not be slowed down. I have generally stayed away from hard chines because of that, and because of the plywood proa that I built and it didn't turn out well. Anyway, I am partial to Rob's make as much as possible out of one piece ideas; in the design I just uploaded, the lw hull can be made from a single 8' wide panel.As mentioned, I uploaded top and side plan views of the modifications from all our discussions. It is in the files->gardner's layouts->toyyot folder of this group. Let me know what you think, even if you don't have time to calculate it.- Gardner
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Rick WilloughbyRick Willoughby03 9796 24150419 104 821
Rick Willoughby
03 9796 2415
0419 104 821
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