General recommendation for slower speed, "Econo Prop"

[Willie12345]

I've been giving some thought to options that might extend motor life and reduce fuel consumption. Two thoughts come to mind that might help extend engine life and improve mileage. Reducing engine speed to some degree should reduce the wear and tear on the engine to some degree. An understanding of the engines torque curve and fuel consumption at give load levels would be helpful in this area. Having said that, I'd be interested to understanding any basic recommendations of prop characteristics that would also aid in reducing fuel consumption and engine wear. High speed operation would be of little concern, with the fundamental focus on engine life and fuel consumption.

Any ideas or suggestions ?

 

[emilsr]

Re: General recommendation for slower speed, "Econo Prop"

If you reduce engine speed by pulling back the throttle, then yea, that'll help with wear and tear on the engine (and fuel consumption). Over-propping to the point the engine turns below the recommended rev range at wide open throttle isn't the answer....no matter what the question is.

 

[Willie12345]

Re: General recommendation for slower speed, "Econo Prop"

"Over-propping to the point the engine turns below the recommended rev range at wide open throttle isn't the answer....no matter what the question is."

How do you find the "right prop" and avoid over-propping ? Let's assume that the optimum engine speed is 2600 based on the torque /fuel economy curves. How do you select a prop given this information ?

 

[steelespike]

Re: General recommendation for slower speed, "Econo Prop"

First of all marine motors are operating like they are pulling a trailer up a never ending hill and as you increase pitch your moving closer to over drive so the motor struggles even more.
Marine motors have an ideal wot rpm range. Lets say a 50 hp motor has a range of 4500 to 5500.It needs to be propped so that wot
operation is within that range if propped toward the top ,usually a lower pitch, it will have lots of power with good hole shot,powerful response to added weight or pulling skis. As you prop a motor closer to the low end of wot rpm range, higher pitch, power is reduced
both from the higher pitch and the fact the motor may be at a lower rpm barely generating its rated hp.
You would need a flow meter to tell the difference between a boat with a good setup and a boat with an excellent setup.
A 3 blade prop has good overall performance with good top end.
a 4 blade will have better low to mid range performance and maintain slow planing speed with less fussiness.
A ss. prop will very generally have overall better performance than an al. prop.Though the difference could be very close.
That 50 hp will likely be faster with a 17" prop, assuming a correct prop, but will struggle more to get on plane,push a load and will likely require more throttle to generate the same rpm as the motor with say a 15" prop that generates more power and quicker hole shot. etc. If you deliberately prop a motor too low it will be lugging leading to premature repair problems besides poor performance.
Attempts at ideal mileage are usually centered around the best cruising speed.usually planing some where from just planing to about 1000 rpm higher. A 50 will use very roughly 5 gph(gallons per hour) at wot,whether pushing a runabout or a house boat.
the ideal cruising speed will use significantly less.
There are so many types of props not enough room here with my endless rambling.
Very basically there are 2,3,4 and 5 blade props in the everyday boating world.Look up props here and on the net and ask us any questions as I'm sure the rhetoric will bury you.
A read about a 16ft diesel boat that made about 30 mpg at its ideal cruising speed,but not very fast.

 

[emilsr]

Re: General recommendation for slower speed, "Econo Prop"

"Over-propping to the point the engine turns below the recommended rev range at wide open throttle isn't the answer....no matter what the question is."

How do you find the "right prop" and avoid over-propping ? Let's assume that the optimum engine speed is 2600 based on the torque /fuel economy curves. How do you select a prop given this information ?

You prop so that the engine reaches it's recommended rpm range at full throttle. The range will be on a placard somewhere on the engine. Once you're in this range, you'll USUALLY get your best fuel economy with a prop that has the least amount of slip at cruise speed.....I say USUALLY as there are a lot of other pertinent factors (hull design, weight, engine type and size, cruise speed....it's a long list).

Most boats get the best fuel economy running at the hull design speed, which is basically a fast idle. That's why trawlers get such good fuel economy for their size/weight; they're designed to run at that speed. With a planing hull it isn't much fun to idle around so you really need to define what speeds you want to cruise at.