Yet another prop question

[wishihad]

Recently repowered my 20' hurricane deck boat w/a 115 etec. The dealer put a 17deg alum. Boat would run 30mph @ 4krpm but only reach 4900rpm @ WOT w/a speed of 38mph. Book says engine should run 5500-6000rpm WOT, so I changed to same type prop 15deg. Now I have to run 4500rpm to hit 30mph and WOT is 5400 @ 37mph.

My holeshot is now spectacular but I lost 1 mph @WOT and I'm assuming I'm burning more fuel, due to hight rpm, to acheive same cruise speed of 30mph.

What should I do?

 

[Silvertip]

Re: Yet another prop question

Two stroke engines do not like to be "lugged" which yours obviously is if it only turned 4900 RPM WOT. Before changing props its always best to work with setup first (engine height). Once that's optimized, then a prop switch may be in order. Your original prop is not a 17 degree -- it's pitch is 17 inches meaning theoretically the boat moves forward 17 inches per revolution. But since it operates in a liquid, there is some slippage. For each inch of pitch reduction the engine will rev about 200 RPM higher. You went up two inches and noticed about a 500 RPM increase which is normal. Keep in mind that all you are doing is increasing RPM. The 1 MPH loss can be due to trim setting, air temp, water conditions, and difference in prop efficiency. Less pitch raises RPM for any given speed. More pitch lowers RPM for any given speed. Because the engine revs faster does not mean the boat goes faster. Remember, you reduced the pitch so you lose two inches of movement with each rev. Your performance changes are as expected. If it were a car, the prop switch you made would equate to shifting to a lower gear so the engine will run faster - and yes, burn more fuel. But you gained hole shot and are no longer lugging the engine.

 

[wishihad]

Re: Yet another prop question

Two stroke engines do not like to be "lugged" which yours obviously is if it only turned 4900 RPM WOT. Before changing props its always best to work with setup first (engine height). Once that's optimized, then a prop switch may be in order. Your original prop is not a 17 degree -- it's pitch is 17 inches meaning theoretically the boat moves forward 17 inches per revolution. But since it operates in a liquid, there is some slippage. For each inch of pitch reduction the engine will rev about 200 RPM higher. You went up two inches and noticed about a 500 RPM increase which is normal. Keep in mind that all you are doing is increasing RPM. The 1 MPH loss can be due to trim setting, air temp, water conditions, and difference in prop efficiency. Less pitch raises RPM for any given speed. More pitch lowers RPM for any given speed. Because the engine revs faster does not mean the boat goes faster. Remember, you reduced the pitch so you lose two inches of movement with each rev. Your performance changes are as expected. If it were a car, the prop switch you made would equate to shifting to a lower gear so the engine will run faster - and yes, burn more fuel. But you gained hole shot and are no longer lugging the engine.

Silver.
tnks 4 the input.
I undestand and realize that less pitch equals less distance covered per revolution. I did try varing the engine height and have it one from the upper most setting. I'm actually very happy that my beast of a boat is obtaining 35+mph. I'm just trying to understand what the impact will be on fuel economy vs engine life with the two diffrent combinations. 30mph seems to be most comfortable speed 4 my hull in smooth to light chop, i don't really care about the hole shot the boat motivates nicely with both the 17 and 15.

So long story short I have 2 questions:
1. WIll I achieve better fuel econmy running 4000rpm with a 17 prop or 4500rpm with the 15 prop? both of which produce around 30mph gps.

2. Assuming the 17 will give me better fuel efficency am I doing harm to my engine if I go with the 17 which will only acheive 5000rpm WOT.

tnks agfagin 4 ur input

 

[walleyehed]

Re: Yet another prop question

The prop that puts you closet to the top of the RPM range will cause the "least" amount of lugging (high manifold pressure/vacume) and will allow the best fuel economy because it is doing less work. The combustion temps will be lower and the engine will perform better over-all.
Economy is not just speed...it is mpg not mph. In the same respect, better speed does not mean better economy, especially when over-propped.

 

[wishihad]

Re: Yet another prop question

The prop that puts you closet to the top of the RPM range will cause the "least" amount of lugging (high manifold pressure/vacume) and will allow the best fuel economy because it is doing less work. The combustion temps will be lower and the engine will perform better over-all.
Economy is not just speed...it is mpg not mph. In the same respect, better speed does not mean better economy, especially when over-propped.

Huh?
Wouldn't or isn't the engine doing the same amount of work if the boat is traveling the same speed, reguardless of engine and prop rpms?

So the question is at what rpm is the engine the more efficent at converting fuel into physical energy. I would assume, possibly incorrectly, that the prop as well as the engine internal rotating componets efficency would decrease as speed increase. Assuming they are transmitting the same amount of energy/work/whatever.

 

[walleyehed]

Re: Yet another prop question

The most efficient RPM changes from 1 hull type to another.
The economy range is very much mis-understood.
If the engine is propped for a max of 5000, and you cruise at 4500, you're going to burn alot more fuel per mile than if you prop for 5800 and cruise at 4500.
Speed comes into the equation when comparing 2 props that achieve the same WOT RPM...then you establish which prop is faster at the cruise RPM. WOT RPM has to be set FIRST.
The closer you are to the top of the RPM range, the better your fuel economy will be at best cruise.
And No, that can't be determined by using 1 prop.
Your best over-all performance AND fuel economy is going to be with an SS prop with some good technology built into it.
If you check the BRP site, you will find test reports with different hull styles...these reports show at what RPM best economy was attained, they also list max RPM achieved...you'll have to find a model in their test reports that is closest to your set-up and take a guess at what your personal best cruise RPM will be, and this is where prop design technology comes into play...a higher tech design will be more efficient at any given RPM.
I hope I haven't confused you more...d