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# Thread: Lower Unit gear ratio question-

1. 5000 RPM  Join Date
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## Lower Unit gear ratio question-

If a l/u has a 1.75:1 ratio is the engine crankshaft turning 1.75 times for each turn of the prop shaft or is it the other way around?  Reply With Quote

2. 5000 RPM  Join Date
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Never mind. Found it. Thanks  Reply With Quote

3. About a Year ago this was a Topic on a post. Heres my Response on Gear ratios.

Matt back in July last year you posted this. https://www.screamandfly.com/showthr...er+Gear+ratios My response was post#5. And here was my response.

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, Can the motor carry the same 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 gearratios while providing a faster top end speed will also show a more sluggish hole shot.  Reply With Quote

4. Gear ratios directly effect the torque a propellor sees. To simplify if a motor makes 100 ft lbs of torque and you have a 1:1 ratio you see 100 ft/lbs applied
to the prop shaft. Same motor with 2:1 will double the torque, but at half the shaft speed.
This is over simplified. In other words, regardless of prop in our typical application a short ratio will have a peakier or quicker acceleration and the taller ratios will have a longer more stable pull  Reply With Quote

5. Originally Posted by RBT Gear ratios directly effect the torque a propellor sees. To simplify if a motor makes 100 ft lbs of torque and you have a 1:1 ratio you see 100 ft/lbs applied
to the prop shaft. Same motor with 2:1 will double the torque, but at half the shaft speed.
This is over simplified. In other words, regardless of prop in our typical application a short ratio will have a peakier or quicker acceleration and the taller ratios will have a longer more stable pull
AMEN.... Torque multiplication makes a whole variable...  Reply With Quote

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