Prop recommendation for a 50 HP Johnson

[weekendfisher]

Hey Guys,
I am looking to replace my prop as it has sustained a few knocks over the years and is missing some metal and looking pretty ugly these days.
It is still the standard prop that came with the motor.
It is for a 94 model 50 HP johnson on a 16ft fibreglass half cabin.

I can't find any numbers on it so i was hoping someone would be able to tell me what sort of size and pitch i should be looking at for this type or motor.

The boat is mainly used for fishing or just cruising with the family and i very rarely have it at WOT so holeshot or top speed are not all that important but if i had to go for one it would be speed.

Thanks.

 

[jestor68]

Re: Prop recommendation for a 50 HP Johnson

You may need to remove the prop in order to find the numbers stamped on the hub.

Without the prop size and WOT rpm it is turning, it would be a wild guess as to what prop to put on there.

But if I were to take that guess, I'd start with a 17" pitch.

 

[steelespike]

Re: Prop recommendation for a 50 HP Johnson

Its important that the motor be able to operate within its rated rpm range whether you only cruise or run wide open.
Usually prop numbers are under the prop nut.
We need the prop size and your present light load wot rpm and speed by gps.
A prop shop can determine the size.
On a 16 ft. half cabin I think a 15" pitch is close to what you need.Consider that a starting point
until we can get some performance numbers from the present prop.

 

[weekendfisher]

Re: Prop recommendation for a 50 HP Johnson

Thanks guys,
The only time i did take a GPS speed reading was 38 knots at near WOT so i was not at WOT but close.
I often don't travel at full speed as i generally have kids on board and i prefer to be a little more cautious.

I took another look last night and did find some numbers on the side of the prop they read 11-3/4X17 I was looking at the back and around the hub nut but this was printed on the side where i had not really been paying to much attention to before.

This prop is definately smaller than the aluminum one i had on my previous boat which was a open 15 ft fibreglass with a 55hp Johnson of the same year.
Thanks

 

[steelespike]

Re: Prop recommendation for a 50 HP Johnson

Your setup can't do 38 knots perhaps it was kilometers?

 

[weekendfisher]

Re: Prop recommendation for a 50 HP Johnson

Perhaps ...
I will need to double check lol...
I have looked over the Iboats website and i have found a 12-3/4 17P being a little bigger would i see any improvements on this size ?
IF it will be around the same i will probably order this prop.
Cheers

 

[jestor68]

Re: Prop recommendation for a 50 HP Johnson

I assume that your motor is a 2 cylinder, small gear case model (?).

If that's the case, you might double check the prop size before you order. A 12.75 diameter doesn't sound like the correct size for the small gear case motor.

 

[weekendfisher]

Re: Prop recommendation for a 50 HP Johnson

Hey,
The orignal is a 11 -3/4-17 so going to a 12 would just be adding an inch in over all diameter wouldn't it ?
I have just used the orignal numbers but gone for a 12 instead of 11.

It is a 2 cylinder model not sure if is the small gear case or not but i would assume so.

 

[jestor68]

Re: Prop recommendation for a 50 HP Johnson

I have looked over the Iboats website and i have found a 12-3/4 17P
Cheers

I was referring to the 12-3/4 X 17 not being a size normally found on the small gear case motor.

 

[weekendfisher]

Re: Prop recommendation for a 50 HP Johnson

Ahh fair enough, I selected small gear case on the site and it listed the 12 from Solas.

Is there a way to be sure what gear case you have without comparing them side by side ... markings numbers etc ?
Thanks

 

[steelespike]

Re: Prop recommendation for a 50 HP Johnson

Assuming your 11 3/4 X 17 is the correct size for your motor it is a small gear case.
Also it appears that the 50 hp 1994 with a large gearcase is a 3 cylinder and a 17" prop is over 13" in diameter.
I think its safe to assume we are dealing with a small gear case.
Are we talking a 12" diameter or 12 3/4" diameter. A 12" could very well be for a small gear case
a 12 3/4 not so sure.If it is listed with other props for a 2 cylinder small gearcase 1994 50 its probably good.
regardless of diameterr a 17" prop should behave like a 17"
If we are increasing diameter by an inch it could produce lower rpm but be less sensitive to venting(cavitation).

 

[weekendfisher]

Re: Prop recommendation for a 50 HP Johnson

Hey
Thanks for that very detailed answer.
I am pretty sure that it is the small gear case as i have been searching for info on the and the 50 HP commercial came with the bigger gear case which would explain why my previous motor had a larger prop possibly.
The page i have been searching props on is this one http://boatpropellers.iboats.com/Jo..._17.00-Pitch_Propellers/58/?*******=603108612
I selected small gearcase and 17 pitch and these were the results.
Thanks

 

[steelespike]

Re: Prop recommendation for a 50 HP Johnson

The Solas Amita has a good reputation for predictability and performance.