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?
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.
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
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.