T&T: Boat on wheels

[Faure, Marin]

>So my question is : what about if a boat was on floating wheels ?


I'm no marine architect and I didn't do all that well in physics in
school.  But I believe that the nature of water is that anything
floating on it only floats because it displaces its weight in water.
(Exceptions are water beetles that weigh so little they are supported by
the surface tension of the water, and hydrofoils and planing boats at
speed which are supported by the speed-generated hydrodynamic force of
the water against the bottom of the hull or foils.)  So if you put a
boat on four floating wheels, the wheels will sink into the water until
they displace the same weight of water as the boat weighs.  At that
point, the boat will float.  But the wheels will not be on top of the
water, so you will still have all the drag of the water on the submerged
portion of the wheels to overcome.

The reason a truck gets so much better mileage is that it does not have
to displace its weight in asphalt and dirt in order to stay on top of
the road.  It simply sits on its tires, so the only drag it has to
overcome are air resistance and the fairly minimal drag caused from the
small area of contact between the rubber and the road.  Of course it has
to accelerate its weight up to speed, keep its weight rolling forward,
and haul its weight up hills, and it takes a fair amount of fuel to do
these things.  But on level ground the engine's not working so hard, and
going downhill it's not working at all.  So you get your 6 or 8 mpg
average.  A boat's engine, on the other hand, never gets a break.  It
has to propel the boat forward under a constant drag load from the
water.  There is no "level water" break, or "downhill water" break.

Now if you filled a boat with helium or some other lighter-than-air gas,
the boat would weigh less in terms of its weight against the water so it
would not need to displace as much water to float so the drag would be
reduced.  There are some downsides to this, not the least of which is--
if you use helium--the crew will be talking in high, squeaky voices all
the time and will be hard to understand on the radio.

______________________________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington