flat bottom motor question?

[gateway]

I have a 12' flat bottom jon, From the top of the transom to water level is around 10 to 12 inches.<br />The 4 hp johnson outboard measures from where it sits on the transom to the plate above the prop is 15". is 15" the right size shaft for this boat? because I still get cavitation at WOT.<br />Thanks,<br />no4njnk

 

[sho305]

Re: flat bottom motor question?

I have run them that way with no problem. Seem to go faster with it trimmed down some from level as my weight made the back of the boat sink more. Is the prop ok?

 

[Joe Reeves]

Re: flat bottom motor question?

No4njnk..... From the top of the transom to water level means nothing. The measurement is from the center top of the transom to the keel (bottom of the boat). Approx 15" = short shaft, approx 20" = long shaft. The cavitation plate (just above the prop) should be about 3/4" to 1" beneath the bottom of the boat. I suspect you have the wrong shaft length engine installed.

 

[gateway]

Re: flat bottom motor question?

Hey sho305,<br />I am 250 lbs and the boat sits a little lower than normal. But the prop is old but, is in great condition. Is there a differant pitch that could make it do any better? sorry, I dont know the pitch it is now. It's pretty steep though.

 

[gateway]

Re: flat bottom motor question?

Hey joe reeves,<br />You think I would need a 20" shaft on such a small boat?<br />no4njnk

 

[sho305]

Re: flat bottom motor question?

Oh, I see what you mean. I am thinking fast boats with the water coming off the bottom. You need to have the cav plate below the bottom a little like he said. Like if you looked at it from the side on a trailer down the bottom, and to the cav plate. Some little motors have another small plate on the front of the shaft a couple inches above the cav plate that works good lined up with/just above the bottom. It does not matter where the water is at rest, as the prop only uses the water from under the boat above a couple of mph.<br /><br />If the prop is not bent or got chunks gone it is ok. Normally they are never changed and the factory sets it up for what people will use it for; likely just what you are doing so it should be the right size/pitch. I am 200lb and I like a wider flat bottom, not the narrow one my wife likes. I like to stand in something wider than a canoe.

 

[gateway]

Re: flat bottom motor question?

Thanks sho305...I think I may need to try a differant prop because the motor came off of a much bigger boat, It was used as a trolling motor.<br />Dont you use a differant prop to push a bigger boat?<br />no4njnk

 

[sho305]

Re: flat bottom motor question?

It depends. You would use a lower pitch(lower gear) and maybe bigger diameter(grab more water if it fits on motor or more blades) for a bigger boat depending on what you can get for it. Most OBs have limited diameters to choose from. But trolling was likely slow or mid speed on this motor and it might not have mattered. They may have put a smaller prop on and then it would rev a lot at high speed, when not cavitating (of course) on your little boat. I doubt they propped it for max performance on the big boat just to troll with, as most people troll slow. Who wants to troll with a 4hp screaming away back there? The prop change would have little effect until the 4hp was run at around 3/4 throttle and above on the big boat.<br /><br />Call the dealer and ask what it should be. The size ought to be on the side of the prop or the hub end, like under the nut. If it was too small it would rev up and be at top speed quickly on your boat, and idle down to very slow speeds. Have to get the air out of there first to find out how it runs.

 

[steelespike]

Re: flat bottom motor question?

Are you sure it is cavitation?<br /> Might be a spun hub. A prop shop should be able to tell and fix or replace.<br /> Im betting it is the stock prop.