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Discussion Starter · #1 ·
Need some help from all you boat motor heads. Have new power on my 86 Trophy 20ft 140 suzuki in place of the 125 force. What are your thoughts about the RPM. I am on my 3rd prop trying to plane under 4600 rpm. The top speed rpm 5900 is still ok, red line is 6200, but all the on line data is that 4000 gives the best mpg. The big question. Is it better to run this 4 stroke at higher rpm or pitch it up to bring the rpm down and bring the top end rpm to the lower range? What is best for the motor. Gas is still cheaper than a new motor. How long can I use the "I'm going out to test the prop"?
 

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I would say prop it to get it down to the 4000 range which you say
is best for MPG. The four stroke should have plenty of torque to
handle the extra pitch. Heck, I've seen several of the new Parker
21 sport cabins with 150 HP four stroke power, which I would have
never thought to be adequate, yet they seem to run fine. Your boat
is much lighter and should scoot along with that 140.

This may be a dumb comment, but also be sure you are ideally
timmed when you are doing your testing. You have not mentioned any
of the pitches you have tried, but I would guess a 17 or 19.
 

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I also wanted to add the importance of the prop diameter. For
example, my prop is the Yamaha saltwater series, which has
a larger diameter than the regular stainless prop. It's a 17
pitch and I have plenty of power, yet max WOT RPM in flat
conditions with a light load is about 5300-5400. You would
think that a 19 would be more of a match for my setup, but
because of the prop design the 17 is the ticket. Since you
have a four blade, which I don't know much about, there a
probably even more variables.
 

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You want to prop for the maximum or close to the engine's RPM at WOT. Load your boat normally like you will run. Then if it doesn't plane at low speed add trim tabs or a hydrofoil.
 

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I agree with Georgia Peach - DO NOT over prop your motor trying to get the boat to plane at 4000 rpm. If WOT is close to the recommended max, leave the prop pitch where it is. Not planing at 4000 rpm is your boat's way of telling you it needs more power to plane it than the boats in the tests your read.

If you check around you may find that your new motor has a good reputation for reliability and efficiency, but is considered to only put out about 125 - 130 HP. (Why the motor companies can get away with outright lies about motor weight and power I don't know)

By some 12" x 12" trim tabs. That will inprove economy without hurting your motor.

Big_E
 

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Discussion Starter · #7 ·
Thanks guys for the help. The tests are over. We added a double cup and just lost rpm at WOT and just a small drop in the plane rpm. The shock is gone. The word from the motor heads are that the motor will run forever at 5000 and is much better for the motor than to lug it down. Now fish look out I'm on the sea again. over.
 

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I couldn't agree more with the above post: always prop your boat to achieve very close to your max rpms at the load which you will normally run. Incidentally, a few years ago I tried a number of 4 bladed SS props and always went back to the three bladed design as it maxed out fuel economy with little performance sacrifice.

Assuming that you don't already have trim tabs, ADD them. TTs allow you to plane at a lower rpm (~500) and to save fuel in the process. Installation is a good DIY project and the tabs aren't that expensive (about as much as a SS prop).

I had great luck with Bennettw as have many on this board. Good company, great service. Incidentally, for your size boat I'd go with the smallest tab size. I had a 22' boat that had a 12x9 and the boat could overreact to small changes in tab settings if you weren't experienced in their use.

John
 
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