Gear ratio differences

[carolina]

Can someone help me understand the gear ratio differences as well as whats the best gear ratio for different applications?
Example 1.62 1.75 ect.... I currently have a 20' bullet 300 promax, I have 2 gear cases both torqmaster 1 of them is Titus worked and stamped 1.75 the other is a stock 225 3 liter 2000 year model. The 1.75 seems to have better hole. Just would like to know more about gear ratio and also for different applications.. thanks for everyone's knowledge. Matt

[carolina]

Pics added 5 pics are "WAS" my current gear case. From my understanding it's a torqmaster it is stamped 1.75 .I know Titus worked it and I really enjoyed this case. The skeg broke 82-83 mph we went for a ride when the boat blew out!!!

[carolina]

The other case is a back up that i need coned. Is Titus still is business and also anyone know what gear this is and type it's not stamped came off a 2001ish 225 efi 3 liter. And we're still not sure what happened to the skeg thankfully we weren't running 100's.. thanks again for any assistance Matt

[90 5.0]

For all intents and purposes you “can” prop to the same effect with changing gears, to a point.

With a 300 on a 20’ bullet most making long runs prefer a 1.62, however if you don’t make long runs often you might find the 1.75 suits you better.

The lighter the boat and load the more you can equalize prop pitch and high gear ratio, as the load and boat get heavier the torque multiplication factor of the higher gear becomes more apparent.

With your specific rig, the length of runs I made and importance of whole shot would dictate staying with your 1.75’s or trading to 1.62’s

[HStream1]

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

[carolina]

Bud thanks for the breakdown this definitely helps me understand a lot more then I knew before ��. Also 90 5.0 thanks for the set up examples.

[rude tim]

You also have the torque factor to consider. 2:1 gives you much greater Torque than 1:62.
use 200 Foot Pounds as an example. 200X2=400 Foot Pounds. 200X1:62=324 Foot Pounds.
The heaver the boat the bigger the difference.