Correct prop size on new boat

rpatton

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I bought a 2013 Bennington 20' pontoon boat with a 2013 Yamaha 90 4stroke on the back from my inlaws. How do I know if the prop is the correct pitch and size. When running at 4500rpm it runs and pulls great but if I bump it up to 5000 rpm or more it acts like it starts to cavatate. The rpm's go up and speed drops. Is this normal. I was on a mercury 60 4stroke this weekend and it ran 5500 and pulled hard and seemed to run faster then my 90. Any thoughts?
 

ondarvr

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The ventilation can be an issue, but there are a few thing's you can do to help eliminate it. Some props tend to ventilate more than others, so that's an option, but good props may cost you $300-$500. Lowering the motor one hole on the transom should help also, and is free with just a little labor mixed in.

​There are some other things that will help, but post a pic off the transom with the motor down so we can see how it lines up
 

Scott Danforth

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setting up a pontoon is difficult. the first step is get the motor height right for the weight you normally run. then worry about the prop pitch.

unfortunately unless you have a jack plate, that means half the time the prop is too deep if you raise the motor so it doesnt ventilate with only 1 or two people, or it ventilates if you normally have 10 people on board and you now only have 1 or 2

was the 60hp merc also on the back of a 21' pontoon with the same number of toon logs? was the merc a big-foot?
 

rpatton

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Scott, you mean I might be to deep? I was worried that he had spun the prop or flattened pitch. He was notorious for running into sandbars. I might ad that I drove this boat when it was new and I really do not remember this issue. Question Reilly is how does a dealer pick the correct prop for a new boat when the buyer picks the option of a bigger motor then what is factory installed on the boat? Am I correct that I should be able to push to wot and the prop will keep the motor within rpm spec without over rev like a traditional boat would be ? This one seems to start to over rev about 1/2 throttle and blow out trimmed all the way down deep in the water. To answer your question, yes the 60 was a big foot . Why do you ask?
 

rpatton

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Yep, that is what I plan on doing this weekend first thing . Thanks
 

rpatton

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I know I asked this before but am I right in that , I should be able to go full throttle and the rpm's should stay between 5-6000 rpm and be pushing the boat with no blowout . Correct?
 

Sea Rider

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Yep, firstly check if prop has already a shot hub, if OK, check OB/transom height, set it at the sweet height. OTHO if prop is found shot won't know if factort delivereed prop is ideal for you boating application or not, as there seems to be no data on that prop as to buy exact same pitch.

t's a must tach any prop factory delivered with any OB on your application, if found under or over pitched will need a prop maximization for OB to work at least middle to max wot revs as ususlly loaded. All factory delivered props installed on any OB are usually medium pitched and some slightly over pitched.

Yams wot range is 5000-5500 rpm, if prop is found in good working order, at 5000 wot revs OB is working on the lugging side, if so, will need to go for at least 1 less in pitch to pull wot revs towards middle or 2 less towards max wot range,

Happy Boating
 

rpatton

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Thanks sea rider, so when I go wot and the rpm jumps to 6000 I either have a bad hub , damaged prop ( there are some good dings and nicks in it ) or the wrong prop? So like I said before first thing is to mark the hub and see if it is moving
 

Sea Rider

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Hard to tell, seems hub is already shot....a hub slip test is required to determine so...


Hub Slip Test.JPG


Remove prop, mark a straight line or 2 dots as in pic, go for a short wot spin as loaded, remove prop and check if marks have moved away from each other, if so, hub is kaputt!! Need a new prop.

Happy Boating
 

rpatton

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Yep thanks. Will it work to mark the shaft and prop instead of removing the prop? Or do I need to remove?
 

Scott Danforth

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you may be too shallow (set for more people/weight) - you can hear when a prop ventilates which would indicate that you are not deep enough. you stated your inlaws hit sand bars, maybe he raised the motor in an attempt to prevent this

you may have a bad hub - do a prop hub test as Sea Rider indicates. this will tell you if your prop hub is ok or not and go from there

to answer your question on how does a pontoon manufacturer pick a prop.... they test. Bennington and other manufactures test a variety of motors to get some rough guidelines that they give to rigging dealers who also water test
 

rpatton

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Thanks for all the great help and I will take the advice this weekend.
 
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