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SternDrive Gear Ratios

 

Prop Shaft RPM

 

Drive Ratio

 

1.36

1.50

1.65

Engine RPM
3500

2574

2333

2121

3600

2647

2400

2182

3700

2721

2467

2242

3800

2794

2533

2303

3900

2868

2600

2364

4000

2941

2667

2424

4100

3015

2733

2485

4200

3088

2800

2545

4300

3162

2867

2606

4400

3235

2933

2667

4500

3309

3000

2727

4600

3382

3067

2788

4700

3456

3133

2848

4800

3529

3200

2909

4900

3603

3267

2970

5000

3676

3333

3030

5100

3750

3400

3091

5200

3824

3467

3152

5300

3897

3533

3212

5400

3971

3600

3273

5500

4044

3667

3333

5600

4118

3733

3394

5700

4191

3800

3455

5800

4265

3867

3515

5900

4338

3933

3576

6000

4412

4000

3636

6100

4485

4067

3697

6200

4559

4133

3758

6300

4632

4200

3818

6400

4706

4267

3879

  In my experience, a boat will run the fastest with a higher numerical drive ratio assuming you can find a prop with enough pitch to limit the engine to the desired RPM at full throttle. This does not take into account any handling or bow/stern lift issues.

The theory is that the faster a prop rotates the more power it takes just to turn it in the water. A slower turning prop with more pitch is more efficient than a faster turning prop with less pitch even though the calculation for theoretical speed is the same for both.

The chart on the left shows the prop shaft speed for a given engine rpm/drive ratio. You can use it to see the effect of changing drive ratio, without changing the propeller pitch. The difference between a 1.36 drive and a 1.5 drive is about 10%, or 500 RPM at 5000. If your boat was running 5000 RPM with a 1.5 drive the prop shaft RPM would be 3333. If you were to change to a 1.36, find the same (closest) prop shaft RPM in the 1.36 column and you see that the engine RPM would drop to  4500. As each inch of prop pitch is about a 200 RPM change, you would have to go down 2 1/2" in pitch to pull the same RPM. Our prop calculator will help with this math.

The overall ratio is a combination of both the upper and lower ratios combined. In the case of a 1.5 Bravo the upper is 32/27 (1.185) times the lower at 19/15 (1.266) = 1.50.

The downside to a higher reduction in the upper is that there is more torque on the lower gears, vertical shaft  and prop shaft. In the case of a Bravo there is also more load on the gear floor.

Mercury has fixed many of the weak areas in the Bravo drive to varying extents with the new regular and X series

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