Gearbox Ratio

[BASS]

Greetings, Does anybody know how to calculate an unknown gear ratio? A few people have commented that my prop looks on the small side and need the gear ratio to do the calculations. Can you use a strobe or similar to do this?

 

[colinmi]

Disconnect the shaft or drive coupling from the engine. Beg, steal or borrow a tachometer. Hold the tach on the center of the crankshaft pulley whist the engine is running, hold the tach on the center of the output shaft on the gear box and do the sums.

Watch your fingers. You can but these tachs for about £50 or so. Seen them in Machine Mart a while ago.

 

[aquaholic]

Best and safest way to check if you have the correct prop is to check your rpm's under normal load against the Max recommended rpm or otherwise known as WOT (wide open throttle)
e.g. if your wot should be 4600-4800rpm and you are only reaching 4000 under normal conditions (subject to the engine being sound) then that would indicate that you are under propped and need to increase the pitch and vicy vercy. However I am only basing this on a petrol inboard as you dont mention engine type, fuel etc

 

[BASS]

Thanks for your suggestions, The engine is a Perkins 6354 fitted the "wrong way round" with a velvet drive gearbox fitted foward. I cannot realease the shaft due to a siezed grub screw on the flange, so the only way to check the revs is at the prop end. Any further thoughts much appreicated.

 

[brianrunyard]

If you can turn the engine with a wrench on the crankshaft, count the turns for 1 rev of the output shaft.

 

[essexboy]

I would suggest an optical tacho, pricey but cheaper than surgery to sew fingers back on !

 

[boatdoc]

do you still have the transmission plate on the box? if so whats the number?

 

[BASS]

No plate on the box I"m affraid.

 

[burgundyben]

Ah....the great...Borg Wornout...

tell us the boat, tell us the prop size and we'll guess at the ratio....

props for the application would be wildly different between say 1:1 and 1.5 to 1 so should be obvious no?

also what shaft size? and what variant of 6.354? how many HP?

for instance, mine are 1:1, 1.25 shaft, 160 hp, 17.75 by 17.75.