pontoon prop pitch

[OnEyeJack]

I re-powered my 27 foot, two tube pontoon boat 4,500 lbs. The Mercury 50 HP 4 stroke was said to have come off a similar boat.
it appears to be about a 10 inch diameter and is a 12 pitch. Anything slightly over idle and the boat cavitates, so I guess can not go faster than 10 mph. Mercury prop calculator says "no result" others say 12 inch 8 pitch.
Can anyone comment on what prop they are using on similar conditions ? I don't want to spend $300 experimenting.
Given enough time shouldn't any prop be able to reach "say" 3000 rpm ?

i-Jack

 

[ThomW]

What motor was on it before? It seems that a 50hp on a 4,500lb, 27 foot boat is moderately under powered. What is the part number on the prop you have? Should be stamped on side of prop and on the end where lock nut goes. That will let us know for sure what you have on it now.

 

[Scott Danforth]

your dealing with a 'toon.

get the motor height correct before worrying about the prop.

are you ventilating or cavitating. ventilation is pulling air from the surface, cavitation is creating air bubbles by shearing the water molecules.

99% of the time its ventilation.

load your boat like you normally run it, look at the AV plate. are you drawing a vortex? if so, drop the motor a bit. toon's need a powered jack plate to adjust for conditions much more than bass boats do.

'toons are different than v-hull boats so the generic prop calculators dont work.

 

[ahicks]

A 10" diameter prop on a 50 hp engine doesn't sound right to me at all. Not on a 'toon anyway.

Is there a chance you have reversed the diameter and pitch dimensions? Could that 10x12 prop you say you have really be a 12x10? If so, that would make MUCH more sense, and possibly not that far off from being the correct prop.

If it really is a 10x12, it could be your issue as it likely doesn't have near the blade area required to hold that 50 hp engine with that kind of load on it.

"Ventilating" usually get tracked down to correct prop and engine height, though occasionally we do hear about something (e.g. tree branch or trash) caught on something in front of the engine, screwing up the water flow getting to the prop.

 

[OnEyeJack]

Thom : previous was a 1968 Johnson 40 hp, with a 2 blade prop. Boat went about 20 MPH then (twice as fast).

Scott : The previous motor would come out of the water going over waves, but then dig right in again. The new motor is mounted as far down as it can without modifying the transom / well. So new motor about 3 inches lower than old, stock cavitation plate well below water line both still and under way; however there is considerable splashing off housing just above stock cavitation plate.

ahicks ; Somewhat guessing on the prop diameter as it is three blades and was in the water, but the 12 pitch is stamped right in the hub. The rest of the part number is faint.

So I'm wondering what other people are running for pitch under similar conditions. And if a cavitation fin thing would help this low performance application or are they somewhat gimmick ?

i

 

[ahicks]

Pitch on similar boats will be in the 8"-12" range.

If you're 3" lower than original, I'd have a closer look at that prop.

 

[WesNewell]

Assuming it's not a command thrust engine, prop selection would be limited to a 13 spline prop. What you should probably be running is an 8-10P prop. And that's not near enough engine for a 4500# boat of any kind, and there's no way a 40hp would push it 20 mph that I know of, unless it had been modified, and I doubt it then. So somethings not right. I wouldn't expect more than 15 mph out of that engine assuming it's in good shape and mounted correctly. And yes, I can see it only getting a max of 3000 rpm on a 4500# toon with a 12P prop. If the engine is in good shape, first thing I'd do is remove the restricter plate to increase the hp to 60 hp. Then I'd change the prop to an 8-10P prop with an 8 being my first choice. Your better option would be to get a bigger engine.

 

[OnEyeJack]

Thanks Wes ;
I don't think its a command thrust, and I don't think its a Big Foot. It may be under powered but it is a second boat just used for putting around when I have too many people. I would like to be able to run the motor at full throttle - as I stated earlier right now it cavitates just above idle. (jeese I wonder if the hub is slipping ) Also I think it's bad for the motor to not blow it out once in a while.

Also I apologize; there is no speedometer but I can tell you the old motor went way faster and towels etc would actually blow around. This set up cant even out run the flies . The 4500 lbs is what I calculated by the displacement of the pontoons in the water, and it does seam much heaver then it looks when I move it around on land. But I have never actually weighed it. Aprox late 80's Sylivan.



Not sure why I cant insert .jpg but similar to these, pretty common to be around 40 to 90 HP.

https://www.google.com/search?q=pon...h=833#imgrc=hRA6ezMMfoLz5M:&spf=1531943367675

https://www.google.com/search?q=pon...h=833#imgrc=p6C43Gzss4fBFM:&spf=1531943367675

 

[WesNewell]

A new Lowe 27'11" toon only weighs 3281# and it's probably heavier than the one you have. And it's rated for a max 300 hp engine. So a little 50 hp engine on the 27' one you have is way way under powered. Try and find an 8P prop for it. And I'm not sure what rpm's you'll get with that. Probably still under the wot throttle range. If you can find a 75hp 4S in good shape, and remove the restricter on it to get it up to a 90 hp, then you could probably hit 20 mph. FYI, with a 300HP engine that new lowe toon with 1000# load is only rated for ~27 mph. Toons aren't fast.

 

[ahicks]

A 40 or 50hp engine on a 27' 'toon may be considered "under powered" in some circles. Others though, including the one I reside in, see them every day and they're doing just fine. You can load them up until the pontoons are barely visible if you care to, and they are STILL able to navigate from one end of the lake to the other over and over, for years on end. You NEVER used to see big power like you see on some of today's boats until recently. A 25hp engine used to do a fine job!

No, it's not going to do well pulling anything, but then that's not a requirement for many boat owners. Some might even have a dedicated ski/wake boat tied up on the other side of the dock for instance?

 

[Sea Rider]

You can run a underpowered OB on almost any boat except liners provided that are running at least middle to max wot rpm range factory stated, doesnt mean that will have a speed demon, probably will only push at just fast displacement speed if heavily loaded. Anyway, it's much fun than rowing anything else that floats on water.

If there's water splashes on front of lower leg on flat calm water cond while toon rides parallel to water level with OB trimmed to neutral position. OB sits high. The best lower leg spot is when at fast displacemen speed or higher speed water flow skims right under small upper water deflector plate and front of vertical sharp edge.

Most OB's correctly height seated will have prop aeration issues when encountering waves, high tumbles there's nothing you can do to avoid that cond, just dance along with boat up and down with minimum throttle to lessen this issue and avoid passengers yelling at you for given them a free dizzy water trip LOL!!

If you can lower an OB to a point that will stop prop aeration while dealing with waves, high tumbles, expect to shower yourself at back transom when combo rides at fast displacement speed on flat calm water cond.

Just prop OB right with whichever 3-4 blade you like to run full wot revs, toon will be on its way much faster if plan boating constantly heavily loaded as stated.

Happy Boating

 

[WesNewell]

My prop choices for your 50hp (or 60) on your toon.
Aluminum- 1311-122-08 Solas Amita prop, under $100
Stainless- 1331-120-08 Solas New Saturn, under $200