Prop size and pitch

[Sea Wolf]

We have an older Prout Quasar 50' catamaran, 4' draft Weight is 38,000 pounds and is 20' feet wide. She's a large heavy boat. We have the most modest sail plan recommended by Prout and still can reach 10 to 12 knots.

My question relates to motoring and power sailing, which we do a great deal of the time. When traveling on the inter coastal we are usually just motoring. With the two engines running (40hp) we can do 8 knots but with only one engine running at 2400 rims we can do 6.5. We only burn about one gallon per hour, per engine. So, running both engines is not economically worth it. She tracks straight with only one engine running due to the full keel and canoe sterns. I can bring the boat up to my redline of 3300 rpm's with only one engine so I know I am under propped. Props are 15RH12's. There is presently 4 inches of clearance between the bottom of the hull and the top of the prop, so there is plenty of room for a larger prop.

We are in the yard having the bottom done. I took one prop off to have it balanced. I had them add one degree of pitch and I'm anxious to see what that will do to the performance. I would appreciate any comments as to getting more speed under power. I would love to be able to get one or two knots while motoring.

Any thoughts will be appreciated.

[Jeannius]

A Quasar fits in the ICW! What's the airdraft?

[Sea Wolf]

We can run the ICW providing the bridges are built to ICW specs. Some are undersized (the Julia Tuttle in Miami, for one, is only 57 ft) and they force us to run outside. There are several on the Gulfside that are undersized.

Our cabintop is 8 ft +/- off the water, the mast is 50 ft and we have 3 ft more in masthead goodies... that’s 61 ft and we add another 2 feet cushion when asked for our height off the water.

[Jeannius]

Ah, that's lower than I expected. I lost vhf and wind instruments off my boat in the Dismal Swamp Canal. Had successfully passed under several other '65 ft' bridges before then so it came as a bit of a surprise when things came crashing down.

[Snowgoose35]

I just put two 16/13 on mine and it seems to work well - 7 knots at 2200 rpm with both engines running (I have 3GM30 Yanmars and they run 1/2 gal an hour each at that revs)
Just to clarify I now have a Quasar 50 despite the user name based on my last Prout

[mikereed100]

You might try investing in feathering props with adjustable pitch. If nothing else you would pick up a knot or so under sail. We have Kiwi's and I have increased the pitch just a bit. I lost 100 to 200 rpm off the top end, but we never motor at greater than 2400 rpm any way. The boat picked up at least .5 knots at all rpms. There is no smoking and the temp remains the same.

[Sea Wolf]

Thanks for your response; it’s exactly what we were looking for. With the 16/13s, how fast can you go on one engine?