Grog

My 271 Formula Fastech Bravo 1 has a 1.65:1 drive ratio. I began to consider the possibility that a lower numerical drive ratio would be more efficient since most powerboaters seem to run 1.50:1 or 1.36:1. A search took me to the BAM website where I found the following quote:


"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."

If this is true, why are more manufacturers using the 1.65:1 ratio from the factory and why isn't the 1.65:1 more popular? Especially since props are widely available to 32"+.
Anyone have an opinion on this?

rdoactive

Torque multiblication and the possibility of broken propshafts?
Also, props around 30" dont have very good planing or low speed characteristics.
Gary

Rebel_Heart

I know some of the Fountains now come with the 1:1.65 ratio. They also have a much higher x-dimension than in the past. The result if a higher top end - makes great advertising! They don't tell you about how long it takes to get on plane or how long it takes to achieve their top speed.

racinfever

Most of the performance boats with 165-1 that i have seen also have to high X dem. causing lots of slip. To keep the motors off the rev limiter they use high pitch props ,causing poor low & mid range performance, the 165 / 1 ratio helps low end. Space your drives back down and use a lower pitch prop. Do you have a 30 or 32 on there now ? check your slip !