1978 150 johnson RPM/prop question

[tranquility]

I have a 1978 Johnson 150 with a 15x17 alum Michigan wheel prop. The boat will run 38 mph at 5300 rpm. However, at that rpm it will ventilate with turns or in rough water. If I trim down to about 4900 or 5000 rpm it will run without ventilation. Also on the hole shot it will ventilate unless I have it trimmed all the way down. This setup is on a 1979 Sterling low profile ski boat 21 feet long.

The Anti cav plate is aprox 1.5 to 2 inches above the boat bottom.

I would really like to get rid of the ventilation problem. And I need to know the max rpm for this motor.

Thanks.

 

[jimmbo]

Re: 1978 150 johnson RPM/prop question

Consider 2 choices;
1) Lower the engine until the ventilation plate is even with the boat bottom

2) ditch the aluminium prop, replacing it with a suitable stainless steel like an Enertia, Viper, LaserII

Max rpm 5500

 

[tranquility]

Re: 1978 150 johnson RPM/prop question

unfortunately, the engine is already mounted as low as it will go. Do you think that just changing the prop could eliminate the ventilation problem?

 

[jimmbo]

Re: 1978 150 johnson RPM/prop question

A performance SS prop will go a long way in reducing the ventilation if not elliminating it. If you noticed I said performance SS, not just a SS version of an Aluminium prop. To function at a raised height the prop will need to have cupping and more rake than the regular style prop.

 

[tranquility]

Re: 1978 150 johnson RPM/prop question

Ok, so of those three props. Which one has the best chance of solving the problem?

Thanks for the info.

 

[jimmbo]

Re: 1978 150 johnson RPM/prop question

Being that your motor has a high transom height, you are going to need a prop with lots of cupping and probably lots of rake. The props I mentioned have both. I currently run an Enertia on my boat. It grips the water better than anything else I have ever ever tried. Enertia is available in 17 inch pitch and possibly in a 16. It is a Mercury prop but with the proper hub kit it will fit your motor. There might be an issue with the trim tab. Raker is also a good prop at the raised motor height, however I think the lowest pitch is an 18. I've also run a Stilleto, simaliar performance to the Raker.

An option in some areas is locating a Merc dealer who has the Demo Prop program. There you can try a variety of props.

http://sites.mercurymarine.com/portal/page?_pageid=126,48417&_dad=portal&_schema=PORTAL