counter rotating drives benefit

[Don S]

Re: counter rotating drives benefit

I agree that getting the right prop is the best thing to do, but you also have to have the right gear ratio. You just can't use the wrong gear ratio and expect to have the best low end and top end performance.

One thing most people don't realize, is props have a range of usefulness (aka efficiency) on typical recreational boats. That is between 17 and 23" of pitch. Above that, you lose the ability to get on plane quickly, because of the extra pitch the engine can’t easily handle. Below 17 and you will have a boat that gets on plane really quick, but no top speed. Your engine also has a wide open throttle speed that is top rpm for the engine, without lugging (overloading) it.

To keep the props in that 17 to 23 range, manufacturers use gear ratio’s in the drives.
As an example, with Mercruiser Alpha drives. A SBC V8 will use a 1.5:1 gear ratio, while the 3.0L 4 cylinder will take a 1.98:1 gear ratio, and the BBC uses a 1.32. But all three may use the same exact 21" pitch prop for best overall performance on the Alpha drives.

 

[Tail_Gunner]

Re: counter rotating drives benefit

I agree that getting the right prop is the best thing to do, but you also have to have the right gear ratio. You just can't use the wrong gear ratio and expect to have the best low end and top end performance.

One thing most people don't realize, is props have a range of usefulness (aka efficiency) on typical recreational boats. That is between 17 and 23" of pitch. Above that, you lose the ability to get on plane quickly, because of the extra pitch the engine can’t easily handle. Below 17 and you will have a boat that gets on plane really quick, but no top speed. Your engine also has a wide open throttle speed that is top rpm for the engine, without lugging (overloading) it.

To keep the props in that 17 to 23 range, manufacturers use gear ratio’s in the drives.
As an example, with Mercruiser Alpha drives. A SBC V8 will use a 1.5:1 gear ratio, while the 3.0L 4 cylinder will take a 1.98:1 gear ratio, and the BBC uses a 1.32. But all three may use the same exact 21" pitch prop for best overall performance on the Alpha drives.

My son is in engineering and as we speak he has finished a hydro dynamic's class. I took a keen intrest in this class far more than what a normal father would do. In the end i was invited out of the topic...i was looking for definitve answers there are none...each application is unique aka that is why we tinker.....Even you ol Oracle...

 

[HT32BSX115]

Re: counter rotating drives benefit

As an example, with Mercruiser Alpha drives. A SBC V8 will use a 1.5:1 gear ratio, while the 3.0L 4 cylinder will take a 1.98:1 gear ratio, and the BBC uses a 1.32. But all three may use the same exact 21" pitch prop for best overall performance on the Alpha drives.

When the Bravo was introduced one year later (1988) , didn't they mate the 454 with the 1.50:1 drive?
Using a 1.47 Alpha drive would be essentially equivalent to 1.50

One thing most people don't realize, is props have a range of usefulness (aka efficiency) on typical recreational boats. That is between 17 and 23" of pitch. Above that, you lose the ability to get on plane quickly, because of the extra pitch the engine can?t easily handle.

I am currently using 26p props with my Bravo III and stock 454. If I were to rebuild and "speed-tune" the engine I would probably try 28 or 30p props.....

It currently ROCKETS up on plane with the 26p props. I don't think there's a real hard and fast rule on prop pitch (maybe it's a little different with a dual contra-rotating prop drive (like a Bravo III or DUO-prop)

 

[Tail_Gunner]

Re: counter rotating drives benefit

Yes performance characteristic's are a very big thing in a boat. There once was a time top speed was the thought of the day however today midrange punch rules for myself...The little 20' boat i run in is setup for about 57..i can prop to 63..64 but found i spend very little time there and the time i do spend is on pins and needles...(debris). However with the 4lbade's i can pop the throttle at 30 and literally jump to the mid 50's i used to just wait for on coming wake's and at pop her literally making the boat jump over the wake and some low speed airborn time...not at alll good for the drive upon futher review.