room for improvement?

[rthomas]

this is on the larger boat in my sig- right now it cruises nicely at about 25 mph @3400 rpm much slower than that it will start to fall off a little, wot is about 38 mph @4800. i would guess that 3000 would be a better rpm for the motor, the prop that i have is a 17" dia and 16 pitch and the blades are very thick, all the edges are .125 and i know this costs me some. should i try a 19ish pitch or try the same prop with thinner blades-(stainless)? my goal is cruising economy, top end isnt a big concern

 

[QC]

Re: room for improvement?

Need the correct model of 350; carbed, TBI, MPI, horsepower etc. There are a few different WOT RPM ranges. Considering it is 1988 boat, I am guessing you are already above your WOT range assuming your tach is right. With some 4800 RPM would be the bottom and going up in pitch would be bad. On others 4800 would exceed the WOT range and going up would be a good thing. Also, there is no guarantee that 3000 RPM will be better on fuel. This is way more about hull efficiency than engine efficiency. If it was about the engine we would recommend cruising at 1500 RPM, which would be really stupid . . .

A good stainless prop is almost always an improvement . . .

 

[rthomas]

Re: room for improvement?

its a carbed 260 hp, im going to guess that the best economy is going to be when the motor runs at its torque peak, (most v8's 3000+-rpm) and when that rpm matches a stable plane (24-26 mph) - is this correct?

 

[QC]

Re: room for improvement?

No. It is about HULL efficiency, not engine. Also, marine engines almost never operate at Peak Torque, so Peak Torque RPM is not really relevant. The only time you are operating any engine at max torque (any RPM) is with the throttle 100% mashed . . . If you had a continuously variable transmission (CVT) you could make these types of adjustments/decisions. The fact is that you have a single gear, so you just can't make it good for efficiency and then good for Power. If you did prop it for cruising at a very economical RPM while lugging more (Peak Torque type deal) Then you would:

a) Burn valves and
b) never get on plane

Are you sure that is a 17" diameter? Seems too big . . . If I were you I would try a three blade stainless of the same pitch or one inch more. I believe your recommended WOT rpm is 4600 max, so there is some room to slow the RPM down, could go as much as 3 inches if your numbers are right, HOWEVER, if she struggles at all to plane as is, this will make it worse. Three blades are most efficient.

You should maybe consider trim tabs if you can't maintain a good plane slower than 26 MPH.

 

[steelespike]

Re: room for improvement?

The best cruising speed could be any where from just on plane to about 2,000
rpm above.Really the most accurate way to determine best mpg/range is a flow meter.