The coefficient of lift for a slender hull is very low. The reduction in drag through planing lift is of the order of 5 to10% at the best. Obviously it is a function of many variables. There is no transition to plane on a slender hull because the hull has low wave drag to start with. The drag reduction can be high with lots of power from a motor rather than a sail but typically the sail drive is forcing the nose down so there is a limit to how far the hull can come out of the water to reduce wetted surface before it is pitched down by the sail drive.
With regard to section shape. The flat bottom usually works out to be faster on a boat designed for relatively high displacement speed. The best examples of those hulls are amas on big trimarans like Oracle:
Note near flat transom.
Foiling moth hull - almost fat sections:
Modern A-class cat hulls with near flat bottoms:
The lowest drag form will have variable flare in the side. The wave drag is a function of the BWL squared so keeping the BWL low reduces wave drag significantly.
The difference in wetted surface between a hull with semicircular section and one with fixed side flare of 26 degrees is 5.2% more wetted surface. However the beam for the same volume on a given length is 95% of the semicircular beam. So there is an increase in wetted surface but a reduction in beam that almost offsets the increase in wetted surface by reducing wave drag.
If you are using the 32 hull function version of Michlet and you do not overconstrain it then you will see that for high speed to length ratios that the hull bottom flattens and the flare varies along the length such that there is more flare in the bow and less in the middle. The stern usually has low flare to reduce the width of the transom at water level. The reduced flare in the middle obviously reduces the BWL. The flare in the foresections and aft od middle reduce the wetted surface. The flare tends to reduce as the speed to length ratio increases in order to reduce BWL. The B/T ratio reduces as well.
For the lightweight low volume hulls I design and build for human power, the flat section with variable flare works out to be faster than a semicircular section because the lowest drag length is shorter for the flat bottom and that reduces hull weight.
I am not certain how the latest sailing hulls evolved but Michlet, 32 function version, certainly produces hull forms near that shape for high speed to length ratios.
You are correct that Michlet does not simulate dynamic lift but you can be confident that the flat bottom will produce some lift when pitched bow up and that results in drag reduction but not much.
On 30 May 2018, at 9:41 pm, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:At what speed will the dynamic lift be enough to provide any significant lift? And will it really come up completely and plane?Michlet can't simulate dynamic lift.
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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