Prop specifications for a 1970-1971 Mercury 7.5 hp outboard

[Racee1]

I am trying to find out the prop specifications for a 1970-1971 Mercury Outboard Motor.
Such as the spline count, the diameter, and the variables or alternate props available. According to a chart I read I may be able to replace the three blade prop with a 2 blade prop and gain a little speed . Any and all support and knowledge would be greatly appreciated. Thank You in advance.

 

[merc850]

Re: Prop specifications for a 1970-1971 Mercury 7.5 hp outboard

Any local dealer that would help you?

 

[steelespike]

Re: Prop specifications for a 1970-1971 Mercury 7.5 hp outboard

Unconfirmed, prop is 9" 8 spline fits 68-85 7.5 and 9.8
Generally a 2 blade is considered faster but a modern 3 blade has better overall performance.
2 blade results would depend on what you are pushing. You might post here what your trying to do.

 

[Racee1]

Re: Prop specifications for a 1970-1971 Mercury 7.5 hp outboard

I have a fairly old 1963 desota aluminum flat bottom boat which is a fair amount slimmer than the average. It measures 14' in length and
4' at it's widest point near the center seat. We carry the basic gear, a marine battery, a six gallon gas tank, an ice chest, and a tool box. I'm not too sure the exact weight and of course My son and I . Both of us average size approximately 225 lbs. combined. I guess I'd estimate the total load at around 400 lbs. I read that a two blade prop may offer a little more speed for light weight boats / loads. I am just trying to learn and possibly upgrade for better performance. It is an old boat and motor however it runs well and serves well to bring my son fishing. It currently has an old (1971) three blade prop which I think has less pitch and also a bit smaller diameter. I'm not sure if this qualifies as a modern three blade prop? I did read that the three blade prop is applicable for more overall use. The material I read indicated that a two blade prop with a pitch of 10" and a diameter of 9" could add anywhere from 2 to 4 mph gain under a small load. I don't know what is considered a small load though. So as I mentioned I am trying to learn what I can in regards to the application and difference. For example would this difference only occur at full throttle and on plane per say. Also would changing to the two blade make start up or initial take off slower due to the lessor number of blades? Thanks for your support and knowledge!

 

[steelespike]

Re: Prop specifications for a 1970-1971 Mercury 7.5 hp outboard

I think your in 3 blade territory. A 2 blade with the pitch you mentioned might work on your boat
with one 140lb driver.Think "overdrive" speed propDon't forget to include the motor and gas in your total weight. A 2 blade isn't good for hole shot with a load nor to make speed with a load.
While your flat bottom should help get up on the water being so narrow will detract from that.
Todays Tracker 1436 with a 53" beam is rated for 3 people 140lbs each. I have a feeling yours would be closer to 2 people.
If you get yourself a tach "tinytach"and a gps and get your prop size and get us your max rpm and speed we can help get you setup for best speed possible.We can also help with your setup might be able to squeeze out a couple of mph without spending any money.