RPM vs percentage of engine power for cruise speed.

[rs2k]

This thread was originally a question about re-propping my cabin cruiser, but I turned it into a more general question about percentage of engine power vs cruise RPM

I have for a while considered 3500 RPM to be a high speed cruise and 3000 RPM to be a reasonable "long engine life" cruise speed for a correctly propped Chevy Small Block marine engine. Does this sound reasonable? I don't believe that RPM is not really a good value to use, but it's all we really have to go by.

Let's say we have two nearly identical boats and engines. (Props being the only difference)
If Boat A's engine and prop combo is propped for max speed at 5000 RPM
and Boat B's engine and prop combo is propped for max speed at 4000 RPM

Than cruising at 3500 RPM on boat A would be at a lower speed and engine power than if you were to cruise at 3500 RPM on boat B.

Would it be okay to increase the cruise speed of boat A to a higher RPM? Let's assume it takes 3900 RPM to maintain the same percentage of engine power as Boat B cruising at 3500 RPM is using.

I would assume that the increase in Boat A's cruise speed would lead to greater fuel consumption, greater wear and tear, and shorter mean time before failures because of the energy it takes to rotate the engine an extra 400 RPM. Any idea how noticeable these negatives would be?

 

[WizeOne]

Re: RPM vs percentage of engine power for cruise speed.

..... I don't believe that RPM is not really a good value to use, but it's all we really have to go by......

Not so! Your ear is better than the tach. The tach takes nothing into consideration except motor revolutions per minute. It has no eyes, ears and feel. What's an effective/efficient cruise speed on one boat will be entirely different on another, motors, props and drive ratios all being equal.

Your best economy is going to be to prop your boat so that you achieve maximum rated rpm at full throttle, then let your senses tell you when your motor is working easy. (assuming on plane).

The ONLY other way would be to have a fuel flow meter and I would bet that I could 'feel' your boat's sweet spot within a few rpm of what a fuel flow meter would show.

 

[wca_tim]

Re: RPM vs percentage of engine power for cruise speed.

There's some really good threads about this where 45auto posted some really good cpomparative data including between engines, fuel economy, etc... Not exactly the same questions you're asking I don't think, but certainly gets to the core of what you're thinking about.

 

[rs2k]

Re: RPM vs percentage of engine power for cruise speed.

Not so! Your ear is better than the tach.

...then let your senses tell you when your motor is working easy. (assuming on plane).

I don't know, I sure like the sound of both engines at full throttle. There's a nice harmonic whine going on between them that I can never get anywhere else in the RPM range.

Your best economy is going to be to prop your boat so that you achieve maximum rated rpm at full throttle, then let your senses tell you when your motor is working easy. (assuming on plane).

Really? I would have thought best economy would be had by propping the boat for lowest rated WOT RPM.

 

[rs2k]

Re: RPM vs percentage of engine power for cruise speed.

There's some really good threads about this where 45auto posted some really good cpomparative data including between engines, fuel economy, etc... Not exactly the same questions you're asking I don't think, but certainly gets to the core of what you're thinking about.

Do you know where that thread is located? A few quick searches haven't found it yet.

 

[WizeOne]

Re: RPM vs percentage of engine power for cruise speed.

....
Really? I would have thought best economy would be had by propping the boat for lowest rated WOT RPM.

I am not sure of what you are speaking. Every marine motor has a specified maximum rpm. This is what you want to prop to. Your WOT should be at that specified rpm.

...and yes I know, they can sound good at WOT, if your WOT is at Max rated RPM, but an experienced ear would tell you that they/it is working too hard and is way above the most economical cruise speed/rpm for your particular hull.

The manufacturers of the engines do not give different 'max rpm' values, on a given engine, for different applications. One size fits all. You just prop it to take into account your hull and or average load requirements.

 

[rs2k]

Re: RPM vs percentage of engine power for cruise speed.

I am not sure of what you are speaking. Every marine motor has a specified maximum rpm. This is what you want to prop to. Your WOT should be at that specified rpm.

...and yes I know, they can sound good at WOT but an experience ear would tell you that they/it is working too hard and is way above the most economical cruise speed/rpm for your particular hull.

The manufacturers of the engines do not give different 'max rpm' values for different applications. One size fits all. You just prop it to take into account your hull requirements.

The maximum rated RPM for my engine and out drive combo (260HP OMC Stringer 800) is 4600, but it says to prop it to 4200 - 4600 RPM. I was thinking that best fuel economy would be had when propping it to 4200 instead of 4600 RPM.

I know what you mean by listening to the engine. I've been around engines for a long time and I picked up a pretty good ear for engine load when flying two stroke powered airplanes. You never wanted to work them too hard for too long. You might find yourself in the middle of a farmers field... or worse.

 

[180shabah]

Re: RPM vs percentage of engine power for cruise speed.

Yes the ENGINE will be more efficient running WOT 4200 vs 4600, however the VEHICLE efficiency will be almost identical. It takes X hp to move your boat at a given speed, and that X hp requires Y fuel, regardless of the RPM.