Getting more power out of my 7.5 Merc

[Craigmac]

Any ideas for getting more power/speed from my mercury 7.5 based on the info below:

- year is unknown but its fairly old (20-30 years)
- it has a 2 blade prop
- it's a 2-stroke

Also, I've found on craigslist a 3 blade prop (from a Merc 9hp, circa 2000) for sale. It's also listed as having 9" diameter and 8 splines. Will that fit? if yes, Will that help get more speed/power?

Thanks.

 

[Bondo]

Re: Getting more power out of my 7.5 Merc

- it has a 2 blade prop

Ayuh,.... What pitch,..??

Welcome Aboard,...

 

[Craigmac]

Re: Getting more power out of my 7.5 Merc

Great to find a solid pool of boat knowledge on the web.

I'm not sure the pitch, but found online how to find it on the prop. So I'll do that this weekend.

Though I'm hoping to buy the 3 blade prop (for $35) before this weekend (if it makes sense).
Does pitch make the most difference (or more blades)? I.E. the more pitch the less power

 

[Bondo]

Re: Getting more power out of my 7.5 Merc

Does pitch make the most difference (or more blades)? I.E. the more pitch the less power

Ayuh,... Props are ALL about Pitch....

 

[Titanium48]

Re: Getting more power out of my 7.5 Merc

Stock props on the old Mercury small twins were 7 pitch 3 blade on the 7.5 hp and 9 pitch 2 blade on the 9.8 hp. Propeller pitch does not affect engine power directly, but does control how fast the engine turns at a given boat speed. Lower pitch is like a lower gear in your car - the engine turns faster at a given speed and you get better acceleration, but possibly a lower top speed if the engine reaches maximum RPM too early.

 

[cdn-pk]

Re: Getting more power out of my 7.5 Merc

i wouldn't use power and speed as the same thing. they have a inverse relationship. The lower the pitch the more thrust is created (for the same rpm) but at a cost of speed. another example is the amount of time it'll take to get on plane may increase if too high a pitch, even though eventually you may have a high speed (MPH).

as for the 2 blade vs three blade do a search around here i'm sure you'll find more info. but 3 blade is usually the best compromise. I can however say that drag (or load) on the motor will increase but vibrations will decrease (provided the props are balanced).

As for if such and such a prop will fit, we need more info, in particular engine model numbers. and prop part numbers help too.

cheers

 

[steelespike]

Re: Getting more power out of my 7.5 Merc

Old Mercury could refer all the way back to about 1947.
There should be a number somewhere maybe the mounting bracket
A 7.5 is capable of speeds around 17-20 mph in an ideal setup.
The size prop to work best for you depends on your boat and the typical load.
If you read the stickys at the top of the page and get us as much info as possible we may be able to help.Assuming the motor can spin the prop up right higher numbers give more speed less power,lower numbers more power final speed would depend on a number of things.