1984 90hp johnson v4 which pitch is best

[tannedman]

Ok i'll start with boat 1991 16ft foxcraft weight 940 pounds tack don't work need to figure out how to fix this boat had a 1991 75hp merc and did 42mph i picked this johnson up cheap from a used boat dealer who stands behind his work so no isues there. There is a stainless steel 21p prop on it and he gave me a smaller 17p as a spare since i haven't had it out for a gps speed test yet is the a prop size thats recomended for the weight of boat and size of moter like a chart should i try a few different size's

 

[Bifflefan]

Re: 1984 johnson v4 which pitch

Re: 1984 johnson v4 which pitch

Man, that info is all over the board.
Whats the HP on the Johnny, what pitch was the old prop?
Fix the tach, first....
Depending on the HP and weight of the new motor is should perform close to the same, if they are close to the same, with the same prop pitch.

 

[tannedman]

Re: 1984 johnson v4 which pitch

Re: 1984 johnson v4 which pitch

Sorry forgot that its a 1984 90hp v4 and the pitch on the merc was also a 21 stainless

 

[ezeke]

Re: 1984 johnson v4 which pitch

Re: 1984 johnson v4 which pitch

A 91 75HP motor is about the same power as a 1984 90HP motor because of a change in rating systems around 1985.

If the propeller is the same pitch, look up the gear ratio for the Merc and adjust accordingly. You need to be able to reach 5800 RPM maximum at WOT to keep from lugging the engine and shortening its life.

The gear ratio for your V4 is 2.0.

For example, if the gear ratio of the Merc is 2.3, you would have to go from a 21 pitch to +/-18 pitch to get the same theoretical performance.

 

[metriccrescentwrench]

Re: 1984 90hp johnson v4 which pitch is best

Definitely get a working tach (quite often a non-working tach is because of a bad rectifier - checked that yet?). That 17 pitch prop is going to be close for you. I've got a 15 ft. boat for which NADA lists empty hull weight at 750 pounds, with an '81 Johnson 90 and 17 inch pitch aluminum prop. It runs about 42 mph at 5600 rpm. The 21 inch pitch prop would be way too much.

 

[Bifflefan]

Re: 1984 90hp johnson v4 which pitch is best

I dont know about the above post, I used to have a 74 85 HP johnny on a 16' starcraft bow rider. With a 19p and 4 people on the boat it still did great and went faster than I wanted to go in it.

The weight is going to make a difference, as the V4 weighs more than the merc. It may make you a bit tail heavy and will handle different.

 

[tannedman]

Re: 1984 90hp johnson v4 which pitch is best

Thanks for the info i will have a tac installed and borrow a couple of his props to determine the proper size i need 5800 max but what would be the minimum

 

[ezeke]

Re: 1984 90hp johnson v4 which pitch is best

The motor should idle at around 650 RPM in gear, warmed up and unrestrained.

Between that and around 4,000 you will probably be on plane and able to reach a comfortable cruising speed.

You may never want to run the engine at 5800 RPM again, but you want to be able to reach it to know that you are not overpropped.

Have you ever run your automobile to its top limit?

 

[tannedman]

Re: 1984 90hp johnson v4 which pitch is best

LOL no i was just makeing sure i have all the base's covered to make sure i have enough to run properly but not to much being used Thanks

 

[steelespike]

Re: 1984 90hp johnson v4 which pitch is best

I think you will end up with a 17 or 19.Give the 2 props a try with a tach.
The 21 will probably be too much.