19p to 17p how many rpms

A

Arcticycle

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Aug 14, 2011
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I put a Solas alum 19x14.25 4 blade on the boat doesn't plane any better than my 21p 3 blade boat will only hit 4100 rpm do you think a 17 same style would put me in my 4400 to 4800 range what would you try next 17 or maybe lower i want it to tow and ski care les about top speed (2150 Capri 5.0 efi alpha at about 6000ft)
 
J

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: 19p to 17p how many rpms

Arcticycle said:
I put a Solas alum 19x14.25 4 blade on the boat doesn't plane any better than my 21p 3 blade boat will only hit 4100 rpm do you think a 17 same style would put me in my 4400 to 4800 range what would you try next 17 or maybe lower i want it to tow and ski care les about top speed (2150 Capri 5.0 efi alpha at about 6000ft)
Click to expand...

Welcome to iBoats.

Assuming the 21P you replaced was a Mercury alum prop, surprised you did not get any change. I would not expect the same wot RPMs. What was the top speed of each? I am trying the same solas this summer from a 14.25x21x3 Black Max Alum. RPMs did change, but it was even noticable in "seat of the pants", ie quicker acceleration.

I suspect you are fighting a drive ratio issue at that elevation. Do you know the drive ratio? The general rule of thumb, is 300-400 rpm with 2" of pitch. So a 17P solas would give you 4400-4500, and a 15P 4700-4800. The latter would be prefered for skiing.
 
A

Arcticycle

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Re: 19p to 17p how many rpms

Thanks john great site! The I never payed attention to the stock setup until this site so I tryed to monitor all this last weekend and my speedo works then it quits the all I noticed was little better holeshot and better handing I would say 37 mph top speed I'm getting a gps next trip ill check the gearing also
 
L

Leardriver

Petty Officer 1st Class
Joined
Oct 7, 2008
Messages
373
Re: 19p to 17p how many rpms

At 6,000 feet in Denver you can go from a 21 pitch all the way to a 15 pitch 4 blade or a 17 pitch 3 blade.

The elevation is a performance killer.
 
J

John_S

Rear Admiral
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Jun 21, 2004
Messages
4,269
Re: 19p to 17p how many rpms

Report back with the data. Also, year/sn of your motor. At this point, I don't know if it is a more modern 260hp, or older 220hp injected motor.
 
A

Arcticycle

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Re: 19p to 17p how many rpms

the boat is a 2001 240 hp fuel injected and the gear ratio is 1.47 is that a lower ratio or higher?
 
J

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: 19p to 17p how many rpms

Arcticycle said:
the boat is a 2001 240 hp fuel injected and the gear ratio is 1.47 is that a lower ratio or higher?
Click to expand...

That would be the standard ratio that comes with that motor. For high altitude, a 1.62 ratio would typically be used. The ratios are a little deceiving. For the 1.5 ratio, your engine turns 1.5 revolutions to make one revolution of the prop. A 1.62 requires 1.62 engine revolutions to turn one prop revolution. So the higher numeric ratio, the lower the actual gear ratio.

That explains why you have to drop into the lower end of pitch on the prop. Save your old prop incase you ever go down to low elevations. Once you recheck your performance data, can go from there.
 
A

Arcticycle

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Aug 14, 2011
Messages
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Re: 19p to 17p how many rpms

Well I was a little higher elevation than I thought 7700ft so I'm goin to order a 15p and go from there thank you iboat members
 
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