Repower

Y

Y-not

Cadet
Joined
May 28, 2008
Messages
18
I am going to repower my boat. I currently have a 70 hp with a 13 1/2x17 prop and wop is 5300 rpm. I will have a 90 hp, what size prop is needed? Both motors are Johnsons. The boat is a 19 ft Carolina Skiff.
 
Irv964

Irv964

Petty Officer 1st Class
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May 3, 2008
Messages
315
Re: Repower

Y-not said:
I am going to repower my boat. I currently have a 70 hp with a 13 1/2x17 prop and wop is 5300 rpm. I will have a 90 hp, what size prop is needed? Both motors are Johnsons. The boat is a 19 ft Carolina Skiff.
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2 stroke or 4 stroke? What are you after, holeshot or top speed? Plan on running AL. or SS?
 
Y

Y-not

Cadet
Joined
May 28, 2008
Messages
18
Re: Repower

My motor is 2stroke, I am not really too concerned about top speed but would like to keep my rpms at the correct level.
 
Silvertip

Silvertip

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Sep 22, 2003
Messages
28,762
Re: Repower

Don't forget that gear ratio plays into the prop equation. Assume two identical 90 HP engines, one with a 2.0 gear set and the other with 2.42. Also assuming both boats were identical, which one would you expect to be faster if both had 19P props? Then there is the issue of weight. The V4 90 is heavier than the 70 3-cylinder. It may turn out you can twist a prop with more pitch -- but I would still start with a 19P.
 
guy74

guy74

Senior Chief Petty Officer
Joined
Jul 12, 2007
Messages
794
Re: Repower

I would start with the 17" pitch prop you have then go from there with what you will learn from that. I run a 13.5X17 on a 90hp Evinrude on the back of a Glastron GX150, with a WOT of 5300.
 
S

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
Re: Repower

When starting blind with a new prop it makes sense to keep the pich toward the low side.If it ends up to light you can still run it and just stay within the rpm range by watchinbg the throttle,at least you should have good hole shot.. If you end up over propped your pretty much done except for a brief test and maybe some really light cruising.
 
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Dhadley

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Feb 4, 2001
Messages
16,978
Re: Repower

Experience tells us you're going to end up with a 19" prop once the V4 is set up correctly and with an average load for that type hull. Guessing tells us that the 70 wasn't set up right or it would have been turning more rpm with a 17.
 
guy74

guy74

Senior Chief Petty Officer
Joined
Jul 12, 2007
Messages
794
Re: Repower

Dhadley said:
Experience tells us you're going to end up with a 19" prop once the V4 is set up correctly and with an average load for that type hull. Guessing tells us that the 70 wasn't set up right or it would have been turning more rpm with a 17.
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It would be nice to know where he boats and what he's looking to do with the boat, set up correctly is kind of a relative term. What works at sea level for a person looking for top speed, doesn't work well at 2000ft for a person towing skiiers and doing other watersports activities. Around here most boats around that size with a 90hp motor run a 13.5X17, I won't say they are set up correctly, but it seems like a good place to start. Also the OP doesn't state the modle years of the 2 motors, so we don't know if they are flywheel or propshaft rated. Just my $.02
 
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