thanks for both responses...my son gave me a book by Dave Gerr about
propellers today...fascinating topic
thanks again
Marshall
"Chris Newport" wrote in message
...
MarshallE wrote:
Hi,

I own a 1942 Wheeler (83 cutter from WWII) and am interested in learning
about prop design. My engines are 250 HP Cummings with Twin Disc Gears.
My
goal is higher speed with lower fuel consumption without causing damage
to
the drives. I know that the current props are 4 blade and generate 10
mph
at 12-1400 RPM depending on current and wind. My current fuel
consumption
is 1.1 gallon per mile. The weight of the boat is 80 tons and is a
displacement hull.

You fuel consumption looks about right for an 80 foot boat at 10 knots
You are pushing the displacement hull a bit hard at 10 knots, do some
trials and plot speed against fuel burn, and MPG. Knots/GPH = MPG
You should find a sweet spot somewhere between 8 and 10 knots.

The speed of a displacement hull in knots is determined by the square
root of the waterline lenth in feet, multiplied be a constant k

k = 1.3 is the max that you can hope for, fuel burn will be BAD
For best cruising MPG k will usually be between 1.0 and 1.1 but it
depends on the hull design. In some cases best MPG may come at lower
values of k, plot your graphs and see how they come out.

For example, for a waterline lenth of 64 feet sqrt(len) = 8
8 * 1.3 = 10.4 knots -- Only achievable with a clean slick modern hull.
8 * 1.2 = 9.6 knots -- Maybe the max for older hulls
8 * 1.1 = 8.8 knots -- Only a tad less speed for about half the fuel
8 * 1.0 = 8.0 knots -- Best economy cruise will be somewhere around
here.

There is probably little to be gained by messing with the props.
If you want to go faster get a new boat with a semi-displacement hull
but be prepared to spend big $$$$ on fuel.