prop change and im confused with the results

[nailem]

my buddy has a a 5.0 mercruiser with a alpha one drive. recommended rpm 4200-4800rpm. he has a 15.5 x 19, 3 blade stainless prop and is only turning 4100 rpm (check with a digital tach)with three guys very light gear and boat trimmed out. top speed is good at 40mph. the boat is slow to plane though. we tried my 14.5 x 17, 3 blade aluminum prop today. the top speed went down to 38mph that part i get but this is where i am a little lost. the rpms stayed the same, and it was slightly SLOWER to plane.

shouldn't the rpms have gone up about 400 or so and it should plane off faster? what do you think?

 

[steelespike]

Re: prop change and im confused with the results

when going from stainless to aluminum all bets are off.What boat is it.
It would seem a 5.0 would run faster on a 19 stainless.

 

[nailem]

Re: prop change and im confused with the results

it is a 1986 23' mach 1. he is happy with his speed of 40mph with the stainless but it is slow to plane and rpm are at 4100 at very best. also it was perfect conditions on the lake. the ss prop and the aluminum are both Michigan wheel props and both are in new condition.

so what you are saying is most of the rules are out the window when switching from ss to aluminum? what would you suggest to get his rpm up?

 

[steelespike]

Re: prop change and im confused with the results

I'm not a pro but::
I wouldn't throw out the rules but stainless to aluminum is more unpredictable.
Also when attempting a performance change its best to stick with similar props.If going back to al. or up to stainless results may vary.
You might confirm the rpm is accurate.motor and tune is good.Throttle opening all the way.If it checks then it would appear a 17" pitch is in order.
The pros might be able to give more technical help.

 

[nailem]

Re: prop change and im confused with the results

tach is dead on. and the boat was run with the 19 several runs then we changed props on the beach and ran again. i could not believe the results so we changed back to the 19 and same thing.

i guess i dont understand what you mean a 17 is in order? do you mean a ss 17? the aluminum is a 17. the only reason we did this change was i had a 17 as a spare prop.

 

[Randybeall]

Re: prop change and im confused with the results

You must look at all the things you changed with the first test. You dropped two inches of pitch, you dropped 1 inch of diameter, and you went from a very efficent SS prop to a less efficent Alumnium prop. I suspect that your slippage numbers went wild with that change. My WAG on reprop for that rig would be SS 15.5 X 17. That will get your rpm up, and maintain the efficency. I would expect stronger hole shot and time to plane though top speed might not change.

 

[nailem]

Re: prop change and im confused with the results

Randybeall, thats what i figured we need to do but i think another ss prop is not in the picture right now for him. we just tried the 17 aluminum because i had one and it was free. im also having a hard time convincing him his boat will perform better with a different prop.

i understand the performance loss with the aluminum 17, it just seems to me and "i know nothing" that the aluminum 14.5 x 17p would have turned/slipped more and thus his rpms should have gone up.

can anyone venture a guess on what we could try in an aluminum prop. like i said i know he is not going to spend the money on a ss right now.

 

[glengold]

Re: prop change and im confused with the results

al prop will flex more, causing lower speed at the same rpm's. don't expect rpm's to change as they would if you stayed with stainless and lowered pitch. if the motor turned the proper rpm's at one point its not the prop, its the motor. I don't blame him for not wanting to buy another prop, he already has a stainless prop! check w/the manufacture about prop ratings. Many people think they are over propped when in reality the motor isn't running at full power. You might be heading down the wrong path......

 

[steelespike]

Re: prop change and im confused with the results

Looks like this may be a dead horse from 2008. I felt they might find a motor issue
and if they didn't a 17" ss same brand would be the solution.