? Prop Selection Question ?

[bds85466]

I have a 1972 Johnson 125 hp. It's on the back of a 1972 16 foot Glastron GT 160 (a small, light, coupe of a boat). I'm currently running a 12 3/4" diameter 17 pitch 3 blade aluminum prop. Our tach is out of comission, but with two in the boat we get it right around 45mph at full throttle. I was thinking about getting a 19 pitch 4 blade prop to reduce vibrations and increase our hole shot. Nothing scientific here, but my general reasoning says that increasing the pitch will give it a higher top speed and adding another blade will give it more torque for pulling people out of the water. Am I correct in thinking this? Does anyone have any recomendations? Is getting a 4 blade prop too much for this boat?

 

[R.Johnson]

Re: ? Prop Selection Question ?

Your 72, 125 engine does not have tilt & trim. I would try a OMC Raker prop, this will give you some bow lift. Try a 17, or 19 pitch. These prop's are not cheap, so you want the correct one. This will depend on your dealer, and how much he will work with you on selection. Probably the best way would be to let a dealer prop the boat. That Raker prop would be about $500 a guess, so you want the right one.

 

[Silvertip]

Re: ? Prop Selection Question ?

Yes -- your thinking is seriously flawed. You currently have a 17P 3-blade and you want to go to a 19P 4-blade. First, even if you stayed with the same three blade prop manufacturer and prop style and only changed from 17 to 19P you'd lose about 400 RPM. You would likely lose top speed and hole shot would suffer IF the current prop is allowing the engine to run at the top end of the rpm band. Since you don't have a tach, you have no idea if the engine is running in its recommended wide open throttle rpm band (probably about 5500 RPM). Going to a four blade removes more rpm, more speed, and may result in the same or even worse hole shot than you had with the 3-blade. The point here is that going from a three to four blade you generally drop an inch in pitch, not increase by two inches. You cannot just slap a higher pitch prop on and expect to go faster. The engine has to have enough power to twist that prop. This is a long answer to a short question but get the tach fixed, then make some wide open throttle runs with average load. Note speed and RPM. Make sure the engine is in tune and mounted at the correct height before you make your test runs. Then think about props.

 

[JB]

Re: ? Prop Selection Question ?

We have a Prop Forum for Prop Questions.

 

[bds85466]

Re: ? Prop Selection Question ?

Your 72, 125 engine does not have tilt & trim. I would try a OMC Raker prop, this will give you some bow lift. Try a 17, or 19 pitch. These prop's are not cheap, so you want the correct one. This will depend on your dealer, and how much he will work with you on selection. Probably the best way would be to let a dealer prop the boat. That Raker prop would be about $500 a guess, so you want the right one.

Well, actually the boat does have trim. I'm not sure if a 500 dollar prop is the right way to go since I could try -- wait -- buy 5 different aluminum props for the same price.

 

[bds85466]

Re: ? Prop Selection Question ?

Yes -- your thinking is seriously flawed. You currently have a 17P 3-blade and you want to go to a 19P 4-blade. First, even if you stayed with the same three blade prop manufacturer and prop style and only changed from 17 to 19P you'd lose about 400 RPM. You would likely lose top speed and hole shot would suffer IF the current prop is allowing the engine to run at the top end of the rpm band. Since you don't have a tach, you have no idea if the engine is running in its recommended wide open throttle rpm band (probably about 5500 RPM). Going to a four blade removes more rpm, more speed, and may result in the same or even worse hole shot than you had with the 3-blade. The point here is that going from a three to four blade you generally drop an inch in pitch, not increase by two inches. You cannot just slap a higher pitch prop on and expect to go faster. The engine has to have enough power to twist that prop. This is a long answer to a short question but get the tach fixed, then make some wide open throttle runs with average load. Note speed and RPM. Make sure the engine is in tune and mounted at the correct height before you make your test runs. Then think about props.

Good points. I will say that the 125hp has no problem... none; in getting the small boat up quickly. A 125 is actually overated for the boat. I guess I'll just get a trolling motor instead.