Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

[redfishsc]

Motor-- 1983 Johnson Sea Horse J8RCT (7.5hp)

Boat-- 14' Starcraft SS aluminum, riveted.


The motor itself is in really good shape, runs well. It was used only as a kicker and never the main motor. The prop, however, is really worn out (oyster rash). Occasionally the motor will make a slight increase in sound (half a second), and the boat hesitate..... which leads me to wonder if perhaps it's cavitating on the knicks/cuts on the edge of the prop. The prop is an 8" X 7 pitch. That model also has available an 8.5" X 9 pitch.

This one doesn't seem to have a hub since it's not through-prop exhaust.... but the center bushing in the old prop had been shoved inward enough that I couldn't get the shear pin out--- it was half buried into the prop... had to drill a channel for it to slide out through. Not sure if that bushing was coming loose, I don't know how it's assembled, but there's NO way on earth it went onto the motor like that. The shear pin also was broken when I finally got it out, but I'm not sure if the pin broke in the process, or broke when it was jammed into the prop (not even sure what would cause that).

Since the motor now pushes a much lighter boat than it once did, I'm thinking (out of my naivete) that the 8.5" X 9 pitch would be more appropriate. I do have a spare 8X7 prop (nearly brand new) from a parts-only 6hp that matches the previous.

Without having a tach to properly check/try this, could you guys help me decide which would be best? I installed the other (newer) 8X7 on it tonight to see if that stops the minor jerking.


BTW the boat will plane out as long as I have a whale-tail on it. Without the whale-tail, it will sometimes plane, sometimes not (the anti-vent plate sits nearly perfectly even with the keel so it's in the right spot).


We load the boat with around 400lbs total, including me and my young son, trolling motor, battery, tackle, etc.

 

[steelespike]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

Counting the boat your at about 625lbs. I think the 7" is a good starting point.
In my opinion your 7.5 should plane easily with just you and your son aboard maybe even fully loaded.
We had a 5 gale on a steel flat bottom boat It popped on plane with a 125lb adult and 2 40lb children
or with a 180 lb adult with a tiller extension. It would run about 10 to 12 mph.
Lightly loaded your motor could run around 17 mph on a light reasonably fast boat.
Is your boat a bare utility; no floor or built-ins?
The jerking: does it feel like you hit something?

 

[redfishsc]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

Yes, the jerking does have a feel to it as if we were hitting something, but definitely not a hard jarring feel. Just a quick "bump", but obviously I wasn't actually hitting anything, I was in 10' of water at the time.

I will give the 7" pitch a try since it's free :joyous:. Thank you for your help.

With the boat loaded at 625, we're doing 12-14mph on the old chewed up prop. I'm not expecting the new prop to gain a lot of speed, but I really would like to rule out any internal mechanical issues with the hesitation.

 

[Sea Rider]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

Induction tachs being so cheap and small is a must install one. My theory understanding prop pitches changed radically when installed one on my 18 horse. Go for one and check which prop gives at wot near or slightly passing middle readings between min and max rpm factory stated for that hores as usually loaded for best boat/engine performance. A spot on tool to instantly check if needing - or + pitch optimization.-

Happy Boating

 

[steelespike]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

If you're getting a little thump it is very likely a worn clutch dog in the gear case.
A normal wear item on a 30 year old motor.If its the old style 2 piece gear case its a fairly simple job.
On some motors you can flip the clutch dog and the worn part won't bother in reverse.
Someone may be along with more details.

 

[redfishsc]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

double post

 

[redfishsc]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

Thanks SeaRider, I've seen stuff like that for $30 and wondered how accurate they were. I may get one if I decide to re-prop.

Steelespike, I appreciate your help. I think flipping it around, if I can do that on this one, is what I'll do since reverse is rarely used.

Is the clutch dog, specifically, part #12 here in the link below? If so, what other parts do you recommend I replace as routine maintenance? Thanks. I'm not as familiar with outboard repair stuff.

EDIT: part #12 on the common drawing for the gearcase. Link won't work, and the link in my above post is so small lol. It's at the bottom right side.

 

[Texasmark]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

If you break that 2 piece lower unit you really need to pay a lot of attention to the oring type (round gasket in a groove) and ensure that you get it cut to fit in slight compression to ensure that it doesn't leak. I think a 7" pitch would be your better bet also with no other info available concerning rpms. 7 1/2 on a 14' boat isn't all that much. I had one, grandpa had one, neighbor had one and they were just on the edge of adequate performance. Course on a flat bottom john boat it doesn't take much engine. I had a 5 1/2 johnnyrude that would move mine along quite nicely. On a round chine semi-v a different story, aluminum or not. A little 9.9 is a different story. BTDT on all this.

Mark

 

[steelespike]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

You don't have the 2 piece gear case.
I've never done that style gear case. As far as additional parts probably seals and gaskets And you should change the impeller if you don't know its age.
Very basically the gears are all ways meshed with the drive shaft, spin freely on the prop shaft.The clutch dog is splined to the prop shaft and slides forward and back to engage the dogs on the gears thus driving the prop shaft.There are probably detent balls and springs involved don't lose them on disassembly.
Unfortunately its been 40 some years since I did a clutch dog.

 

[redfishsc]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

OK guys thanks for the help on this. If the nearly-new 7 pitch scavenged from my other motor doesn't fix it, I'll assume the clutch dog is messed up. I may be lucky in having the clutch dog from the other motor fit it, many of these parts are interchangeable. I'll have to look it up to see.


Texasmark, you are right that a 9.9 would be a better choice, but this motor was a good deal. I paid about 1/3 of what a similar 9.9 would have cost, and with our budget as it is, and the type of fishing we typically do, the 7.5 will suffice. In fact a lot of the fishing we do would be done just fine with a trolling motor and two batteries.

 

[Texasmark]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

OK guys thanks for the help on this. If the nearly-new 7 pitch scavenged from my other motor doesn't fix it, I'll assume the clutch dog is messed up. I may be lucky in having the clutch dog from the other motor fit it, many of these parts are interchangeable. I'll have to look it up to see.


Texasmark, you are right that a 9.9 would be a better choice, but this motor was a good deal. I paid about 1/3 of what a similar 9.9 would have cost, and with our budget as it is, and the type of fishing we typically do, the 7.5 will suffice. In fact a lot of the fishing we do would be done just fine with a trolling motor and two batteries.

Sounds great. We don't know what you don't tell us.

Mark

 

[Texasmark]

Re: Selecting prop for a 7.5hp Johnson (1983) on a 14' Starcraft tinny

You don't have the 2 piece gear case.
I've never done that style gear case.

You're absolutely right I had a brain fart and was reminiscing about some old '50's engines I had apart. Forgot about the '83 notation in the title.

Mark