Help, Wrong prop on my Johnson

[wmbreck]

Hi,

I have a question about a boat we bought last year. It is an old Larson fiberglass - I think American 150 model. 15 feet long. Nice shape and everything works. The only issue is that it came with just a 28 hp Johnson SPL long shaft motor. We knew it was underpowered. The former owner used it just to fish and we will upgrade the motor at some point.

Here is the question. GPS says it reaches about 18 mph with one person and drops all the way to 10 mph with 2 people. I knew it would be slow but not that slow. But I just found out from the owner that he had replaced the original prop with one that would be slower for trolling. Motor never really seems to reach RPM's you'd think. I was wondering what we could expect if we can get the normal prop or a high performance one back on it. Even 3-4 mph would really be nice???

Thanks if you can make a guess for me.

Bill

 

[RicMic]

Re: Help, Wrong prop on my Johnson

You really are going to need the engine rpms to come up with any kind of a guess on what to do. If you are going to a bigger motor at some point, you will probably want a tach anyway. What size is the prop on there now?

 

[wmbreck]

Re: Help, Wrong prop on my Johnson

Soory but not sure what size. Boat is on Lake Ontario and I'm home in Jersey now. That is why I was hoping to find if it was worth buying another before I return there in 3 weeks. All I know is that the owner 'supposedly' changed props for trolling purposes and, if so, I can't imagine that would not cost us a little bit of top end performance. Even 10% more would be helpful and worth the price of a prop since we don't have cash for a 50 or 60 hp right now.

 

[RicMic]

Re: Help, Wrong prop on my Johnson

It would likely be a waste of money to buy another prop, without more info, you could just end up with another one the same size you have now. It dosn't really add up, if the previous owner changed props to troll slower, he would have to have gone with a lower pitch, if that was the case, then the motor would be over revving now with a light load. It sounds like the pitch is to high and the motor isn't developing full power, but that would have made the motor troll faster. I wouldn't waste money on a new prop, without knowing what size the current one is and the rpms.

 

[wmbreck]

Re: Help, Wrong prop on my Johnson

Thanks. Hard to grasp when you are new to all of this.

 

[steelespike]

Re: Help, Wrong prop on my Johnson

The 28 is good for about 33 mph on a light reasonably fast boat.I'm pretty sure we can do better than 18 with the right setup.Do you know the year of the motor?
When you get to the boat look on the barrel of the prop near the base of the blades or under the prop nut for the prop numbers.You could get a Tinytach about $47 and easy to install.The tach for your motor will fit any 2 cylinder 2 stroke.
The antivent plate(just above the prop) should be about even with the bottom of the boat.Make note of it.
With the tach and your gps make a couple of lightly loaded runs.
Adjust the trim out (prop away from the boat will tend to raise the bow and improve speed to a point. trim in it will lower the bow it will plane easier but will lay down flatter tending to slow.The idea is to get just the right trim.Weight placement as well may help.Move your passenger around for the best trim.
Is it a tiller motor?