Small motor props

[Aaron Lariscy]

How many different props can you get for a '74 7.5? When I bought mine the guy told me it was a "speed prop" it has three blades and I know I have seen one with a two blade prop but a google search does not turn up much.

Also for this motor is 90psi a good compression reading for this motor it is even across both cylinders, this little thing seems lethargic and I am trying to investigate all the options...

 

[Bondo]

Re: Small motor props

How many different props can you get for a '74 7.5?

Ayuh,... By What manufacturer,..??

90psi is very Low...

 

[Aaron Lariscy]

Re: Small motor props

Mercury, what would it take to get the compression back ringing it or more?

 

[eavega]

Re: Small motor props

90psi is very Low...

Really? for a 7.5 HP 1974 motor? I've had motors run very reliably with 60 PSI. 90 is probably fine, given the variations between testing instruments and test techniques. On a motor that size I would assume a pull start, so there is even more variation based on technique. The telling thing is that the compression is even on the two cylinders.

Rgds

 

[steelespike]

Re: Small motor props

If you look around the net,ebay etc I think you will find a good supply of props.
A 3 blade prop works better all around than the 2 blade.The 2 blade might be a little faster with a lightly loaded light boat but as weight goes up the 2 blade fades.

 

[Aaron Lariscy]

Re: Small motor props

So in your opinion 90 psi is good for this motor? Seemed low to me as well but it is even across both cylinders. As far as props go I am looking for a lower pitch because this boat is always pretty loaded an eBay search turned up pretty much no items what key words should I be using.

 

[steelespike]

Re: Small motor props

I also agree 90 psi is pretty good and being even is important.If you want to try to improve it you could try a decarb treatment with something like seafoam just follow the directions.
As far as key words start simple "mercury prop" (propeller)probably too many results but will expose the most results.Many times a vendor will list the applications,years and hp.
The more specific your key words the more specific(limited) your results.

 

[Aaron Lariscy]

Re: Small motor props

But I am curious how many different pitches there will be for such a small HP motor. Is it worth getting a tach and checking rpms or am I not going to have enough prop choices to worry about it

 

[Art Bernard]

Re: Small motor props

You're not going to have a ton of choices with reguards to prop, but I'd say go with a 3 blade. The 3 blade will run with less vibration and give you better holeshot.

Art

 

[steelespike]

Re: Small motor props

I would say typically about 4 sizes may be available.Produced at some point,not necessarily all in production now. You probably know a iboats lists 3 props 2 two blade props unavailable, a 7" 3 blade available. Without your boat type,size weight,and the present rpm and speed theres no way to know if the few sizes would do you any good.If a prop chage is indicated a prop shop may be able to repitch your prop or modify it.Changes to a little prop may be limited.
I can tell you a 1/2" to an inch change could have a strong affect.

 

[Aaron Lariscy]

Re: Small motor props

The boat is a 1236 that stays pretty loaded my wife and I two coolers deep cycle battery three gallons of gas 4 poles and a small tackle box. I don't have a speed loaded but yesterday with just me and a battery it did 12 up river and 15 down I have since installed a jack plate to get my cav plate even with the bottom of the boat. As far as tachs go how well do the knock off tiny tachs on eBay work. I hate to spend money on a real one then find out I have no prop options or that the prop work is too expensive.

 

[steelespike]

Re: Small motor props

So it must bew good for about 13 or 14 on still water.?
The jack plate is a good move;raise it to a point just before venting becomes a problem.
If the motor was severely low you might pick up 1 or 2 mph.Should plane slightly easier.
Careful weight placement will help and you may have to change the trim setting.
Chack that the throttle opens all the way.I have a feeling your stock prop is pretty close to what you need with your load.
I thinbk I would stick with the new tinytach that replaces all three models.

 

[Aaron Lariscy]

Re: Small motor props

So it must bew good for about 13 or 14 on still water.?
The jack plate is a good move;raise it to a point just before venting becomes a problem.
If the motor was severely low you might pick up 1 or 2 mph.Should plane slightly easier.
Careful weight placement will help and you may have to change the trim setting.
Chack that the throttle opens all the way.I have a feeling your stock prop is pretty close to what you need with your load.
I thinbk I would stick with the new tinytach that replaces all three models.

The anti cav plate was almost 2" below the bottom of the boat now I have it about a 1/2" above but I haven't gotten to run it since. I may look into a tiny tach but right now I am just thinking it's not worth the trouble due to limited prop choices. I don't think paying for custom prop work is worth it it would more than likely cost more than what the motor is worth. I am just going to run it 7.5 for a while and likely upgrade to a 9.9 in the near future.

FWIW I just checked because I realized I hadn't yet and I already have a 7 pitch prop which is the lowest I think is available. So that pretty much answers my questions.

 

[steelespike]

Re: Small motor props

It would appear so.Work on the setup.with the steps I suggested.
Check that throttle opening; after 37 years the wear alone could be causing partial opening.
Please let us know how raising the motor works.

 

[Aaron Lariscy]

Re: Small motor props

It would appear so.Work on the setup.with the steps I suggested.
Check that throttle opening; after 37 years the wear alone could be causing partial opening.
Please let us know how raising the motor works.

Will do sir I am very interested in seeing how raising the motor helps as well. I am hoping it helps a good bit with such a small motor and boat little changes make a big difference.

 

[Aaron Lariscy]

Re: Small motor props

Just an update since there seemed to be some interest, the jack plate makes a drastic difference in performance. I started by just getting it level with the bottom of the boat which was 2" and it gave me about a 1mph increase but alleviated some very twichy steering issues I had. I then shimmed it up another inch and gained another 1mph but my shim job began to fail before I could do much more testing. Tonight I raised it two more inches for a total of 4 inches and Ill see what it does tomorrow.