1973 Johnson 65 hp prop Question

[Slippery When Wet]

I have a 73 65hp Johnson on a tempest 15ft ski boat,the prop that is on the motor has more than one ID number(op51681-op36232 and op37403)the boat pops out of the water and on plane in about two and a half boat lengths and runs at 32 mph,with one person or three,we do moderate tubing with light weight people the rest of the time its a runabout,I feel that it could use more pitch and would like to gain mph,is this feasible and what prop should I be searching for?As a edit on the hub itself there are markings 13 and a quarter X 17 and some numbers that are hard to see looks like 232764 but unsure.

 

[steelespike]

Re: 1973 Johnson 65 hp prop Question

13 1/4" (diameter) X 17"(pitch)Do you have your rpm at 32?Without rpm and speed you can't tell what or if you need a prop.Also can't tell if your new prop is at the best rpm.A TinyTach ir about $47 and easy to install.They have a site.
From your description it appears you could go up in pitch.Is the prop Aluminum?
al. is not magnetic.

 

[Slippery When Wet]

Re: 1973 Johnson 65 hp prop Question

Thanks for the comeback the motor is running at 5075 rpm @32 mph with two people in the boat and full tank of fuel.And yes the prop is aluminum and appears to be original.

 

[steelespike]

Re: 1973 Johnson 65 hp prop Question

Your rpm seems low for the speed.You may want to confirm the tach is accurate.
Perhaps borrow a shop tach. Does the boat respond to up trim?
Does the bow come up and speed increase or does it lay down flat like it is glued
to the water?Does it have a foil or trim tabs?
It does appear a 15" prop would help your rpm which needs to be closer to 6,000.
A 15 would get you to about 5,500.Less than a 15 would seem extreme for a 15ft ski boat.Is the boat glass or aluminum?Dry weight? Is the bottom clean and smooth?

 

[Slippery When Wet]

Re: 1973 Johnson 65 hp prop Question

Your rpm seems low for the speed.You may want to confirm the tach is accurate.
Perhaps borrow a shop tach. Does the boat respond to up trim?
Does the bow come up and speed increase or does it lay down flat like it is glued
to the water?Does it have a foil or trim tabs?
It does appear a 15" prop would help your rpm which needs to be closer to 6,000.
A 15 would get you to about 5,500.Less than a 15 would seem extreme for a 15ft ski boat.Is the boat glass or aluminum?Dry weight? Is the bottom clean and smooth?

I will borrow a tach to check against mine, reposition of the motor trim makes very little mph difference I can gain two mph trimmed up but the boat wants to bob the bow,trim down 32 mph and flat on the water,no foil,no tabs,it is a fiberglass boat,clean bottom,I will check the weight,heck Iam not even sure what model this boat is but its a blast,thanks for your response.Rob

 

[Frank Acampora]

Re: 1973 Johnson 65 hp prop Question

The boat sure looks like a Glastron GT 150 but the color is wrong for a 1973 model and it does not have a GT 150 emblem or medallion on the deck.

At any rate, it will be rated for 90 horsepower so if you want to replace your 65 in the future, that is what you can hang on it.

Double check your tach---on that hull, with a 17 pitch prop, the engine should be turning more than 5000, but 32 MPH is about in line with what speed you would expect. OMCs are good, but 65 is only 65, is only 65. There is just so much it can do.

And BTW: If you don't have the capacity plate which will show model and year, Go over to ClassicGlastron.com and look for your hull.

This is a '73 with a '73 Johnson 65

 

[Slippery When Wet]

Re: 1973 Johnson 65 hp prop Question

Thanks Frank that boat sure looks like mine,I more or less inherited this boat from my father in law,he was not sure what it was as he bought it second hand eon,s ago and it spent the last ten or so years sitting in a boat house in northern on.The color is a chocolate brown disco metal flake that appears to be the original color,I have not able to find the capacity tag but just got the boat and I will search out under deck and in the rear for a clue,the papers that he got when he bought it said it was a tempest but looks like a glastron too me,more pics to follow.Rob