Finding the Right Prop

[Sureshot]

Hello,
I have a 1986 14' Bayliner Capri that has a 2000 2-stroke 50 HP Johnson outboard on it. The boat weighs 700 lbs. The motor came off a pontoon, so the prop is probably not the right match for my boat. The current prop is an aluminum 3-blade 13" pitch / 12.5" diameter, part no. 775713. When I push the boat WOT, it reaches around 6000 RPM, which I think is actually over-revving since my manual indicates 5500 is the max. What I'd like to figure out is what pitch would be optimal for my boat. The top speed I seem to get is in the mid 20's MPH but I'd like to improve that if possible. Any help would be great, thanks.

 

[Sureshot]

Re: Finding the Right Prop

Bump

 

[louiefl]

Re: Finding the Right Prop

What else are you doing with the boat? A taller prop will affect acceleration, but since you are over revving the engine you could go to a 15" pitch. Could you go to a 17"? Not sure.

 

[Sureshot]

Re: Finding the Right Prop

Maybe pulling a tube. Mainly just cruising though. I was thinking 17 but is there any easy way to calculate exactly what prop pitch would get me around WOT 5500 RPM?

 

[louiefl]

Re: Finding the Right Prop

In 2000, thre were 2 different 2-stroke 50HP Johnsons, the regular one which had a wide open throttle range of 5000-6000 rpm and the 50 SPL which had a range of 4500-5500 rpm. The final drive ratio for both engines is 2.42:1. There are prop pitch vs engine rpm calculators on line (search this forum for some links), but according to my calculations, a 13" pitch prop, 2.42 drive ratio with 12% slip should get you to 26.9 mph at 6000 rpm.

A 15" pitch would be 31 mph at 6000 rpm and a 17" pitch would be 35.1 mph. At 5500 rpm you would get 28.4 and 32.2 mph for the same set of pitches. The limiting factor is horsepower, if you put too tall of a prop on the engine, it may not have enough power to get to speed and the engine will be lugging - not good for the long-term health of the engine (theoretically a 21" pitch will get you 43.4 mph at 6,000 rpm). The weight of the boat, hull design, number of passengers, etc will affect the ability of the engine to achieve you goal. Also, your boat may not be stable at high speeds, so be careful chasing speed.

Think about a car with a manual transmission. In first gear you might get 25 mph at redline and will accelerate rapidly. This is the prop you have now. Now try starting the car from a dead stop in second gear; the engine will bog down and inital acceleration might be poor, but eventually you will get up to maybe 35 mph. This is going up to the next taller prop. Now try starting in 5th gear, the engine will lug and acceleration (if you can get moving) will be horrible. This is what happens if you over-prop the motor - not enough power to accelerate.

http://www.piranha.com/enginelist.php?id=4576

The link shows prop choices for your engine. It looks like the recommended diameters range from 13" to 14" with pitch ranges from 11" to 21". you say your existing prop is 12.5", check the clearance around the prop to see if you could accomodate a 13-1/4" or 13-3/4" prop diameter. I've heard of prop retailers that will let you test drive props provided you put down a non-refundable deposit that would go towards the purchase of a prop. I'd try the 15" pitch and see if it meets your needs and gets your engine to 5,500 rpm. If you have no problem getting to 5,500 rpm, you might try a 17" pitch and see if you can get to at least 5,000 rpm and the boat is stable at that speed.

 

[Sureshot]

Re: Finding the Right Prop

Thanks Louie, great info. Sounds like there will be a little bit of trial and error determining 15 vs. 17 but the right fit will be one of those. Appreciate your input.

 

[jestor68]

Re: Finding the Right Prop

Or you can just use the iboats prop finder to select a 12.25x15 prop that will drop your WOT rpm by 300-400 rpm and gain some speed.

That looks like the best compromise for good all around performance, and a couple brands are under $100.

 

[Sureshot]

Re: Finding the Right Prop

I'll check that out as well Jestor, thank you.

 

[steelespike]

Re: Finding the Right Prop

I agree a 15" seems like a good bet should have plenty of pep yet stay within rpm rating.

Is the antiventplate(just above the prop) about even with the bottom of the boat?

 

[Sureshot]

Re: Finding the Right Prop

It sure is, within .5 inch maybe. So if increasing the prop by a couple of inches (13" to 15") will take me down 400 RPM and I'm 6000 RPM WOT, that would put me around 5600 WOT. My range is 4500 - 5500. Is that ok to be over just slightly?

 

[jestor68]

Re: Finding the Right Prop

It sure is, within .5 inch maybe. So if increasing the prop by a couple of inches (13" to 15") will take me down 400 RPM and I'm 6000 RPM WOT, that would put me around 5600 WOT. My range is 4500 - 5500. Is that ok to be over just slightly?

It's OK. When you put the family in there, it should be right there in the upper portion of the rpm range, with a little in reserve to pull a skier up.

 

[Sureshot]

Re: Finding the Right Prop

Thanks, appreciate all the input.