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| Advice on Transmission Ratios |
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daycoming
New member
Username: daycoming
Post Number: 3
Registered: 03-2011
| | Posted on Sunday, April 03, 2011 - 05:34 am: | 
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Hi, hoping somebody can help.
The old engine from our boat was a 300 Bedford Diesel with a 2.2:1 gearbox ratio.
We have found a replacement engine which is a Ford Leaman with gear box and a ratio of 1.9:1
what I need to know is would the different gear ratio make much difference the boat in its younger life was a prawn trawler it is now a pleasure fishing boat.
Looking forward to hearing any advice.
Regards
Phil. |
oslin04
Senior Member
Username: oslin04
Post Number: 59
Registered: 10-2006
| | Posted on Sunday, April 03, 2011 - 10:53 am: |
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Hi Phil,
As a Marine Engineer I have done a rough culculation for you, your current 2.2 to 1 box is giving you a shaft speed of 454 rpm at 1000 engine rpm, forgetting the point in the answer,
The 1.9 to 1 box will give you a 526 rpm shaft speed at 1000 rpm, again forgetting the point in the answer,
so with the engine running a 3000 rpm the shaft would be running at 216 rpm faster on the 1.9 to 1 gearbox for the same engine rpm, and will require slightly more power to reach the same engine revs as you are getting on the 2.2 to 1 box,
Depending on the new engines bhp you might need to reduce the propeller pitch by 1 inch, only if the new engine does not reach its max running speed or near to it, but if the new engine is more powerfull then you need not worry, but try it first, propeller sizes are not an exact science, they are trial and error in most cases
I hope this helps
Regards Keith |
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