'cruise' defined (my version)

[tomcat rio]

. i would call it the rpm where the torque curve flattens out. for tne pinsular , its around 2900) this should be the best nmi/gal...increasing rpm after this point will increase speed but nmi/gal drops off.
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it seems to be quite an arbitrary term, though.

[tomcat rio]

i gotta learn to proof stuff.

see link,

http://www.peninsularengine.com/marine400ta.html
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pretty much all engine manufacturers publish similar data.

[jcollins]

Good information Tomcat. Thanks!

[Mariner]

Cruise speed is the speed at which you, the owner, prefer to travel most of the time. There are a lot of factors that can go into determining cruise speed/rpm, such as planing state, efficiency, comfort, and speed expectations. I cruise at 10 knots. The boat is capable of running faster for extended periods of time, but above this is where I feel the fuel efficiency drops off to the point that the marginal increase in speed does not justify the considerable increase in fuel consumption.

[tomcat rio]

...at the rpm where your torque curve flattens out.

[Mariner]

I cruise at about 2000 to 2,100 rpm. This burns 10gph and achieves about 10 knots.

[tomcat rio]

what's your fuel flow and speed at 1700 rpm where the torque starts to flatten? i would guess the nmi/gal is better than at 10 kts. i think this has been refered to as 'low cruise'...

[tomcat rio]

...i would bet that the 966 foot pounds of torque might be more than enough to turn a prop with a larger pitch/diameter

[Mariner]

Our most efficient speed is idle. Our least efficient speed is WOT.

At idle (700rpm), we make 4 knots at 1gph (4nm/g). At WOT (2800rpm), we make 16 knots and burn 24gph (0.6nm/g).

I'm not sure what the speed is at 1700 rpm, but I know the fuel consumption is about 6gph. Somewhere I have a spreadsheet and graph that we put together. I'll have to dig for it.

[tomcat rio]

my gut feel is that if you have the same torque at 1700 as at 2100. you could use that 1700 rpm torque to rotate a larger prop which, although mass would be higher, is not rotating as rapidly and might produce the same thrust(speed) as the current prop at the highter rpm. the advantage, of course would be the lower fuel flow at the lower rpm...
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if diameter is a clearence limitation, a pitch increase might produce similar results.
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i'm certianly no prop guru. this is just my understanding of the theory in gerr's book.

[Mariner]

The general concensus on prop pitch selection is that you want the highest pitch prop that still allows your engine to achieve it's maximum operating rpm. If you open the throttle and the engine goes above your max rpm (or you reach max rpm and still have throttle left), then you can switch to a high pitch prop to achieve a higher maximum speed. Our engine is computer controlled, so I don't know if it can turn higher rpm's or not with that prop on it. But remember, fuel economy is really based on weight and hydrodynamics more than anything. It takes a certain amount of energy to push X pounds through the water at X speed. Changing the gearing (prop pitch), doesn't really affect that.

[tomcat rio]

well, if two cars are doing 60 mph and one is in 3rd gear and one is in 5th gear, both may be producing the same amout of energy but i would bet the one in 3rd gear is burning more gas.
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it would certianly be nice if you could order several props, try them all and return the less efficient ones.

[Mariner]

Not if they're going uphill. Resistance plays a big role in efficiency and gearing. The risk of overpropping a boat is that you can put too much resistance on the engine, thereby not allowing it to turn the rpm's it's designed to for a specific fuel consumption rate. If I over prop, instead of turning 2100 rpm at 10gph, making 10 knots, I might find that I'm only turning 2000 rpm and making only 9 knots...but still burning 10gph.

My inclination is to trust that Albin took the time to select the appropriate propeller for our boat when they designed it.

[Mariner]

And good prop shops will allow you to test various propellers on your boat. Sometimes they require a deposit, sometimes they ask that you buy the propeller, and return it for another one. But the basic pincipal is the same, you get to try out as many as you like if you have the patience and time to do so. Obviously, the truly limiting factor is the fact that you have to swap out the props, which is no small task on boats like ours.