Trying to select a prop for my 3.5HP Mercury 4 cycle on my Bass Raider 10E.  Maybe some of you guys have some guidance.  

Speed isn't the primary concern here (oh yes it is  ) but optimal WOT RPM for this motor.  

The factory prop for this motor, sold under a few different badges is a semi weedless 3 blade 7.4" diameter and 7" pitch. 

For emergency use I bought a spare 7.25"X6" which I guess is the factory prop on the similar 2.5HP. 

There is an aluminum prop available that would fit (I think) advertised as 7.4"X5.7".  

That's it, 3 props.  The plastic props don't look to be suitable for thermal shaping while the aluminum might.  

We normally run with at least 400 pounds including motor.  

Once with 500 pounds with a 29 battery and 30 trolling motor and once just for a cute video with one person but no data for that run.  

I will try again with the lightest load I can (after the hurricane).

 

Here are my data points with the 10E, both on the same day, same water and wind conditions:

7.4"X7", swept area (blade minus hub) of 40 sq.in., 4840rpm (WOT), GPS speed 5.4 mph. 

7.25X6", swept area of 38 sq.in., 6000 rpm(WOT), GPS speed @ 5500 rpm 4.8 mph.  

 

The 7.4"X7" has 4.5% more swept area, 16.7% more pitch, and gets 12.5% more speed, but runs 24% less rpm at WOT (13.6% less 5500/4840)  

The smaller prop just doesn't load up the motor. Speed at 5500rpm was about the same as 6000rpm.  

I'll probably buy the 7.4X5.7 aluminum and try to bend and cup to something between 6 and 7 pitch.  

My goal is to get something around 5500 rpm and suck as much power out of this 3.5hp as I can 

 

Advise?? 

 

 

Went through similar with both a 6hp and a 9.9hp 4-stroke on a larger boat, and in both cases, erred to the side of higher rpms, ideally ‘in range’, over speed. Maybe custom (repitch) the smaller diameter prop but with a 6.5” or similar pitch and try to balance the best of both (speed, rpms)?

The smaller prop is a cheap ebay item, so low risk to work on it.  Just doesn't seem to be the right plastic material to repitch.  I'm looking for something that will get up into the proper rpm range without bumping the rev limiter.  

With a lighter load (one person) the bigger prop might rev a little higher but usually run as 2 two person adventure boat

 

Any room to play with engine height on that thing, or is it sitting where it needs to be already (waterline vs cavitation plate)? Might be able to block it up an inch or two depending...

Being a 'Pontoon' hull there's plenty of water to the motor.  I have it raised 3/4" and don't have room on the transom to go higher.  Making a riser is not out of the question and I might do that in the future.  

Went ahead and ordered another cheap plastic prop just in case my Re-Pitch didn't turn out.  

Took a 7.25X6 and got it to something close to 6.5 using a heat gun. Each blade looks to be even.  

The blade tips were originally about 1/4" inches above the hub center.

Now the hub is about 3/8" above blade tip contact.  

 

It will be a few days before I can test my work.  

 

 

Slip seems high for this prop design.

The two data points I have, Heavy Load?, 

Speed Vs pitch
7.25X6, 4.8mph@5500rpm (part throttle) 
5500/2.18=2523rpm = 14.33mph
4.8/14.33=0.335
 

7.4X7, 5.4mph@4860rpm, WOT
4860/2.18=2229rpm = 14.77mph
5.4/14.77=0.366

 

Got a chance to take the Bass Raider out in the river and test the re-pitch.  

Ran up and down river with an average of 5.6mph (two people). 

Swapped to the factory prop and 5.4mph again.  

 

The tachometer was all fogged up and I could not make out the rpms other than the smaller diameter stayed at 4 digits.  Did not go over 10000 (5000x2) so it loaded up the motor better.  

 

Raised the motor just a little more with a thicker wood shim.

I hope "over 10,000 rpm" is a typo or math error.  I would think 5,500 or maybe 6,000 would be the max for that motor. 

Sorry, thought I had mentioned that this Tach always seems to revert to 1P1R mode.  Why?

10,000 displayed = 5000rpm, I think.  

The small prop before re-pitch would rev to the limiter, display 12000, but only get to 4.8mph.

The factory prop got to 96xx displayed, about 48xxrpm and took us up/down river at an average of 5.4mph. 

Now, the re-pitch is faster and the motor doesn't over rev.  

I'll dry out the tach and try to get rpm reading from the 7.25X6, re-pitched to 6.5 to 6.7. 

Being plastic it will probably relax a little.