[PCW] A question of weight

[Gary Stavrou]

One of the sacred cows of cruising power catamarans is that they  
should be as light as possible.
I'd like to understand why!

If we look back to Newton's First Law of Motion which, put simply, says:
A body will remain at rest or move at a constant velocity unless  
acted upon by an external force.

Let's consider a fine-hulled displacement-type cat say 45 feet long  
with a displacement of 8000Kg (17,600 lbs).
And let's pretend that the water is totally calm, that there is zero  
friction between the water and the hulls and that there is zero air  
resistance... this is purely hypothetical, of course, but it will  
help me make my point.
You start the engines, turn the steering to dead ahead, and put the  
motors into gear.
The boat will move ahead and accelerate at a rate dependent on the  
throttle setting.
When you reach your preferred cruising speed of, say, 16 knots you  
turn the motors off!
Now, according to Mr Newton, the boat will keep traveling at 16 knots  
in a straight line without any added power until an external force is  
applied.

Lets now compare this with a similar, but heavier cat of say 13000Kg  
(29000 lbs). It will take more engine power, or perhaps a little  
longer, to reach 16 knots but again, when the engines are turned off,  
the boat will continue traveling.

Let's come back to reality now. We all know that if you turn off the  
engines the boat will slow down and stop.
This is because external forces are indeed applied. These are primarily:
(1) friction between the water and the hulls
(2) the effect of wave action
(3) windage.

So in both the light and the heavy boats, the only reason that we  
need power is to overcome 1-3 above.
The heavy boat will sit a little deeper in the water and will have a  
little more wetted area.
The windage is about the same in both boats.
The wave action should slow down the heavy boat less, because it has  
more inertia.

If you've followed me so far, you can see my dilemma.
Seeing that once the cruising speed of 16 knots has been reached, it  
takes about the same amount of power to push a light boat as it does  
a heavy boat, why is everyone obsessed with light boats?

I must be clear that my example of a heavy boat is NOT a light boat  
that is overloaded and out of trim. No, I mean a properly designed  
displacement cat which is built in heavier materials; say aluminium  
compared to composite.
Also I am ignoring planing (or even semi-planing) type hulls which  
require power to lift the boat out of the water.

Any thoughts, anyone?
Gary