Next size? Johnson 115

[egscharf]

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]

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]

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.

 

[egscharf]

Re: Next size? Johnson 115

The data is gps. Remember, I dropped .50/inch diameter.

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.

 

[steelespike]

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.