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Bertram31.com General Bulletin Board
Re: Cummins Gear Ratios
Posted By: Vic Roy In Response To: Cummins Gear Ratios (Bob Higgins)
Date: Monday, 30 January 2006, at 8:53 p.m.
Most of us with Cummins 6BTAs are running 1.5 gears - due to the lower rpms and high torque they will spin a 21" prop under a B31. Most of the Yanmar guys are running 2:1 on those high rpm engines. As for trolling, my 6BTA 250's with the 1.5 gears and 21x22 three blades will idle (650 rpm) on both engines at about 5 kts, one engine about 3. I fish marlin, so 900 rpm pushes old AJ along at about 7 kts. The 300's do about the same but get about another knot or 2 at cruise since they are governed at 2800 vs. the 250s at 2600. The 330s are 3000 rpm engines, but no one without a thick wallet runs them above 2600 for any time. Rule of thumb on the 6BTAs: the 210s - turbo, no aftercooler, rated at 2600, can run them at 2500 all day; the 250s, turbo and aftercooler, 2600 rated, run them all day at 2400; the 300s (CPL 970), rated at 2800, run them like a 250; the 315/330, a 3000 rpm engine that can be run for fairly long periods at 2700. The Holy Grail is to prop the engines to run at or above (better above, say 5-10%) their rated rpm WOT with a full load on board. Bogging down a diesel with too much prop is a sure way to kill them early.
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