7.5hp Merc prop problem?

[projecthog]

Hello folks,

I did a quick search for anything on a '73-7.5hp merc outboard, as one was handed to me, but I seem to own the only one in existence.

First Merc OB for me too!

I had no problem starting it, as a matter of fact it runs very quiet and smooth with lots of pep, and everything seems to be just peachy, except maybe for a bit of age wear on the throttle gear and vertical shaft, which I think is very reasonable for the old '73.

Took it to the lake, put it on a 12 footer, hooked the tank with fresh mixed 50:1 gas to it and proceeded out from the jetty.

All was well and still is, but there is some slipping going on as in a clutch gone bad in a car.
I asked some questions and found out that some props may have a bonded rubber bushing or buffer in them.

That's new info for this old shearpinnin rope puller!

Question;
-Is the prop for that year and model a rubber bonded one?
And;
-If I need to buy one, are there a few pitches to pick from?

The boat I am going to use it on is a featherlight 12ft aluminum and I'd want to step it up a touch.

The rubber bonded thought seems to be the only feasable one for the slipping feeling which occurs at a bit higher throttle setting while underway.
Slow it down and it will behave, in reverse it does it worse, but still moves the boat.

I had no time to dissassemble anything as I was called out on a trip, I'm currently out in NM, and while looking up causes on the net I thought I'd throw it out here.

I can't find anything on my particular motor or the prop other than the year by serial #. which equates it to a 1973.

My limited knowledge with "newer" props is exactly that.., limited to "nill" although come to think of it, my 140 I/O must have rubber in there too, I know the drive coupler does.

After looking around a bit for other 7.5's I have yet to come across one.

Do any of you have knowledge of props breaking the bonding and making it slip?, I can't think of anything else that would produce the same result.

The motor itself looks to have very little time on it.
The serial# is 1077513, which depicts a Canadian motor of 1973 vintage but should be identical to US motors I would think.

Any takers?
Thanks, PH.

 

[huntin dawg]

Re: 7.5hp Merc prop problem?

You don't have the only one. I have one too.
Sounds like your prop needs to be rebuilt. You should be able to find a prop shop that will replace the rubber hub.
There are a few props available for this motor. I picked up a new 8 pitch prop on Ebay. The stock prop for it is 7 pitch.
It's a great motor. Enjoy yours.

HD

 

[projecthog]

Re: 7.5hp Merc prop problem?

Hey..
Thanks for the info, just got home and read what you said, Thx.

I was told they were good motors, I have this one and it does indeed run fine and smooth too.
I got out the tools and took the prop off.
The splined sleeve is fine but the rubber insert is toast.
Its not apart or anything, but you can tell it is worn out.
I put it in a vice and turned the prop separate from the splined sleeve, so now I know.
I'll take it to town and have it fixed or exchanged for another one with a bit more bite pitch wise, as the light weight of the boat should allow for a step up.
Thanks loads, Take care,
John.

 

[bob1340]

Re: 7.5hp Merc prop problem?

Hey..
Thanks for the info, just got home and read what you said, Thx.

I was told they were good motors, I have this one and it does indeed run fine and smooth too.
I got out the tools and took the prop off.
The splined sleeve is fine but the rubber insert is toast.
Its not apart or anything, but you can tell it is worn out.
I put it in a vice and turned the prop separate from the splined sleeve, so now I know.
I'll take it to town and have it fixed or exchanged for another one with a bit more bite pitch wise, as the light weight of the boat should allow for a step up.
Thanks loads, Take care,
John.

Subscribed.
Let us know what you did. I just got a '69 model and the pitch on the prop looks very mild to me. I have no pulled it off yet to check the rubber bushing, if it has one. I did notice it has a lot of play when in gear. It's like a 15 degree flop back and forth. This makes me nervous. :redface:

 

[projecthog]

Re: 7.5hp Merc prop problem?

Ok,
I bought a prop from a dealer that had a box full of used props, and replaced the one that was on the motor.
Just because the failed one on the motor was a 7" pitch doesn't nessessarily mean it is the right one, so I asked and 7" inch pitch is standard for the 7.5 hp of 1973.

The bonding of the rubber insert on the prop spline had let go of its grip on the prop's outer casting and ended up slipping when power was applied.
What I like about THAT over a prop that is pinned with a shearpin is that it will still get you to shore under low power settings, beauty!

I sent the old prop away to get re-done and paid 50 bucks for the replacement, which is a USED one not a rebuilt one (yet).

I also looked at a new higher pitched prop (9" pitch) which has a higher angle to the long axis and should produce more speed and hopefully the same high end RPM and a faster boat overall, all things being equal.

The 9"er is a two bladed prop (and $144 which makes me hesitate to buy it) , and that should result in less friction in the water or less drag and should give similar or better high end RPM and more speed as the theoretical distance travelled by the blade in one revolution is longer then the standard 7" prop.

Whether the speed gained by that pitch is worth it is food for thought as I am not into racing with my tinfoil 12 foot and doubt that the word "racing" could even be applied in my case.

Hope that gives you a little more on the subject like you asked, I think my motor works decent and propells my boat with me in it at a decent clip, well enough to say I am a happy camper with my motor even with the added cost of the replacement prop.

Take care,
John.

 

[projecthog]

Re: 7.5hp Merc prop problem?

I just re-read your post and realized you had said that you had a 15 degree play in the rotation.
That to me would make me think that there may be a gear lash problem, and could mean worn drive gears.
In my opinion when all things work according to the way they should there should be very little play, maybe a few degrees but not 15 for sure.

