Next size? Johnson 115

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egscharf

Seaman Apprentice
Joined
Dec 2, 2012
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30
I have a seaswirl 1850 wa with a johnson 115 SPL. Was getting 27mph at 4400 rpm with a 13.75 x 19 prop. I switched to a 13.25 x 15 and am now running in the correct operating range (now at 5000 rpm WOT) but the boat is slower, around 22 MPH. What should my next move be? Thinking maybe a 13.25 x 17. What do you think?
Thanks!
 
jestor68

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: Next size? Johnson 115

Your motor's WOT rpm range is 4500-5500 rpm. It's power peak is at 5000 rpm(115 hp @ 5000 rpm).

Using the 150-200 rpm per inch of pitch rule, you are in the range with a 600 rpm change for 4 inches of pitch change.

So based on what you are getting per pitch change, I'd expect that a 17 would drop about 300 rpm compared to the 15 and pick up 2-3 mph.

I should point out it's better to have the motor turning at the high end of the range with a light load, so that it will be able to still get up to speed with a heavier load.
 
Silvertip

Silvertip

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Sep 22, 2003
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28,762
Re: Next size? Johnson 115

I think you need to get a better speed reading -- such as with a handheld GPS. And make sure the engine is properly tuned and making full power. This combination doesn't sound right.
 
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egscharf

Seaman Apprentice
Joined
Dec 2, 2012
Messages
30
Re: Next size? Johnson 115

The data is gps. Remember, I dropped .50/inch diameter.
Silvertip said:
I think you need to get a better speed reading -- such as with a handheld GPS. And make sure the engine is properly tuned and making full power. This combination doesn't sound right.
Click to expand...
 
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steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
Re: Next size? Johnson 115

Assuming your Seaswirl is a 2000 it's 19 ft and weighs about 2350 dry weight. What year is your rig?
I'm thinking you should be running about a 15" prop but should be turning about 5500 rpm at about 35 mph. I think either one or more of your numbers are wrong or your setup isn't right or perhaps your power is down.
 
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