flexibility

[greg crosby]

I have a 1988 3.7 221 starcraft islander. I am looking for all the gas mileage I can get. I have a 3blade merc prop 19" pitch.
The book says 4200-4400rpm WOT. It runs top end at 4500 loaded. The 3.7 I,m told was designed to be a high torque engine? I want to go to a 4 blade prop. I would like to keep the rpms at 4200. Is this a reasonable idea? I'm wondering if a 4 blade 18" pitch would bring the rpms down a bit and give a lower rpm cruising speed? Is there any reason a cupped edge would improve performance on a 4 blade prop? Where is the best place to buy a 4blade prop? Thank you.p

 

[BigDfromTN]

Re: flexibility

I have a 1988 3.7 221 starcraft islander. I am looking for all the gas mileage I can get. I have a 3blade merc prop 19" pitch.
The book says 4200-4400rpm WOT. It runs top end at 4500 loaded. The 3.7 I,m told was designed to be a high torque engine? I want to go to a 4 blade prop. I would like to keep the rpms at 4200. Is this a reasonable idea? I'm wondering if a 4 blade 18" pitch would bring the rpms down a bit and give a lower rpm cruising speed? Is there any reason a cupped edge would improve performance on a 4 blade prop? Where is the best place to buy a 4blade prop? Thank you.p

You are already over reving the engine. Do not drop down in pitch at all. If you simply want to get better mileage, Go to the next higher pitched prop and/or add some cup to the one you have now. Cup acts as an increase in pitch and helps the prop "bite" the water yielding less slip.

Is there a reason you want to go to a 4 blade?

I just went to one on my 24' pontoon due to having issues with prop slip under heavy loads. It really helped and as of now I am very pleased.

 

[steelespike]

Re: flexibility

delete

 

[steelespike]

Re: flexibility

What is fully loaded? Your lightly loaded wot rpm and gps speed will help tell how your setup is working.
Wouldn't hurt to describe your overall performance,hole shot.
A marine engine is allways straining like a car hauling a trailer up a never ending hill.
The motor needs to be able to rev freely within its rpm range.
Once you establish the best performance an increase in pitch might help mileage.
But careful prop selection and setup will likely do as much for you and maintain overall performance.
A simple increase in pitch might increase speed and mileage at a given throttle setting, but hole shot may be slower,
Will likely reduce rpm and top speed. The boat will fade faster as weight is added.Kind of like putting that car into a higher gear.
The difference in mileage of a well setup boat and a perfectly setup boat would have to be measured with a flow meter.
In fact if you want to track your mileage there are flow meters that will calculate mileage as you go.