1978 Johnson 150 prop question

[tranquility]

I am currently running a 15x17 aluminum prop. Making about 4600 RPM. I get some ventilation during turns and coming out of the hole if I'm not trimmed full down.

I am thinking that a stainless prop may help some. Question is which one and what size. I've been looking at standard 3 blade OMC 15x17 in stainless. As I think a few more RPM would be good. This is an older 21 ft. low profile ski boat. Should I be considering a four blade?

Also as this is an older boat with not to big of an investment I am not interested in spending much more that $150. I should be able to find a used prop in this range.

Any input please.

Thanks.

 

[JB]

Re: 1978 Johnson 150 prop question

Prop questions go in the Prop Forum, tranquility.

I will move this for you.

 

[tranquility]

Re: 1978 Johnson 150 prop question

Sorry and thanks.

 

[Dhadley]

Re: 1978 Johnson 150 prop question

Depending on exactly what hull design you have will have a lot to do with your choice. But something doesn't sound just right. A 17 on a ski boat should get more rpm than that with a V6. You're at least 1000 rpm short. That means you'd have to be at least in the 12-13" pitch range and that just isn't right.

Are you sure the motor is running OK? Are you sure 4600 is all you get?

 

[tranquility]

Re: 1978 Johnson 150 prop question

That's a good question. The boat has not been in the water since last summer. Now that I'm thinking about it the max RPM may have been as high as 5200. I will get a chance to check it this weekend, so I will get back to you on that.

What is the max RPM I should aim for with this engine?

 

[Dhadley]

Re: 1978 Johnson 150 prop question

If it's been sitting that long I wouldn't even start it until after the carbs are cleaned. And start with all fresh 87 octane fuel. You'd like to be around 5700-5800 with an average load.

 

[tranquility]

Re: 1978 Johnson 150 prop question

Ok, ran the boat and had a great time. Turns out my memory did not serve me well. At WOT trimmed right to ventilation height it made 5400 RPM and 42 mph. This is a low profile open bow hull built by Sterling. 21 ft. I'm currently running a 15x17 alum. So I'm thinking a 15x17 stainless will end up with just a litlle more rpm. I found one thats 14 3/4x18. Whats the opinion here on size? Again the problems arise coming out of the hole, ventilation unless its trimmed full nose down and also ventilates in turns.

Thanks
Bryan

 

[jimmbo]

Re: 1978 Johnson 150 prop question

How old is your aluminum prop? In the 70s Omc aluminum props were usually uncupped. If your present prop isn't cupped and you put on a cupped stainless of the same pitch you could have some very shocking results. Cupping is like adding pitch and thereby will lower rpm. It also enables the prop to grip the water better resulting in less slipage which would lower rpms. Sometime people have been running props that were running partial blown out thereby allowing the motor to overrev, They then go buy a Stainless prop with more pitch to lower the rpm. The new prop being have better bladeshape and most likely cup now runs with out blowing and the rpms drop much lower than expected.

 

[tranquility]

Re: 1978 Johnson 150 prop question

It appears that the current prop is cupped and in good condition.

 

[Dhadley]

Re: 1978 Johnson 150 prop question

The next question is what's your intended use? Typically a ski boat has a pretty good hook. If you're going to be mainly skiing then there's no need to look at props like a Lightning or TXP. If you're not going to be skiing as the main use and want as much top end as possible then that's a different prop.

 

[tranquility]

Re: 1978 Johnson 150 prop question

Mostly hauling my kid's on the wake board and the tube.

 

[Dhadley]

Re: 1978 Johnson 150 prop question

I would take a good look at the Stiletto line of 4 blade props. Either the Bay Pro II or Bay Pro III.