10ft dinghy, 15hp johnson prop reccomendation

[nabiul]

What is the stock prop for a 1994 15HP johnson?
I bought this motor, had it out a couple of times and I think I spun the hub.

The prop is a 9.5x12.5. It performed great with just me in it @ about 425lbs total loading, was doing about 25mph, but with my friend @ about 650lbs loading the hole shot was terrible. It took forever and a lot of rocking back and forth to get on plane and finally when we were planing we hit about 21mph. Did this twice then went fishing at trolling speed; when we tried to plane again later the prop just kept slipping around mid throttle.

I'm looking at new props and there is a huge number of choices from 7 to 11" pitch.

 

[steelespike]

Is this an inflatable dingy?

 

[Sea Rider]

That 10 footer dingy whether inflatable or hard hull achieves more hull drag due to short size and too much boaters weight when trying to get out of the whole. Could go one less pitch than actual spun one, will have better hole shot with weight along slight more top end speed.

Happy Boating

 

[nabiul]

Is this an inflatable dingy?

Yes.

That 10 footer dingy whether inflatable or hard hull achieves more hull drag due to short size and too much boaters weight when trying to get out of the whole. Could go one less pitch than actual spun one, will have better hole shot with weight along slight more top end speed.

Happy Boating

One less is 11.5 which is higher than the top of the range available of 11; what about 7,8,9,10 those have to be made for a reason.

 

[steelespike]

You said the original prop was a 12.5 And it made 25 mph,Is that a guess?Because that calculates at about 5900 rpm.
Which is ok for a 15. but as you reduce pitch the rpm goes up.And on a relatively small motor rpm goes up easier.
It will improve heavy performance but you will need to watch you don't over rev.I wouldn't go any less than a 11".
Is it an inflatable? A ridged hull can be forced beyond their intended speed.

 

[nabiul]

40km/h ~ 25mph as measured by a car gps unit, they aren't that accurate but close enough. 5900rpm! Yeah a 15 was probably not meant for a single person on a light inflatable boat. I really want to know what the stock prop pitch was from the factory.

The boat will be at 650lbs most of the time so hole shot is very important. Maybe the prop was slipping earlier and affecting hole shot...........

 

[flyingscott]

your motor is only rated to 5500 rpm

 

[Sea Rider]

That's why is a must tach original factory delivered prop while lightly loaded, will know max wot rpm. In case you spun the prop can go for same one in the event was working ok or for a lower pitch prop. Used to have an Evi 96, wot range was 5-6K, only had 3 available pitches to choose from. At that time with a 10 footer sib and 2 up performed excellent with delivered prop pitch, we were not that heavy as we are now, definitely needing a prop maximization if still wanting using that size sib now a days.

Try an 11 with you alone and yourself with boarting mate, record both wot tests to check rpm difference achieved with added weight. It's not same boating with one heavy guy than with 2 same on a small inflatable which needs be top air inflated for hull to drag less. Those extreme lower pitch prop you mentioned are for powering heavier boats, probably won't plane them, but will have better fast displacement speed than with a higher pitch prop. Anyway will need an indiuction tach to play correctly with prop pitches and don't blow the engine in the intent.

Happy Boating

 

[steelespike]

your motor is only rated to 5500 rpm

Using a number of sources the E/J 15 hp is rated for 81-93 5500 - 7000,74-80 5500 - 6500, 97 - 2005, 5000 - 6000.

 

[Sea Rider]

OMC really liked playing with max wot revs through the years as opposed to other Jap brands. If an OB screams at 6 imagine at 7 K.

Happy Boating

 

[nabiul]

What do you guys think of 4 bladed props?

http://boatpropellers.iboats.com/Jo...3-093-11_Propeller/23/6156/?cart_id=483890752

 

[Sea Rider]

Which ever 3-4 blade prop you go for will be a starter prop, will need to be tached at wot to see if revs around max rpm range with 2 heavy guys, can go from there maximizing another prop to suit your boating requirements much better if needed as selected prop will not guarantee it's the perfect choice untill tested on a wot run.

Happy Boating