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
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.