Stern,
You argue for advantages of shorter hulls:
Actually I haven't yet, I've only pointed to the fact that drag graphs are curves and that therefor an optimal point can be found. This directly implies that a longer hull isn't always better from a drag point of view. Of course the same can be said for a shorter hull. The trick for a boat designer, among others, is to find this point and balance it against other considerations.
The true advantages of shorter hulls are in fact :
-1- it is easier to build a lighter hull
-2- it is easier to build a stronger hull
-3- it is easier to build a stiffer hull
-4- the boat feels more responsive to steerage
-5- can be build light, strong and cheap with plain materials.
Actually points 1 to 3 are strongly dependent on length of the hull and the size of the crosssectional area. The fact that the M20 despite Marstrom best (carbon autoclave) efforts can't hold its weight down to 108 kg while homebuiders can made dependable 108 kg F16's from ply shows this in undeniable terms.
There are of course also drawbacks. But others covered those already.
Wetted-surface decreases, hence surface drag decreases.
However, to preserve volume (bouyancy), you need to increase cross-sectional area. This leads to increase in form drag (pressure-drag). Even if your length/width ratio still is low enough to keep wave drag low, there is still increasing form drag.
That is not entirely true. The overall drag will ONLY increase when the increase in wave-making drag (absolute sense) is more then the decrease in wetted surface drag. If it does not then the overall drag will actually reduce when making the hulls shorter. But because of the curved nature of these dependencies you will find that reducing hull length beyond at certain hulls length will increase drag again.
Also you can see both things happen at different boat speeds, go take a look at Gareths plots again. You'll see the 5 mtr hull having less overall drag at low and high speeds when the 6 mtr has less overall drag in the mid range of hull speeds.
By now we have enough data to proof that any rule using the word "always" is wrong. Therefor a longer hulls is not *always* faster because it may not always have less overall drag.
Hence, reducing length is not automatically a optimal solution.
I never said it was automatically an optimally solution, that is only the way you intepreted it.
The consequence of your arguments for shorter length cats, Wouter, would be to cut off a feet or so from the stern of your excellent Taipan 4.9. Or switch to a 12-14 foot class.
No it is not, you are trying to insert the word *always* again.
But this thread is not about boat length. Let us return to the original question!
What is possible today?
Are the Super Taipan or the M20 the fastest cats around?
No, the Eagle 20 carbon is, same weight as the M20, same hull length but it has a jib as as such it is faster over a significant spectrum of windspeeds. The only exception being the light winds as here the jib is not of much help.
Wouter
