[PCW] cs hull semi-displacement?

[Malcolm Tennant]

Chuck Johnson.

Dear Chuck,

The mathematical definition of planning based on monohulls of course is:- If 
the vessel exceeds a speed/length ratio[in imperial measurement]of 2 then, 
ipso facto it is planing.  By that measure all the CS hulled boats are 
planing.

However if you accept the physical definition that to be planing then the 
mass of a hull must be dynamically supported by the downwards thrust of the 
water deflected off the bottom [newtons third law] rather than statically ie 
floating. Then the CS hulls definitely are not planing.

When we are designing a CS hull it is of little consequence whether it 
operates in planing mode or displacement mode. What is important is that 
when a CS hull is operating on the interface between the two fluids, air and 
water, the point of maximum resistance ["the hump"] will be at approx Froude 
number 4 and that for any CS hull with a particular set of parameters F4 
will be at a particular speed. For this same particular set of parameters we 
will need a certain amount of power to achieve a particular speed. And that 
there will be a particular value of F number beyond which it becomes counter 
productive to try to proceed. Assuming of course that you can get the 
engines into the hull anyway.

In other words, it doesn't actually matter, when designing a CS hull whether 
one considers that it planes, semi-planes, semi-displaces or operates as a 
full displacement hull. What does matter are the numbers.

Regards,

Malcolm Tennant.


MALCOLM TENNANT MULTIHULL DESIGN LTD
PO Box 60513, Titirangi.
Waitakere 0642
NEW ZEALAND
Ph: +64 9 817 1988
e-mail: malcolm at tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com