Dialed in?

[Ccfbailey]

Would like some input from the experts. I have a Lund Rebel 1650 xl ss. 90hp merc 4stroke. Boat came with a 13.75x15p 3 blade prop. Dealer admittedly put the wrong prop on. 6200 rpm and 35mph/GPS. They sent me a 13x 17p 4blade Spitfire to try. Got out today and its like a different boat as you would expect. On plane in a blink of an eye and 42mph. 6050 rpm. I expected the rpm would be lower but I can't base that on much. Fuel tank was full but it was just me and my son (220lbs) plus a bit of gear. Very happy with the performance but will gladly accept recomandations.
Thanks

 

[Sea Rider]

Re: Dialed in?

Check wot min-max rpm factory stated for that engine. If achieving near max rpm at wot with last prop tested, experiencing good hole shot and speed, I would stay with it.

Happy Boating

 

[Ccfbailey]

Re: Dialed in?

Thanks Sea Rider.
Merc specs that motor at WOT at 5000 to 6000 rpm. Looked at test results from similar boats running my engine and they all used a 19p. Some 3 blade and some 4. After running this prop I'm definitely staying with a 4 blade. Just wondering wether to change size or not to get rpm down a bit. Should it bother me that I'm running a tick higher than 6000 at wot?

 

[Sea Rider]

Re: Dialed in?

50 rpm passing max rpm at wot is not big deal, rpm will decrease accordingly when boating with heavy champs or more loaded than before. Counting with a tach, throttle a bit less to be inside 6 K at wot if that worries U.

Happy Boating

 

[steelespike]

Re: Dialed in?

Assuming a 2.33 gear ratio the numbers don't quite add up.
Calculator says can't go 42 at 6050 rpm.
10% slip, a reasonable number, indicates 37.6 mph.
Gps should be accurate so maybe the tach should be reading higher.
What rpm and speed were these test motors running are they 2.33 gear ratio?

 

[jestor68]

Re: Dialed in?

Would like some input from the experts. I have a Lund Rebel 1650 xl ss. 90hp merc 4stroke. Boat came with a 13.75x15p 3 blade prop. Dealer admittedly put the wrong prop on. 6200 rpm and 35mph/GPS. They sent me a 13x 17p 4blade Spitfire to try. Got out today and its like a different boat as you would expect. On plane in a blink of an eye and 42mph. 6050 rpm. I expected the rpm would be lower but I can't base that on much. Fuel tank was full but it was just me and my son (220lbs) plus a bit of gear. Very happy with the performance but will gladly accept recomandations.
Thanks

Your rpm is as high as it is because you dropped .75" of diameter with the Spitfire. Considering that your 17 pitch Spitfire acts like a 18 pitch 3 blade, your slip is about 7%; not unrealistic for the Spitfire on a light boat.

Your Merc 90 could easily turn a 19 Spitfire at 5500-5600 rpm. Your motor is rated 90 hp @ 5500 rpm.

The Solas 4 blade 19 for that motor is a 12-1/2 diameter, which would turn right about 5600 rpm.

 

[Ccfbailey]

Re: Dialed in?

That's a yes to the 2.33 gears. I'm just using a GPS app on my iPhone so might not be accurate. It matched the speedo at 35 mph with the 15p and matches it at 42mph with the 19p. They could both be wrong...who knows haha.
Went back to the tests and 3 similar boats to mine with the 90hp all used 19p 3blade.
Lund impact 1675 42.4@5900 rpm. Laser 13.25x19 ss
Crestliner fish hawk 1650 44.3@6000 rpm 13x19 al
Crestliner super hawk 1600
43.5@ 5980. 13x19 al
All pretty similar results. Wish they tested with my prop though.
A friend has a 19p trophy 4 blade ss. Not sure the dia. Will try to borrow it and see what happens.

 

[jestor68]

Re: Dialed in?

The Trophy Plus is a 13.75 X 19, while the Spitfire is a 12.5 X 19.

I invite you to visit the Mercury prop selector and put in your numbers. You will see that it comes up with 19 pitches across the board for an rpm of 5500-5600; which is the ideal WOT rpm for that motor. It calculates with a 19 pitch, you should get 43.9 mph @ 5500 rpm.

While the 17 is good for having reserve power for carrying a heavier load, the 19 is better for your lighter load; proving once again that prop selection is a compromise. You can prop for light load, heavy load, or something in between.

 

[Ccfbailey]

Re: Dialed in?

Thanks for the input! I did that prop selector when I had the 15p 3blade on and it suggested the 17. Never thought to check it against this prop. I'm thinking I'll keep this prop and get a 19p trophy plus... Then I'll have a spare. Love the 4blade on the big water of the Great Lakes!!

 

[Ccfbailey]

Re: Dialed in?

Your rpm is as high as it is because you dropped .75" of diameter with the Spitfire. Considering that your 17 pitch Spitfire acts like a 18 pitch 3 blade, your slip is about 7%; not unrealistic for the Spitfire on a light boat.

Your Merc 90 could easily turn a 19 Spitfire at 5500-5600 rpm. Your motor is rated 90 hp @ 5500 rpm.

The Solas 4 blade 19 for that motor is a 12-1/2 diameter, which would turn right about 5600 rpm.

You mentioned my rpm is still high because of the diameter of the new spitfire. The Solas 4 blade 19 is 1/2" smaller than the 17p spitfire. Will the higher pitch drop the rpm?
Hope I'm not bothering you with this. I enjoy learning the technical aspects of this and trying different things.
Thanks

 

[Sea Rider]

Re: Dialed in?

It's always good to have at least a extra prop for different activities or if going light, heavy loaded, nobody runs at full wot all the time, but if needed your engine will run safely inside factory rpm parameters.

Happy Boating