10% decrease in top speed at WOT on Maxum after changing prop with same dimension

[Timboat]

This year I changed the propeller and hub on my 1997 Maxum 2300 Powerboat (Merc 5.7L Alpha 1 Gen 2) and now I have a 10% decrease in top speed at WOT.
I understand that the different diameter and pitch of propellers can change top speed and acceleration, etc, but my problem is that I replaced my existing propeller with a brand new propeller of the same dimensions.
Previously I had a 15.25 x 15 Aluminum 3 blade prop (which was probably the original with mercruiser drive) I replaced it with Michigan Wheel 3 Blade Aluminum Propeller 4-3/4" Gearcase 202 Hub Kit
(Size: 15 Pitch 15-1/4" Diameter) for $133 online
With the old prop, my top speed in ideal conditions, half a tank of fuel 2 passengers, smooth water, etc is 37mph at 4700rpm at WOT
With the new prop, my top speed in ideal conditions, half a tank of fuel 2 passengers, smooth water, etc is 33mph at 4700rpm at WOT
This week I reverted back to the old (original) prop which is dinged up and missing a few chunks, and it still performed at 33mph at cruise at 4100rpm.
My question is, why is the new prop NOT performing the same and not better than the old prop?
Could the hub make a difference? I had the newer slotted design on the new on vs the old on the original.
Could the aluminum be cheap and substandard and losing integrity at high speeds?
Could I have over tightened the new one?
How do I get my extra 10% off performance back?! Thanks!

 

[alldodge]

Things don't add up
Both props are the same size and pitch, only difference is the old one is dinged up
Old prop runs 37 mph at 4700 rpm last year
New prop runs 33 mph at 4700 rpm this year

Put old prop back on and runs 33 mph at 4100 rpm

Initial thoughts
Your tach is going wack-o and maybe your not using GPS for speed. Either that or your trimming up to much and loosing speed

The old prop having dings on it will have more slip, so your speed with the old prop might have slower but the motor would have higher rpms.
If we assume you have a 1.5 ratio drive, the last year your slip was 16%. This year with the new prop your slip is 25%. This year the old prop your slip is 15%.

 

[Texasmark]

Just because the dia and pitch are identical by no means indicates a similar prop. Apples and apples are the same bran, same model, same dimensions.

Your problem could be as simple as your original prop was cupped and the current isn't. That reduces the effective pitch at WOT about 1" and reduces holding power which increases prop slip as calculated by AllDodge. 1/15 x 37= .67% or 2.5 mph difference there.

Rake or no rake for another, owing to bow lift or less bow lift, changing speed for a given amount of power from the engine due to more of the hull in the water and increasing drag.

On the hub thing, if worried, mark a line on the spline all the way out to and including the barrel of the aluminum part of the prop. Take it out and run it and come back and inspect your line. If intact, no hub slip. If not aligned, problem would be hub. Course if the hub were slipping you should see wear marks having had it out for a good run.

On the prop nut torque, slightly possible with a pressed in rubber hub but not with props with plastic/brass inserts. I would assume torque for that engine would be in excess of 55 ft-lbs which was the torque for my 90 hp engine with a 4" gear box, but 55 ft lbs in itself would add a lot of pressure to a pressed in rubber hub.....would doubt the new one would slip anyway, even finger tight..

 

[Timboat]

Those replies gives me a place to start. I researched further found that the new prop is this one:
http://boatpropellers.iboats.com/Michigan_Wheel-Vortex_XHS-Propellers/992002/ And does have a cupped design. But I understand your point about comparing apples to apples.

As for my speed testing I am using only GPS, infact I even use an app on my phone with a decimal point (XX.Xmph) on the speedometer so I can really trim the drive up or down and maximize speed at lower increments
As for my tachometer it very well could be the problem, as sometimes I have to give it a good tap to get it back working again, but that is usually just a lower RPMs and in this testing it have been testing at WOT.

Is there a way to find out the exact propeller the mercruiser drive was installed with and buy the exact same one?
Or could I would be open to buying another propeller that could give me back that lose of performance? I am assuming better quality propeller would cost me more than the $86 this one cost.

Thanks

 

[alldodge]

Is there a way to find out the exact propeller the mercruiser drive was installed with and buy the exact same one?

Unless you can find some numbers on it which can be translated into a find, then were guessing.

I still go back to your original prop last year did 37 and this year does 33. Something is missing

 

[Timboat]

Yeah, I tend to agree with you. I wish I had some way to go back in time and get a solid benchmark test with the original prop last year to be 100% sure I was going 37mph at 4700 WOT. I am also thinking to check some other perfromance killing issues like too much growth on the hull and perhaps my engine needs a spark plug replacement, wires, carb cleaning, etc. Regardless thanks for all the help. I will keep investigating.

 

[Texasmark]

Yeah, I tend to agree with you. I wish I had some way to go back in time and get a solid benchmark test with the original prop last year to be 100% sure I was going 37mph at 4700 WOT. I am also thinking to check some other perfromance killing issues like too much growth on the hull and perhaps my engine needs a spark plug replacement, wires, carb cleaning, etc. Regardless thanks for all the help. I will keep investigating.

Glad you tossed in some other common variables. Things do wear over time and need a little TLC. Grin