Prop change from specification

[kloobs2305]

I bought a 1988 Sea Ray 270DA (twin 4.3l 175hp) 3 seasons ago and found out last year that sometime in the past the prop was changed from spec'd 14x19 to 15.25x15's.

The boat ran well but was getting around 1.25mpg and it seemed that I had to run at higher RPM than normal (>75%) to get reasonable cruising speed. (ie 25mph,3700 rpm)and so I thought that going back to the spec prop 14x19 I could get much better speed/mpg.

When I did this, my WOT RPM came down from spec 4500 to 4200. On initial start my WOT RPM would not get above 3700RPM until after planing which could take up to 15seconds If I had 4 people on board. I did get higher speed at equivalent RPM but couldn't cruise below 23mph/3600 RPM because the boat would come out of plane. Fuel efficiency was no better, and perhaps slightly worse.

I went back to the old props, now planing is 5 seconds as before with a lot of people, I can hold plane at 3300RPM/20mph, cruise at 224-25mph at 3600-3700rpm. Assume fuel efficiency is same as before.

Does this make sense or should I look at the engine to determine if I have a problem there?

Any comments?

Thanks in advance.

 

[steelespike]

Re: Prop change from specification

This is a twin installation right?
I'm no expert but it would seem the spec'd props should perform better.
Maybe not like the 15s but better than they did.
Maybe a compression check would be a start and a tune up with a double check of specs.
I'm assuming a 270 is a 27 footer.with twins 1.25 mpg doesn't seem terrible
considering a 50 etec on a 700 lb boat gets best 9 mpg.at 4,000 rpm.
Different animals but I think you can see a comparison.