Stein,
I always considered the "semi-planing mode" to be akin to "a little bit pregnant"
In aerodynamics and hydrodynamics lengthening the object by adding a virtual object is possible, the aerospike rocket engine is using this principle very effectively, but it isn't happening with cat hulls. Look up aerospike rocket for more info.
For such a virtual hull to work it needs to excert a raised pressure onto the vertical stern itself. As in real life the sterns are only experiencing ambient airpressure it results that no vitual hull is effectively present. The theory of virtual hulls applied to cat hulls is now nothing more then fitting the errornous formula to "explain away" conflicting real life data.
I'm sorry.
Most of our beloved catamarans sail in the semi-planing mode.
....
Another way around the displacement speed, is to make the hulls so narrow that they only create very small waves.
I'm of the opinion that beach catamaran hulls are not of the "semi-planing mode" type if I could bring myself to ever accept such a concept, but rather that they are of the long narrow type with only relatively small wave-systems. I can actually SEE the truth of the last statement when I sail my own F16 while there are absolutely no indications of any planing.
By shortening the hull, you may as Wouter, has pointed out, reduce friction drag. However, increasing hull cross-sectional area to maintain volume, increases form drag. One part of form drag is wave drag.
When the reduction in wetted surface drag is bigger then the increase in form drag then going for a shorter fatter hull is more attractive from a drag point of view. Clearly this is not always the case, but just as clearly we can conclude that this isn;t an impossibility either.
Most sailboat classes have a maximum length.
HAVE YOU EVER SEEN SUCCESSFUL BOATS THAT ARE SIGNIFICANTLY SHORTER THAN THE ALLOWED CLASS LENGTH
To be really honest, I know of no boats that are shorter then their maximally allowed class lengths. In effect I have no data points of this kind. I put this down to nearly everybody believing that longer is always better.
The first real recognision of the fact that shorter make be less draggy, that I personally know of, was made in the F16 class. That is with one exception as Phill Brander once told me that such a thing was tried by a homebuild A-cat at one time. Not further data is know to me about this experiment.
But also my point is worded a little bit differently. Is wrote that when you really reduce the overall weight (boat+crew) for a given setup that it is best to also shorten the hull length by some amount over ONLY reducing the width of the hull. Most boatshowever are far to heavy to even consider such a move. Thereis of course a transition point somewhere. Also the only class that ever made a weight reduction big enough to have encoutered such a phenomenon are the A-cats. The F16's are still relatively young although the level racing with the F18's despite being shorter with a smaller rig does hint at the theory being right. They can only be as fast despite these "disadvantages" is the overall drag of the boat was reduced by at least the same amount. Now either the hull drag component is too small to matter in relation to other drag factors like the rig or the hull drag was reduced by at least 15 % as well.
Wouter
