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

Gary,

You're certainly correct regarding the physics, but I think you're
ignoring a couple considerations.

In your example you're increasing displacement by 65%.  That's
substantial.  Either your draft will be much deeper (with a
corresponding decrease in wing deck clearance, etc.) or you'll have to
significantly increase the beam of the hulls to compensate - think
Archimedes instead of Newton.

Either way you solve it produces more problems.  Deeper draft will yield
a lot more skin friction (not a little) for a boat traveling 16 kts.
Wider hulls mean a much bigger bow wave that you have to overcome at 16
kts.

If you want a 7 kts boat, then adding displacement won't matter nearly
as much.  But when you get into the higher speeds the nonlinear
mathematics of the resistance factors start to work against you.  But
you can overcome them pretty easily with larger motors and more fuel.

Bob Deering
Juneau Alaska

Weight is important, in that hull resistance is directly proportional to
weight. In other words, a heavier boat takes more power to push it.

If you consider an automotive equivalent, these days car makers often
publish a cd (coefficient of drag) which indicates how streamlined the
vehicle is. These range from about 0.2(very smooth sports car) to 1.00 (flat
plate).  But this doesn't tell you the total drag. That calculation involves
cd multiplied by FRONTAL AREA. Thus a Hummer will have more drag than a
Mini, even if the cd is the same.

Boat hulls are similar, but the interaction with water is more complex.
Still, FRONTAL AREA, or if you like maximum cross sectional area, is a key
factor in total resistance. Frontal area is related to displacement.  A
heavier boat will need fatter hulls to keep it floating right, and these
will have greater frontal area.  The alternative is to keep the area the
same and increase the length. Long skinny hulls have less frontal area than
short, fat hulls of the same displacement.

Weight matters because as the boat moves, it has to constantly push an
amount of water equal to its own weight out of the way. It takes power to
push that water.

If this was not true, you could use the same size engine on a 100' boat as
you would use in a runabout.

Heavier boats have a place, and keeping a boat light is tough. It is human
nature to keep adding gear and equipment to fill the available space. A
heavy boat can carry more fuel, water, fuel, and the comforts that make life
aboard more self sufficient.  So, if it goes slower, why worry.

Kind Regards,
HYPERLINK "mailto:designer@aviadesign.com"Grahame Shannon
HYPERLINK "http://www.aviadesign.com/"Avia Design Group Inc.
4884 Skyline Drive
North Vancouver, BC
V7R 3J3 Canada

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