Mercury Lowerunit Gear Ratios / Performance

[Bayside]

Good day, I need some help in deciding which gears to get for my Sportmaster LowerUnit. I’m stuck between the 1.62, and the 1.75. I would like someone to explain what performance each gear would give. I have a 21 ft shadow with a 300xs. I would like some help in helping me pick the right gear set for my boat.

[RiverJunkie702]

What was in it before? What pitch prop are you running? If you for instance had the 1.75 in it before and your prop was perfect at the max recomended rpm range. If you went to 1.62 you’d likely have to switch to the next lower pitch size prop. You understand?

[Bayside]

The Lowerunit originally is a 1.75. But I blow the Lowerunit. Now I‘m about to rebuild it but my friends was telling me I should try a 1.62 Ratio and I’ll get better result in speed but I does use 30 pitch chopper and a 28 signature prop. I spoke to the mechanic and he told me that if I use 1.62 I’ll get some more speed but I wouldn’t to able to carry a loaded boat of people, once I’m sailing and I power down I wouldn’t be able to catch plane as quick as with the 1.75. So base off the conversation I’ll rebuild it to the 1.75

[HStream1]

Heres a response I posted a couple years back when this same question popped up.

There are 4 lower merc gear ratios.

2:1, 1.87:1, 1.75:1, 1.62:1.

It works like this:
2:1 is two engine rotations to one Prop rotation

1.87:1 is 1 7/8 engine rotation to one prop rotation

1.75:1 is 1 3/4 engine rotation to one prop rotation

1.62:1 is 1 9/16 engine rotation to one prop rotation

In theory heres the difference, and Ill use 5000 RPM and a 26 pitch prop as a base line for the example.

The formula to calculate Theoretical speed is:
RPM X Pitch =
Ratio X 1056 =

2:1
5000 x 26 = 130000 = 61.5 MPH @ 0% slip
2 x 1056 = 2112

1.87:1
5000 x 26 = 130000 = 65.83 MPH @ 0% slip
1.87 x 1056 = 1974.72

1.75:1
5000 x 26 = 130000 = 70.34 MPH @ 0% slip
1.75 x 1056 = 1848

1.62:1
5000 x 26 = 130000 = 75.99 MPH @ 0% slip
1.62 x 1056 = 1710.72

So as you can see in theory by using 5000 RPM and 26 Pitch as a Base line the different ratios show around a 5 MPH difference. Now many variables come into play here, Boats weight, Hull design, Prop efficiency (% percentage of Slip), Can the motor carry a taller pitch pitch prop all the way through its maximum RPM range?

Now lets look at it a different way changing only prop pitch by 2 (ex 24 P vs 26 P) for this example ill use 1.87:1 as the base because as shown in the theory above the different ratios equate to around 5 MPH.

5000 x 26 = 130000 =65.83 MPH @ 0% slip
1.87 x 1056 = 1974.72

5000 x 24 = 120000 =60.76 MPH @ 0% slip
1.87 x 1056 = 1974.72

As you can see by merely changing the prop by 2 pitch sizes the MPH difference is almost the same @ 5 MPH difference.

Again many variables come into play and the above is based on Theory not real world results.

Also the tighter gear ratios while providing a faster top end speed will also show a more sluggish hole shot.

To calculate Slip take Theoretical Speed minus Actual speed = % slip.