I originally started to reply to H.W in another thread, but it was far off topic. And then when I started to discuss Bucketlist, which was even more off topic, I decided this needed its own topics, so here we go!
There are some questions for Rob below!
I made a series of simulations in Michlet to investigate into this last winter. The following graph shows the ratio between drag and displacement, so lower is better. But that is not all to it. Because since drag goes up with the square of speed, I have divided the drag with square of speed, to "normalize" the numbers. So that is what the graph below shows.
If the only source of drag is the friction from wetted surface area, the curves in the graph below should be horizontal. This seems also to be the case with the very light boats - almost horizontal, so wave production has low impact. But the heavier boats shows a large increase in drag at different speeds at or below "hull speed", which you can see in the graph. This is most likely because a heavy boat will produce a large wave which I guess the boat needs to "climb" to be able to reach a higher speed. And that would require quite a large sail or a strong breeze.
The purple curve for example, shows the drag of a 10m 4 tonne hull. It shows quite a pronounced hump in the curve.
On a 10m hull (33'), Freude number is more or less 1/10 of speed in m/s, or 1/20 of speed in knots. At Fn=0.4, where the hump starts, is roughly 0.4*20 = 8 knots with this waterline length.
We can also see that on very light boats the friction from the wetted surface area dominates. So it might not be optimal to have a too light boat. It depends a bit on the sail area it can carry I suppose, especially in light winds.
The optimal seems to be about around 1 tonne for a 10m hull (yellow), from the looks of these graphs. (Given that it has enough sail area to use all the righting moment the weight provides.)
This gives us a simple formula: D = L^3, where length is in meters, and displacement is in kg.
(Two hulls complicates things, but I guess that if we are pushing the boat and it starts to "heel", it will sail mostly on the lee hull, so to me the results for the single hull are the dominant for sailing in some kind of breeze. And the catamaran result are dominant when running on motor or in light winds.)
And it seems like "my theory" aligns with the real world regarding this as well. I don't know of any 32' boat faster than the Marstrom M32, which has a lightship displacement of 550kg, and should have a racing displacement of about 900 kg with a crew of 4 and some gear. 9.6^3 = 880 kg, so pretty much spot on my formula.
We can also look at the A-class catamarans. 5.5^3 = 166kg, which is more or less the weight of the boat with crew (80+80kg).
This also takes us to why the proa seems to be a good concept compared to a catamaran. When heeling, weight is transfered to the lee hull, so the lee hull should be longer to not start producing a larger wave and drag. But I think that in light winds, the trimaran is most likely the best, with only the wetted area of one hull in water. (On my Hobie 14, I sometimes use the trapeze on the lee side of the boat, to lift the WW hull out of the water and decrease wetted surface. Its a bit hard to sail it properly from that position though, so I'm not sure if it has been faster.)
The original Bucketlist would have been an extremely light boat. My formula says 12^3 = 1700kg. According to Robs measurements, Bucketlist was on target to have around 500kg lightship, 700kg with crew. So less than half of my formula. Which means between the red and blue curves above. So it looks like it would have been completely without a "hump" at hull speed, it would just go linearly faster with windspeed until it runs out of righting moment. But would it have been too light? Too much wetted surface for the sail it could carry? I guess we will never know.
Rob, I think it was very sad to see Bucketlist shopped into pieces! Now we can't see how it would have performed! It would also have been interesting to see if it would plane on the lee hull! I think Bucketlist would have been the lightest boat in the world for its length, so I think there exists no real world data with regards to planing on that kind of narrow, flat bottom, very light hull. If it would plane, it would get rid of some of that extra wetted surface area, and sail even faster. Was it because of all new foiling boats getting popular, that you stopped development?
Will the new one (triscarph) be as fast? I don't really understand the Triscarph configuration. Will the waves cancel out in some way? Or will it be as draggy as two very short hulls with bad length to displacement ratio until it foils?
And I should add that I have never sailed on a larger catamaran, only small beachcats. And I more or less only have the theoretical knowledge of boats from reading ship theory on the internet and running Michlet simulations during the snowy winters in Sweden.
/Björn
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Posted by: =?UTF-8?B?QmrDtnJu?= <bjornmail@gmail.com>
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