Gear ratio

opunui · Post by **opunui** » Aug 27th, '08, 11:22

What is the difference between a 1:5 to1 vs a 2:1 in shaft rpm from your transmission? Thanks again.

Harry Babb · Post by **Harry Babb** » Aug 27th, '08, 11:37

1.5:1.......Shaft will turn .6666 times engine RPM
2:1..........Shaft will turn .5 times engine RPM

Example 2000 Engine RPM....1.5:1 gear....shaft will turn 1333 RPM

2000 Engine RPM....2:1 gear......shaft will turn 1000 RPM

Harry

In Memory Walter K · Post by **In Memory Walter K** » Aug 27th, '08, 11:59

Usually the 2:1 tranny turns a higher pitched prop at the lower RPM's

CaptPatrick · Post by **CaptPatrick** » Aug 27th, '08, 12:02

Selecting the proper gear ratio is also dependant on which engines, rated RPM, & horse power you're coupling the transmissions to...

For example, the 320 - 330 HP Cummins 6BTA needs a 1.5:1 ratio, but the 315 Yanmar 6 LP/LY or the 4 LH needs a 2:1 ratio. Basically, low RPM engines need the 1.5:1 & high RPM engines need 2:1...

Br,

Patrick

opunui · Post by **opunui** » Aug 27th, '08, 12:06

Thanks Walter and Harry. one more ?. if i were to put Yanmar 315s in my 31 what is in your opinion the best gear ratio. the boat is light no gen set or things like that. I would like to see a wot of 32kts. but if possible be able to live bait on the first click.Thanks again so much.

CaptPatrick · Post by **CaptPatrick** » Aug 27th, '08, 12:13

Uh, hello....

315 Yanmar 6 LP/LY or the 4 LH needs a 2:1 ratio. Basically, low RPM engines need the 1.5:1 & high RPM engines need 2:1...

mike ohlstein · Post by **mike ohlstein** » Aug 27th, '08, 12:25

Although I'm fairly well convinced that the Yanmar 230 has enough grunt to go somewhere in the neighborhood of 1.85 to 1. I think that it would allow for the use of a square prop (20X20) while maintaining cruising speed and proper engine loading, without burning the prop tips.

Just my 2 cents......

CaptPatrick · Post by **CaptPatrick** » Aug 27th, '08, 13:18

Mike,

Are you getting any burning with your setup?

scot · Post by **scot** » Aug 27th, '08, 13:53

I realize that HP "is what it is" but, doesn't torque at the shaft increase by adding reduction? I was under the impression that 2:1 gear basically doubled the torque at the prop? 1:5 added 50%, etc. and this was why boats with stepper reductions were less effected by load, or sea state.

Assuming the engine/gear combination are within reason and prop pitch + shaft rpm + slippage provides the desired hull speed.

Should I check into rehab?

opunui · Post by **opunui** » Aug 27th, '08, 15:19

Cap'n Pat if you look at the time you posted and my next question that was posted you will see why I missed your reply. Thanks though.

Bruce · Post by **Bruce** » Aug 27th, '08, 15:38

Some of you guys must be using chinese calculators.

Harry Babb · Post by **Harry Babb** » Aug 27th, '08, 20:56

Some of you guys must be using chinese calculators

Bruce......how did you know??

[img][img]http://i147.photobucket.com/albums/r289 ... abacus.jpg[/img][/img]

Harry

Charlie J · Post by **Charlie J** » Aug 27th, '08, 21:13

iam running 1.5 trannys with the yanmar 230LAH ST, on a bahia mar
mike you have the same engines on your fb whats your ratio

mike ohlstein · Post by **mike ohlstein** » Aug 28th, '08, 13:19

CaptPatrick wrote:Mike,

Are you getting any burning with your setup?

No. 20X22 four blade 2:1

nic · Post by **nic** » Aug 28th, '08, 23:07

Opunui,

We have Yanmar 6LPs 305hp (315hp with lower Cetane rating here) with 20 x 23 four bladed and we get 30.5knots wot regularly, but tell everyone 32 at the pub and get away with it.

Locals are getting better top speed results with smaller dia props and suggested 19 x 24s but we went for the size most commonly recommended on this site and are very happy. Massive grip on the water...idle is 6knots approx.

The most important thing is to max at 3900+ revs, we get 3940 & 3960 or so, for long engine life.

Nic

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