Your prop may be fine, but I would try to move it by holding a socket on the nut and try to rotate the blades to see if the outer body will move,
then you'll know that there is a "slip" going on and that'll tell you its no good.

I am not super experienced with these things so my remedy or trying to determine bad or good operation of a prop may have some flaws in it, but it worked for me so it may also for you.

Gear lash excess is wear, so you would need to determine how bad it is and go from there, if that is indeed the problem.

I think of gears seals and bushings/bearings with excess of lash.

Hope that helps, Regards, John.

 

[bob1340]

Re: 7.5hp Merc prop problem?

I got out the tools and took the prop off.
The splined sleeve is fine but the rubber insert is toast.
Its not apart or anything, but you can tell it is worn out.
I put it in a vice and turned the prop separate from the splined sleeve, so now I know.John.

I pulled mine off the other day and it looks good but I want to check it further. What did you use for a shaft to put in the vise? I was thinking of using a bolt and tightening a nut up on it.

EDIT: Never mind. I found a 3/8" ratchet extension works perfect. The bushing is strong and good.

 

[bob1340]

Re: 7.5hp Merc prop problem?

I just re-read your post and realized you had said that you had a 15 degree play in the rotation.

Gear lash excess is wear, so you would need to determine how bad it is and go from there, if that is indeed the problem.

I think of gears seals and bushings/bearings with excess of lash.

Hope that helps, Regards, John.

I am in the process of taking it apart. I don't have a service manual yet. When in gear the rotation is butter smooth. I drained the oil, it was water free, dark, but no chunks of metal in it. This could be a deal breaker for me as I really don't want to dump a lot of money in this motor. Heck, I only paid $175.00 for it and a 12" fiberglass yacht. The service manual alone is gonna eat up most of my $100.00 budget!

 

[projecthog]

Re: 7.5hp Merc prop problem?

For all I know about outboards in particular, you may not have any prop problems at all, and the 15 degree lash may be acceptable, it is just my own off the cuff thought that that may be a bit too much.
I used the method I described for checking the prop, and after that I borrowed a shaft from a buddy who had a lower end apart for parts.

I think these smaller motors are a blast, I am now working on another one to buy, this time a 9.9 supposedly in good condition with no time on it as it was solely a kicker for a 22 foot starcraft cruiser of '79 vintage.

Take care,
John.

 

[projecthog]

Re: 7.5hp Merc prop problem?

Well Bob,

The price was right for the motor, but the budget is a one time spender lol.

My own manual sits here as it arrived a few days ago, I haven't even looked at it yet.

I love looking around for deals that look promising and interesting to work on, but sometimes you allow yourself to get took and then the fun is more expensive.
I thought for sure I had bought another yard display model after I bought this 7.5 (for peanuts that wouldn't even make a squirrel look), but it turned out good and I'm happy with it.

Keep us posted if you find out any more about your motor and if there are more problems then just a little wear.

Thanks, John.

 

[huntin dawg]

Re: 7.5hp Merc prop problem?

The 9"er is a two bladed prop (and $144 which makes me hesitate to buy it) , and that should result in less friction in the water or less drag and should give similar or better high end RPM and more speed as the theoretical distance travelled by the blade in one revolution is longer then the standard 7" prop.

Higher pitch lowers wide open RPM by about 200 RPM per inch of pitch.

HD

 

[projecthog]

Re: 7.5hp Merc prop problem?

Higher pitch lowers wide open RPM by about 200 RPM per inch of pitch.

HD

Ok, Thanks for the info HD,

I figured that the two bladed prop would give less friction and therefore keep the same RPM or close.

That's a good guide line to go by, so from a 7" to a 9" pitch would drop my RPM about 400 RPM?

I wonder what problems (If any) that would bring with torque and labouring? Can that work for making a faster boat, or would it be minimal and not noticeable?

Can you give a good example as to why a pitch change should be done?

I imagine a bigger weight factor could demand a lesser pitch so as to accomodate the motor output.
Am I right to assume that it works that way?

Thanks for the reply, regards,

John

 

[bob1340]

Re: 7.5hp Merc prop problem?

How do you figure what pitch you have?

 

[huntin dawg]

Re: 7.5hp Merc prop problem?

Ok, Thanks for the info HD,

I figured that the two bladed prop would give less friction and therefore keep the same RPM or close.

That's a good guide line to go by, so from a 7" to a 9" pitch would drop my RPM about 400 RPM?

I wonder what problems (If any) that would bring with torque and labouring? Can that work for making a faster boat, or would it be minimal and not noticeable?

Can you give a good example as to why a pitch change should be done?

I imagine a bigger weight factor could demand a lesser pitch so as to accomodate the motor output.
Am I right to assume that it works that way?

Thanks for the reply, regards,

John

On a small motor like the 7.5 HP a small change in pitch can make a big difference. More pitch will give more speed to a point until it drops max RPM so much the motor labors too hard. The 7 pitch is great for the 7.5 HP. I get just shy of 17 MPH with my 7.5 on my small jon boat. That's pretty darn fast. I have an 8 pitch spare that I've been meaning to try just to compare RPM and speed but haven't gotten around to it.
You might be ok with the 9 pitch since it's 2 blade. It all depends on what you are powering with it.

HD

 

[huntin dawg]

Re: 7.5hp Merc prop problem?

How do you figure what pitch you have?

It should be stamped on the prop.

HD

 

[bob1340]

Re: 7.5hp Merc prop problem?

It should be stamped on the prop.

HD

Yup, it says P7 1a right on it.