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21st May 2013, 06:11 PM #16
Make the daggerboards long. It is easy to cut them down later. The biggest risk to the performance is not having them long enough - because you will never find out what the boat can do if they are the correct size.
I was a bit disappointed with the initial performance of BETH so I made a new daggerboard about 300mm deeper and it totally changed the boat. It was really fun to sail upwind against other boats with the longer board.
We did the same with the OzRacers - made the boards quite big. And they sail really well.
A wide boat that is likely to be slowed by waves needs bigger boards in general.
Make them big ... you won't regret it!
MIK
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21st May 2013, 06:54 PM #17
Quenet Yann's SKROWL Carl Cramer's boat of the week.
However Mik, you were limited to this option because you could not increase the width of the daggerboard case with Beth. Seeing as this boat will be no speedster, fatter boards would increase lift at lower speeds. This boat is short so in waves it will pitch a lot, so would it not be better to go the shorter, fatter route with the board? (ie. thickness relative to chord)
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22nd May 2013, 04:36 PM #18
Hi Bruce,
Certianly would go to 12% foils for something a bit slower - but it is always a problem of bulk and cost. I do suspect that using a template for shaping and getting the shape right is almost more important than the actual section chosen. That's why I go for my flat sided foils.
My experience is that length overcomes almost everything. Though if tooo much it can give excessive heeling moment for body hiked boats.
MIK
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27th May 2013, 11:28 PM #19
foils
I wonder if extra long foils are so useful on a slow boat.
(and Skrowl is supposed to be a slow boat... My project Skrome will - should ? - be muuuuch faster!!)
John Welsford prefers short and fat foils for slow boats.
Another question for our projects, both with twin foils, is : symmetric / asymmetric boards?....
The answer will change with the stability, I think.
for my boat, if the water ballasts are enough, I'll put lightweight asymmetric boards, and symmetric weighted boards if I need some more ballast .
(I didn't yet made the stability calculations)
there are still many questions ... /
Eric
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27th May 2013, 11:47 PM #20
Howdy Eric,
My thoughts are ...
symmetric or asymmetric boards won't make any difference to stability.
Note that despite the opportunity that there are very few catamarans with asymmetric boards. If they worked even a fraction of one percent better than symmetric the racers would have asymmetric boards as standard.
If you did go asymmetric it would be important to have one board out of the water all the time. this makes for much work when tacking
Slower boats do need more effective foils than faster ones. There are two main ways - one is more general area. The other is more depth.
Of the two, depth is the more powerful way of making effective foils. On a dinghy size I would estimate that adding 200mm (maybe adding 20 percent of area) of depth is like adding 50 percent to the width. A relatively minor increase in depth is often the only choice once the boat is built. My general thought is that it is not much more work to make longer foils and only a slight increase in materials. And it is much easier and simpler ... and instant to reduce the length by cutting. But there is a lot of work involved in making a completely new longer set when disappointed with the boat performance.
Best wishes
Michael
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28th May 2013, 06:43 AM #21
Twin foils
Of course symmetric or asymmetric boards don’t have anything to do with stability, but a lot with the pain to lift one when tacking, if ballasted (no problem to lower the other one, just let it free). So my “rule”: “ballasted => symmetric & both down, unballasted => asymmetric & one down - one up” (even if, on my boat, with unstayed masts, self tacking sails and no jib, it will be very easy to tack).
The main reason for twin boards is to get rid of this flat but bulky “cabinet” in the middle of the boat (I like to stretch my legs!!). Another advantage is that the slots would not be in contact with the ground (I spent once a whole day to get pebbles out of the slot of a cast iron keel with a centreboard inside). And, obviously, twin boards make sense on such broad and flat boats.
But then came the question of symmetry (or not)...
Maybe perfect, long and symmetric profiles would be the best solution, but this could be experimented later … if the ballast question is answered!
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