1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

It also says Michigan PJ89-Cup on my spare.

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

Depending on load.....a 17p prop will probably be your best choice. You need a tachometer to find the right prop though.

I have a 16' SS also with a 1977 75hp Evinrude....and a 17p prop is a good choice for my rig. I'm have a 15p prop for when I carry heavy loads or pull a tube. I have to throttle back a tad with the 15p cause I can rev to 6100rpm with it.

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

I average about 2-3 people in my 16 foot boat. Occasionally I may pull a tuber, but rarely.

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

A 17p prop seems to be a good all-around prop. Only a tach can verify if it's right for you. Hooking up a tach is simple and only costs about $60-70 bucks.

Running your motor in its optimal RPM range will ensure it's longevity, so a tach is a must IMO.

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

What about Diameter? What difference does this make? Mine appears to be 14 inches and just barely clears the what is above it.

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

Without a tach we're just shooting in the dark. Also without a prop size its very hard to do a prop selection.
Guessing by a comparison is a gamble But the 77 75 does have the same ratio and the boats compare well. So a 17" is a fair gamble.
As far as diameter It is a secondary consideration. We may get a slight clue with your 14" diameter prop.
Genearally as pitch goes up diameter goes down. and as pitch goes down diameter goes up.
First thing is to measure your diameter.
Measure from the center of the bore to the tip of a blade and double it.
if its worn try to be sure its the longest blade.
We'll need the diameter of bonairlls' prop.
The props here at boats on a 17" pitch the largest diameter is 13 1/4"
15" props are 131/2 to 13 3/4, 13" are 13 3/4 to 14"
It would appear your prop is a 13 or 15" pitch not a 17.
Of course the best way to solve this is to have a prop shop measure it for you.
Be sure to get a tach for the best performance and the health of your motor.

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

steelespike said:

We'll need the diameter of bonairlls' prop.

Click to expand...

Here's the info on my setup and performance numbers:

16' SS (restored, so no waterlogged foam etc)
1977 75hp Evinrude (145-150psi in all 3 cyls)
12 gallons gas
2 people (330 lbs total)
2 marine batteries and trolling motor
casting deck (30-40lbs I guess)

13 1/4" X 17p aluminum prop= 5750 rpm @ WOT 33-36mph depending on chop/wind etc

13 3/4" X 15p aluminum prop= 6100rpm(this is 300rpm over recommended max) @WOT 31-33mph depending on water conditions

Motor is manual trim btw.

Re: 1979 Johnson 70 HP Sea Horse on 79 Starcraft SS 16 ft

Good to know.
The 13 1/4" diameter 17" pitch of your prop makes me think he may have less pitch if its really a 14" diameter prop.
Of course the prop he has on his desk is a spare so it may not even apply.