"Gene Kearns" wrote in message
...
On 4 Oct 2003 07:21:34 -0700, (roy) wrote:
Am looking at a new boat approx 20-21 ft in length with 15 degree
deadrise, sort of a flats type hull shape for shallow water. The
manufacturer rates the transom for up to 200 HP, the dealer claims
that 150 is plenty and that anything larger is a "waste of money" in
both initital cost and operating costs. He also claims a speed
increase at WOT of only 2-3 MPH.
I have looked over the Yamaha website for performance on specific
power packages but can't seem to find the same hull with two
different
HP performance tests.
My previous experience indicated that by having maximum rated HP on
the transom that one could turn a larger prop, say a 21 inch pitch
while the less powerful engine would force the prop pitch down to a
19
or less.
I realize that at WOT a 200 HP will burn more fuel but it is my
intent
to operate for the most part in the 30-35 MPH range on a hull that
will offer in excess of 50 MPH. Will the reduction in RPM on the
larger motor to achieve a given cruising speed provide adequate
additional fuel economy and performance increase to justify the
increased initial expense?
50 HP isn't going to make a difference of a mere 2-3 mph. Bear in
mind that not only will the HP change, but so will the pitch of the
screw. You will most likely be turning fewer RPMs at your desired
cruise with the larger engine due to this.... and, again, burning less
fuel. If you really want to burn max fuel and go flat out... you have
the option....
You are in the "driver's" seat.... if the dealer wants to sell the
boat, he must let you sea trial both models. My guess, extrapolating
from your description, is that he has a smaller engine he doesn't want
to get stuck with through the winter.
Be from Missouri...... say, "Show me....."
The rule of thumb is that max speed is proportional to the square root
of hp. If you increase the HP by a factor of 1.333, you increase the
top speed by about 14 %. If the top speed is about 50, that is 7 mph.
But if you are only going to go 35 because of waves or other factors,
then that should be taken into consideration. The bigger motor won't
burn less gas at the same speed fer sure. Maybe not much more, but not
less.
del cecchi