Gareth,

Please don't feel rushed, of course this is not a life or death matter. I can wait a few weeks if that would be better for you, I'm sure the other can so too.




I'm assuming that Blue line = 5 mtr boat, green = 5.5 mtr hull and red = 6.0 mtr. hull. is that correct ?



I also see that you've extended the drag plots to 12 m/s = 24 knots. Something interesting is happening at the far right of the graph as you have mentioned yourself in earlier posts. In the far left the same can be seen. Here the hull with less overall wetted surface area sees less drag. For larger regions at these extremes the differences in drag are to small to favour one hull over another. That is if the colour coding is the one as I described above. But having said all this, it is only found in the extremes.

I'm looking forward to the individual wetted surface drag and individual wave-making drag plots. From the plot for the overall drag is almost appears that Michlet use a base drag level for a given displacement and then adds a wave-drag component.

Is it also possible Gareth that you specify the hull shapes that you have used in producing the plots ? Are the scaled versions of eachother or do they share bow wedge angles. I'm thinking along the lines where the I-20 front of the hull is identical in shape to that of the shorter I-18. Stuff like that.

The difference in hull drag (wave+wetted surface but without daggerboard drag) between 5 mtr hull and 6 mtr hull appears to be 14 % at 10 knots. Including the daggerboard drag (based on miss Nylex data) will reduced the hull drag difference at 10 knots to just shy of 10 % which would be roughly a difference of 5% on the overall drag of the whole boat (based on miss Nylex data).

A 5 % drag difference at this spot would roughly translate into a speed difference of 2.5 % neglecting other limiting factors like minimal angle of attack considerations. A measurement based handicap system like Texel predicts a speed difference of 5 % in such a case, as a direct result of a longer hull line length.

This still seems consistant (in the rough) with real life experiences. It is my personal opinion that the long hulled lightweigt boats are unable to consistantly sail to their Texel handicap numbers. Surely their are more causes for this but this hull drag factor may indeed be one of those.

Assuming Michlet produces dependendable results, these plots show that relatively short hulled lightweight boats don't suffer much at all at very low speeds (light winds) and high speed, but do have more to make up for in the medium range speeds. That is indeed my own experience with my own F16 as well, as I've stated quite a few times in the past.

As a side note. I also suggests how adding a spin to the beach cats helped them to close the gap between short and long hulled boats. By increasing the overall speed of all boats, hull length is significantly less important factor in determining the overall drag of the boat. Maybe this equalizing in overal does also help equalize performance between crews of different weights but with the same skill level. All things that many of us have experienced in reality to be true.

Very interesting, Gareth, thanks for your efforts.

Wouter