Gareth.

Those are very interesting plots. For now, I accept for arguments sake the Michlet software produces dependable results. This does not mean that I accept it as such at a later time as well. I've found that results produced by engineering software products can easily be wrong, while the the basic components of the software can be proven to be right in themselfs. I won't go into detail.

So the plots. Actually I don't see much that contradicts the model that I presented. I even see the miss Nylex data reflected in the plots; 15% and 22% drag (of total drag) due to wave-making drag and wetted surface drag at about 10 knots.

The local bulb in the blue plot between 2.5 m/s = 5 knots and 3.75 m/s = 7.5 knots does correspond to the Froude's law prediction that around speed = 1.33 * hull_length^2 the wave-making drag increases significantly.

The wetted surface drag look like power relationship with 1.8 as the factor. I expected a factor very close to 2 and 1.8 is not to far away at all. Also I had to measure the coordinates of the screen.


But I do have some issues with the plots are presented.

-1- We are discussing 105 kg overall sailing weight platforms and not 150 kg platforms. I don't really think that a 150 kg 5 mtr (16 foot) platform qualifies as a lightweight catamaran. It certainly does not qualify as a "shorter extreme lightweight hull"

Can we do this excersize again but now for 105 kg platforms ?

-2- The drag from the daggerboards, a large factor compared to the other two, is not included in the overal drag plots. The immediate effect of including this will be that the green and blue lines will lay significantly closer together in a relative sense.

-3- I never wrote that ANY shorter hull would be less draggy then longer hull with the same displacement. In the past I mentioned that there would be a transition point, thus implying that for one given set of hulls going shorter was better while for another set of hulls going longer is better. The only way to discover this relationship is to plot the drag plots for more then 2 examples. At least 5 examples (different) hull length is needed to go a somewhat go feel for the curved nature of this behaviour.

With only two example it is possible that the lowest drag hull lies between the 5 mtr and the 6 mtr hull for example. Without more then 2 data points one can not tell at all whether the relationship is linear or curved with a possible optimal length.

Is it possible that you work out the plots for 105 kg boats having hull length; 6.5 ; 6 ; 5.5 ; 5 and 4.5 ?



-4- You wrote :"As you can clearly see wave drag is far from insignificant, and is in fact larger than WS drag up to around 9 knots"

However I clearly see the wave-making drag to be smaller then wetted surface drag in the speed ranges : 0 to 3 m/s (= 0 to 6 knots) and 4 to 10 m/s (= 8 to 20 knots). Additionally I never claimed to be insignificant. I said it was much less important that wetted surface drag in these regions. This is also clearly visible in the plots. In the high speed range the blue is significantly lower then the red line; so here it is obvious. But even in the low speed range the difference appears to be noticeable. The lines lay close together that is true but proportionally the red line is significantly higher then the blue line and that ratio is important. It is important because a high ratio says that hull drag is mostly made up out of wetted surface drag. Each component may be rather small in an absolute sense, but RELATIVELY the wetted surface drag is noticeably bigger. By about 20 to 100 %


-5- Also can you produce plots where the individual wetted surface drag is plotted for the 5 different lengths. The same for the wave-making drag.


Thank you.

Wouter