First boat, no prop...confusing motor. Please help.

[azjeep]

So I bought my first boat....craigslist is awesome. I got a 1980 Skeeter 16ft Bass Boat with a 1979 Johnson 115 on the back.

So I looked it up and found that a 13 spline prop works. So I got one on ebay. I get it delivered and take it out of the box and try to put it on...FAIL.

Apparently I have a 15 spline shaft for the prop.

1st question, how is that possible?
2nd question, what prop do I buy? I think this isnt an exact science and you need to experiment with a few props till you find the correct one, but where should I start?

 

[Barramundi NQ]

Hi welcome aboard.
is the lower unit the original? Its possible that the PO repaired or replaced it with another, hence the spline count is not as written in the specs. Was there a prop on it when you bought it?

 

[Starcraft5834]

az.. look at the prop you have.. numbers stamped (likely) on the collar.. last digits mean pitch...might say 19, 21, or something like that...the only way to tell if you even need another prop is to report your GPS speed at WOT- (wide open throttle) and what that rpm reading your getting... your engine will produce a max rpm range (you can look that up) at WOT. if you are within that range now with the prop you have.. there's really no need to change the prop, unless it's all knicked and dinged up...if you cannot reach your engines max rpm range, it could be the prop has to much pitch, or it could mean your engine is ill..or a combination of both, or other things can contribute....report back if you can on your current performance numbers and you can get some usefull suggestions......enjoy the boat!

 

[steelespike]

It may be a V6 gear case try to measure the gear case diameter where it meets the prop.
If you pull the plugs and rotate the motor by hand to count the revolutions for 1 revolution of the prop,
will give us the gear ratio. You may have to mark flywheel and prop to be accurate.

 

[azjeep]

Sorry for no response guys. I thought I would get an email notification when someone responds. haha.

There was no prop on it when I bought it. PO took it off.

 

[YoungMind]

It may be a V6 gear case try to measure the gear case diameter where it meets the prop.
If you pull the plugs and rotate the motor by hand to count the revolutions for 1 revolution of the prop,
will give us the gear ratio. You may have to mark flywheel and prop to be accurate.

Or maybe it is a gear case from a newer V4???:noidea:

Definitely start where Steel said to figure out what gear case was put on there.

 

[inthedirtagain]

A healthy 115 on the flat hull of a 16ft bass boat should really fly!

 

[ono_55]

I have the same boat but a 1985 I have a 115 mercury on the back it turns a 21p alum prop with on problem and yes I does fly
with that 115 on the back.

 

[azjeep]

Ok, I finally was able to do this. If I turn the crank on the top of the motor, it turns the prop shaft just over 1/2 way. ...so i'd say a 1.8:1 ratio.

 

[steelespike]

It seems it would be more accurate if you turned the motor till the prop turns once.
My info indicates 90 -115 are 2.00 and the 150 on up are 1.86.

 

[azjeep]

OK, I would say its much closer to the 1.86 than the 2.0.....

That said, is there a way I can be sure? I took a Popsicle stick and marked one end and taped it to the prop shaft. Then I turned the flywheel one full rotation. The Popsicle stick turned a little over half way....

not scientific by any means but if I were given those two choices, I would say the 1.86:1.

 

[HT32BSX115]

Howdy,

Remove the spark plugs and have a helper turn the engine exactly 10 turns and count prop-shaft turns. The more turns you "do", the more accurate you'll be unless you can easily view 1/2 (0.5) , 1/4 (0.25) or 3/4 (0.75) , turns of the prop-shaft for any full number of crankshaft turns.

Then just use your calculator. prop-shaft turns divided INTO crankshaft turns....... It will be absolutely accurate!

Fractional turns on either are ok if you can accurately determine them. 15.5 divided 7.25 etc (it's easy to see 1/2 turn on either the crank or prop.....if you have a 3-blade prop installed 1 blade is 0.333 turn! on a 4 blade, 1-blade is 0.25 turn etc........)

That said, is there a way I can be sure? I took a Popsicle stick and marked one end and taped it to the prop shaft. Then I turned the flywheel one full rotation. The Popsicle stick turned a little over half way....

This works only if you can accurately determine how far past 1/2 turn (180 degrees)

If you use a builders protractor you can actually measure the number of degrees past 180........ let's say 225 degrees. That's more than half but less than 3/4

180 divided by 360(1 full turn) = 0.5

270 degrees divided by 360 = 0.75 (3/4)

so, ...........225 divided by 360 = 0.625 (turns)

BUT, .........It is FAR easier to just turn the crankshaft MORE revolutions until you see the prop-shaft stop at a easily recognizable spot (like 1/4, 1/2, 3/4 etc)

Then "fat-finger" it into your calculator!


Cheers,

Rick