Gear Ratio

[CHB RON]

Does anyone know the gear ratio of a Perkins 4.236 85 HP dual engine in a CHB 35? I'm trying to figure out what prop I need.

 

[DavidM]

Even if someone reports the gear ratio in their CHB with that engine, I wouldn't base a prop purchase decision on that info. This was CHB from 30 or so years ago remember.


But you can easily figure the ratio with a phototach. Rev your engine up to about 1,000 rpm and measure the engine rpm and then without changing anything measure the prop shaft rpm. Divide the two to get the ratio.


David

 

[dhays]

That info may also be on the transmission somewhere.

 

[Ski in NC]

Do Dave's trick, but on both. Lots of BW gears out there with 1.91:1 ratio on one side, 2.10:1 on the other.

 

[Aquabelle]

Eric.....why did/do they do the different ratios?
Thanks in advance for explaining.

 

[Ski in NC]

It was a byproduct of getting counter rotation. Both engines and gears spun the same direction, and the bell shaped reduction units on the back of the gears had planetary gearsets for both reduction and to swap rotation. Apparently they could not get the gear tooth counts right for exact same ratios. Some ZF/Hurth boxes have the same issue. Can be annoying.

Not an issue if engines are counter rotating, but that creates annoyances of it's own.

 

[Aquabelle]

Thanks as always Eric