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Badbubba88

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Hello all,

I'm running a 1986 15 hp 2 stroke Evenrude on a 1972 Alumacraft deep-V at W.O.T. i'm getting 18 MPH an 5,450 RPM using a Aluminum Solas 9 1/4 x 10" prop 13 spline. Looking to get the RPM in the 6500 range and wanting a little help as this is the first J-Boat project. Was told after research the rpm range is 5,500 to 7,000 and ideal range for max hp would be 6,250. i need more speed to get on plain with 2 people maxed out at 12 mph and i'm already over propped so what is me best option for pitch to correct my situation. With 2 ppl hull and 2 batteries trolling motor plywood decking and gear I'm at approximately 700 lbs including the 15 hp motor. where as just myself in the boat with same setup I'm getting 18-19 mph. Help
 

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That's a hefty load for a 15. A lot of times rpm is down a bit because the boat isn't planing well. Have you considered a hydrofoil? They give a lot of stern lift and help pop up on plane and since it's up on top skipping along, the rpms come up a bit too. I've had one on every 14' boat I've owned.

Sent from my CLT-L04 using Tapatalk
 
I forgot to mention it has a hydrofoil bolted on and is 52 inches wide. I'm more or less wanting to insure which direction in prop I should go. Only upgrade i could see working is a SS 9X lower pitch just dont want to get into a large prop collection before finding one that will work best. Will try that site and see what they recomend and update. And yes I agree the second person really puts a damper on getting this old boat going.
 
Prop wizard suggests an aluminum 9x9 but really wont help much considering I'd only gain about 200 rpm at wot. Starting to think a SS 9X8 OR 9X7 is my only option. Or am I wrong in thinking 200 gain per inch dropped in pitch?
 
For a point of reference, I have an 2017 F25.
I went from a 11-1/4 pitch to a 10-1/2 pitch
(both Yamaha OEM) & gained a bit more
than 700 RPM. Here's the thread:

https://www.tinboats.net/forum/viewtopic.php?f=3&t=44985
 
I appreciate that link to your other thread. Unfortunately it did not give me a sure rout to go. Being the hp different and two stroke vs 4 stroke
 
Yeah, it's not a direct correlation, similar (as in smaller HP) though.
Based on my experience, I'd expect you to gain more than 200 RPM per inch of pitch change.

(Nice looking ride btw)
 
I agree... I'm betting on 300 to 400 per inch and an drop of 200 rpm going from a aluminum to a SS prop. So a gain of 1200 rpm and a loss for switching prop materials making a total gain of 1000 rpm. I'm just wanting to verify my thoughts to insure I'm not going to make a mistake in selection
 
Badbubba88 said:
I appreciate that link to your other thread. Unfortunately it did not give me a sure rout to go. Being the hp different and two stroke vs 4 stroke


Here’s my experience with a mid 90s Evenrude 15hp 2stroke. Pushing a heavy load in a 14’ boat I was lugging the motor with the stock #10 prop. I dropped from a ten to an 8 thinking a 9 wouldn’t be enough. The 8 was too much. Went to a 9 and it was just right. I gained 750 rpm with a one inch drop from 10 to 9.


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awesome this is the info i was more so needing. i just placed an order for a SS 9 1/4 x 9 and will update after the test run. thank you all for the input it has helped me in many categories.
 
I'm gonna follow this thread. I have a '80 15hp Johnson on a 14' v hull. Your load #'s are very close to where I am with 2ppl and gear. I have the stock 9 1/4 X 10 and can get 19.3kts solo but only 14.7kts with a 200lb passenger. I have another stock prop that I can take to a local prop shop and have it pitched down to a 9 1/4 X 9. If that works better I'll invest in a 9 X 9 4 blade. I need holeshot over top speed. I fish alot in San Diego Bay and it has a lot of traffic and no speed limit unless posted. Lots of LARGE Navy ships, inconsiderate yachties (yeah lets run WFO before we have to slow once out in the ocean) and the real wake generators - Tugboats. You have to be ultra aware there because the wakes can be large and be ready to light the motor and deal with them.
 
I've placed my order and wont get the new prop till the 2nd-7th do to shipping. I'll have rpm gain, speed, and whole shot recorded once installed. Really hoping this is a one shot thing instead of having to drop down to a 8 pitch.
 
kdgrills said:
Yeah, it's not a direct correlation, similar (as in smaller HP) though.
Based on my experience, I'd expect you to gain more than 200 RPM per inch of pitch change.

(Nice looking ride btw)


Thanks :D
 
UPDATE

Installed the 9.25 x 9 pitch SS prop and gained right around 750 Rpm and went from 18-19 mph to 21 at 6,250 rpm putting me at peak hp. Ran into a super cool angler at the ramp as I was going to leave and had him jump in to see what it would do with a second person... was at 12 mph and gained 7 puting me at 19 and I'm just blown away with the results...... thank you to everyone for all the input and guidance. To top it all off a was able to treat the guy by putting him on a bunch of snook at my local spot and managed to pull in a nice juvenile tarpon. Cant wait to see if I can get a little more out of this old boat buy playing with the trim now that I have the Jack plate. Tight lines everyone!
 

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