what prop size and pitch

[Jeff Walkowiak]

1998 evinrude 50 hp on a 16 foot ponton boat,, current prop is only 12.5 by 13,, rpm at wide open is around 5700,, there is 1/2 in clearance from the end of the blades to the cavitation plate so it seems like a 13.5 blade size would be all that would fit before hitting the engine. Thanks

 

[steelespike]

Re: what prop size and pitch

I'm not sure what your after??Your rpm seems fine do you have speed figures?
Nornally you select a prop by pitch;the diameter follows design.A prop intended for your motor should physically fit.If diameter is too big it is probably intended for a larger motor.

 

[Jeff Walkowiak]

Re: what prop size and pitch

I was looking for the best all around prop for that engine on a pontoon boat , having a serious issue with weeds causing cavitation above 3500 rpm, we seem to have a lot of sea grass free floating in the water this time of year,, it causes the prop to cavitate constantly and makes going any place a real pain in the butt, I made some weed deflectors for the lower unit that work up to 3500 rpm as I said but not over that, before I did those the weeds would collect at any rpm and I wonder if a 4 blade would be better or a slightly bigger pitch would maybe work better,, +1 inch would reduce the rpms to around 5500 and that would be acceptable for this engine, Also still working on a better deflector , if I can get it to 4500 rpm without fouling the prop I will be satisfied with that as I would not be pegging the engine and the boat would seem like it is making some progress.

 

[steelespike]

Re: what prop size and pitch

Its possible a 4 blade would give more blade area so not as easily fouled.
might also operate at slightly lower rpm.
or would it catch even more weeds.I wonder if a stainless prop would chop the weeds some? Are you saying the deflector is intruducing ventilation at about 3,500 rpm?I wonder if placing it farther forward might help?
There was a post years back where a guy put a potatoe fork in front of the gearcase to protect it from rocks he seemed surprised that it caused venting problems.

 

[Jeff Walkowiak]

Re: what prop size and pitch

the deflectors are not causing any problems at all but they don't seem to do any good over 3500 rpm,, in areas where there is no sea grass floating the boat performs perfectly with the deflectors no issues there,, however we have sea grass that is about 1 foot long and 3/8ths of an inch wide that is stronger than heck,, it fouls the prop, most likely due to the pontoons pushing the weeds toward the center of the boat unlike a conventional boat that pushes them out and away from the prop. for some reason it doesn't seem to take many weeds for the engine to cavitate, it also seems to get worse on an outgoing tide as it pulls the weeds from the grass flats out in to the channels, I need a second prop anyway so I just thought there may be a better one for this boat than the 12.5 by 13 that is already on it. perhaps a 4 blade with smaller blades but more pitch per blade would be less likely to foul .

 

[steelespike]

Re: what prop size and pitch

What you have is techically ventilation; cavitation is a different animal.
A stainless prop resists venting may take more before fouling causes ventilation.
I would think that the fouling problem is more about speed than rpm.
also if you lower the motor it might resist venting better.

 

[Jeff Walkowiak]

Re: what prop size and pitch

I had the prop 3.5 inches lower than it is now and it did the same thing,, I believe the engine is actually still about 1.5 inches lower than it should be but it all comes down to weeds,, in clear water with no obvious weeds it works just fine, when weeds are present it will cavitate the minute I drive over them,, and this isn't a lot of weeds I am talking about,, I can run my p w c's in the same water with the same weeds suspended in them without any problems and we all know a pwc will foul and cavitate pretty easily, a lot quicker than a prop would.
I have tried different angles with the trim and it makes no difference, sharpened the leading edges of the prop to the point I cut my hand on it, still made no difference.
If I can get behind a regular boat and follow his wake I can go for a while without fouling the prop but eventually the regular boat leaves me way behind and the weeds work back to the trail and get in the prop eventually..
Boating most of my life and never had a issue with weeds like this .

 

[steelespike]

Re: what prop size and pitch

Have you talked with other pontooners?There is no doubt the pontoon is sensaitive to ventilation there is so much turbulance it doesn't take much to set them off.My 18ft with a 88 50hp Evinrude notch 2 almost no venting notch 3 is faster but venting can be a pain.I haven't played with it much
so don't know its best performance.

 

[Jeff Walkowiak]

Re: what prop size and pitch

well it ticked me off so I just finished a second weed deflector attached to the bottom of the engine well directly in line with the lower unit of the engine, this should deflect weeds, dolphins and the occasional submarine.

 

[Jeff Walkowiak]

Re: what prop size and pitch

tried the new deflector yesterday,, works pretty well, I ran at 5000 rpm and deliberately aimed at weeds floating in mass,, a lot more than I would usually encounter, the engine would jump 500 rpm when plowing through the weeds but not bad enough to make me stop and put the engine in reverse, to remove them , staying at 4500 rpm I had no issues with weeds at all.
Nice not to have to continually stop to remove weeds all day, actually never had to stop even once to remove weeds.

 

[steelespike]

Re: what prop size and pitch

Sounds like you may have a cure to market.
Bump

 

[twg67]

Re: what prop size and pitch

Do you have any pic's of your weed deflector as I am having a similar problem.
Thanks

 

[Jeff Walkowiak]

Re: what prop size and pitch

problem solved,, went with a 15 inch pitch 12.25 diameter 3 blade prop,, now top speed rpm is 4600 ,,top speed increased by 1 mph, weeds will sometimes bring the prop up a couple hundred rpm but seem to clear themselves pretty fast and the engine sounds a lot better using this prop,, the w o t operating range for that engine was listed at 4500 to 5500 rpm, so now I am in the lower end of the range . nice not to have to keep stopping to remove weeds.