Prop confusion!

[cheaperts]

I have a 20' Lowe pontoon (2 logs) with an 89 40 hp Johnson on it. Motor runs well but is getting kinda tired, I want to repower it with a 70 hp looper but till the right one comes along we will putt along with the 40 horse.

A week ago we spun the old prop that was on it, it was a 3 blade Michigan 12.5x13p. Boat does not have a tach but I have never felt it was getting 5000 rpm, boat on a good day would get 12 mph on gps, usually around 10 mph.

After researching here and elsewhere I decided that I would do better with a 4 blade and less pitch. I found a bargain on ebay for an 11.5X10p. Put it on tonight and took a test drive, boat actually seems much smoother accelerating, but top end is less and f I open it full throttle the engine actually has surge, never did that with the bigger prop. If I pull the throttle back slightly it runs smooth but we are at 8-9 on the gps. Does the engine have a rev limiter that would cause it to surge? Any better recommendation on prop size/pitch/blade count?

Advice anyone?

 

[Old Screwball]

Re: Prop confusion!

If the surging is your rev limited on the motor, the prop is pitched too low. You went from a 13 pitch to a 10. Good Luck.

 

[Silver Eagle]

Re: Prop confusion!

When I had my 21ft bow rider I tried a four blade prop The motor was a 5.7 I/O The motor seems to lug on getting out of the hole. It took longer to get on plane and my top end wasn't as good as the three blade prop. It took it off and just keep it as a spare.

 

[Jeep Man]

Re: Prop confusion!

A 4 blade prop will not give you a speed increase. It SHOULD give you better hole shot. If you had a 13p 3 blade previously a 12p 4 blade would give you an equivalent rpm. Without a tach, having a number of props of different pitch to try is the only way to determine which one is right for your motor. BTW, a pontoon prop usually has a wider blade to help push the bigger load as opposed to a conventional hull. If you are considering a motor change, look for one designed for a pontoon which will have heavier, lower ratio gears and a larger pontoon prop.

 

[steelespike]

Re: Prop confusion!

Numbers with the 13' pitch appears to be reasonable though rpm is really low.
Guessing 25% slip and 12 mph , 2.42 ratio could be about 3400 rpm
Of course without accurate rpm web real;y can't tell anything.
With the 10" 9 mph at 25% slip 2.42 ratio nets 3000 rpm so really not making sense.
If we guess 5000 rpm 25% slip 2.42 ratio 10" we get 14 mph.25% slip is a reasonable slip for a pontoon.
Without an accurate tach we really can't tell anything.You may be able to borrow a shop tach.
With an ideal setup a healthy 40 could produce speeds approaching 20 on a lightly loaded pontoon.
If we assume your speed is right it would appear either your motor is not healthy not reaching a reasonable rpm or your slip is very high causing the motor to over rev from the slip.

 

[BigDfromTN]

Re: Prop confusion!

Assuming you don't want to spend a lot of money on a prop for an engine you may be taking off the boat....

Do you have a prop shop near you? If so, ask them if they can add some cup to the blades which will effectively increase the pitch a little.

Just a thought.