Cruising RPM's on OMC 5.0?

[Mahoney]

I was just wondering what your best MPG(not gallons per hour)RPM is with a 5.0 V8? I am just getting mine back into shape, its an old engine. got the 8th cylinder working yesterday(defective plug) and was going to take it for a spin in a bit.<br /><br />I found with the 7 cylinders at about 3000 rpm, I was only going about 22-23 mph(its a heavy boat) but just inching up to 3200 RPM, my mph shot up to 25-28, and 4000 rpm was getting about 35 out of it.<br /><br />Without spending $400 on a fuel meter, whats your best bet for cruising RPM,s to get the best MPG?

 

[tommays]

Re: Cruising RPM's on OMC 5.0?

the only way i ever new for sure was filling up at a gas dock running a 40 mile round trip and filling up again at the same dock<br /><br />other wise you need a flow meter<br /><br /><br />it depends so much on how the boat is proped out and loaded were the best spot really is<br /><br /><br />tommays

 

[Mahoney]

Re: Cruising RPM's on OMC 5.0?

I kinda figured that. Is it in your experience, there seems to be a "sweet spot" in marine engines? Basically a point where increasing just a few RPM's will get you a larger increase in MPH vs. any other range in the RPM band? <br /><br />I don't have any other powerboat experience to base this off of, but it is what I found out.

 

[tommays]

Re: Cruising RPM's on OMC 5.0?

there is deffiently a spot were you will get much better fuel use and usally you can run a littel above or below that and stay close<br /><br />but theres also a spot as you go up in RPM you will start to use a LOT more fuel<br /><br /><br />tommays

 

[Mahoney]

Re: Cruising RPM's on OMC 5.0?

yah, I am finding that out, just got back from lake. Boy what a difference 8 cyliders makes over 7. 4000 rpm ~33- 34 MPH to 41+ @4500 and it wanted to push a bit higher too

 

[rybad]

Re: Cruising RPM's on OMC 5.0?

I remember reading somewhere that cursing speed should be whatever WOT - 5000-6000 RPM gets you. For example, I have twin Cobra 5.7 on a 29' Four Winns, I usually cruise about 3400 RPM which pushes me to about 30mph. This burns about 24-26 gallons per hour.<br /><br />-Dave

 

[rybad]

Re: Cruising RPM's on OMC 5.0?

Sorry, I meant to say "cruising speed", not "cursing speed".. cursing speed is subjective.<br /><br />-Dave

 

[Richard Petersen]

Re: Cruising RPM's on OMC 5.0?

It really does not make too big of a deal about engine type or size. IF it is not a race type engine. Merc V6 220 hp 3500rpm is best. Twin Merc 135 hp OB's 3500 rpm is best. This for fast planing bow riders and off shore consoles from 18' to 25'. Look up some full boat tests by Boating Mag. American ( GM ) engines breath best at that rpm in general. Wacky non-V hulls, all bets are off.