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Bertram31.com General Bulletin Board
Re: Bigger shafts with bigger hp
Posted By: Al Gillen In Response To: Re: Bigger shafts with bigger hp (Vic Roy)
Date: Thursday, 22 December 2005, at 9:55 p.m.
Vic, Your statement regarding gear ratio needs further qualification. The reason why people get away with 2:1 gears on 1.5" shafts is because they are hooking them up to high speed Yanmar motors. The 2:1 ratio gear hooked to a 3,800 RPM Yanmar converts the 300 HP into shaft turning at 1,900 RPMs at WOT.
Put that same 2:1 ratio on a 6B @ 2800 RPMs and that shaft wouldn't have a chance. What lets the 1.5" shafts survive the 6B is because most of us run 1.53 ZFs, which means the shaft is transmitting 300 HP at 1830-1960 RPMs (engine speed of 2800-3000 RPMs). Basically the same loading the Yanmar puts on it.
Since the shaft speed ends up the same on either combination, the wheels end up being basically the same regardless whether it's a Cummins or Yanmar motor. If you were trying to prop a 6B @ 2:1, running your shaft at 1,400 RPMs you'd need a huge prop - one you simply couldn't get under a B31 - to load down that 6B so that it only turned up 2800-3000 RPMs.
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