New pontoon owner question

medic7156

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Jun 20, 2011
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8
I just purchased a new to me 20' pontoon with a 40hp Mercury outboard. I believe the year is 1995, it's a 4 cylinder and the prop is pretty chewed up, I can't read any numbers. I should add I don't have a tach, unknown optimal RPM. What would be the diameter and a good pitch prop to buy as a replacement? Would a 4 blade be better than 3? Any help would be appreciated. Thanks.
 
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Georgesalmon

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Apr 14, 2012
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1,793
Take off your prop and look under the nut. Numbers are often stamped on the hub. I'm a fan of 4 blades for pontoons. I'm thinkin a low pitch such as 11 or 13 maybe. FWIW
 
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ahicks

Captain
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Sep 16, 2013
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3,957
Without knowing what rpm you're engine is turning, and what size prop is on it now, there is no way you're going to know if the engine has the optimal prop on it. Any change or suggestion will be a shot in the dark.
 

1983 ercoa 21'

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Jan 12, 2014
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A guess in the dark would be a four blade 9 pitch but as mentioned above without knowing your rpms at wot and current pitch it's hard to get you where you need to be?
 

lmuss53

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Sep 9, 2008
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My 20 footer with a 50 hp OMC is running a 13p 3 blade aluminum, wide open throttle runs at 5000 rpm plus a little. I would suggest an 11 or 13 for your boat, 13 if you run light or 11 if you load her down with people and gear. Do not expect this old girl to be a speed demon, 12 to 17 mph range depending on load. BTW the Tiny-tach type of tachs work well on small outboards, and don't cost a lot. Good luck and enjoy your new boat.

On low HP pontoons I use 3 blade aluminum props because they are cheap, and I don't think high dollar Stainless or 4 blade props make a significant difference in performance. but that's just one guys opinion.
 
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medic7156

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Jun 20, 2011
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Thanks for the info. What diameter does that motor take? I've slowed down from boats with IO's, not sure about a little 40hp outboard. Thanks again!
 

lmuss53

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Sep 9, 2008
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Get some information off the motor so you know the year and model, then search the boat sights and for props, there is a prop section at iboats that has your prop in it, Put up a couple pictures of the motor from side and rear, someone will be able to get you real close as to year and model. I know where to look on OMC motors not so much on Merc's. There is a stickie post at the top of the Merc motor section with FAQ's, I think finding your motor year is covered there. You want the biggest diameter 3 blade prop you can turn. Having said that I'll say if they had a 13 1/2 inch diameter prop for $90 and a 13 1/4 for $60 I'd buy the $60 prop,
 

Silvertip

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Sep 22, 2003
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28,755
Stop thinking "diameter" and take a step back. If you have a 4 cylinder merc the first thing you need to know is what the lower unit gear ratio is. To take advice on pitch from someone running a Johnson would likely be a mistake since the two engines may have different gear ratios. Then you need to make some wide open throttle runs to determine what the maximum observed engine rpm is. Make those runs with what you consider an average load. If the engine cannot reach at or near the top of the rpm band, you need a prop with less pitch. If the engine tends to over rev, you can use a prop with more pitch. You buy a prop for your specific engine. Diameter takes care of itself because as pitch changes, diameter will increase or decrease slightly. You should also shop for "pontoon specific" props which are easily identified by the very large rounded blades (commonly called elephant ears). They work better for navigating in reverse and provide optimum push.
 

Jeep Man

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Oct 17, 2008
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If it is a Bigfoot edition, your optimum prop is likely a 14x11. If it is not, then the diameter will be smaller. Again, if not, look for a wide blade (elephant ear) or have it cupped by your prop guy. A 4 blade will give you a little better holeshot and better reverse control but it WILLNOT give you a speed increase. Your pitch will be in the 9 to 11 range regardless and if you do want to go to a 4 blade, what ever your best 3 blade pitch is, drop it by 1 for a 4 blade prop. You need to match your prop with the engine WOT. A tach is crucial. A mismatched prop can seriously damage your motor.
 

Mi duckdown

Commander
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Apr 14, 2007
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2,575
Why not rebuild the prop you have and go from there. tach is necessary to answer prop questions. it is all guessing without tach, and exspensive swapping props. check out tiny tach.
 
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Silver Eagle

Master Chief Petty Officer
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Mar 16, 2010
Messages
852
Write down the serial number of the motor and take the prop to a Mercury dealer and just get the right prop.
 
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