Shaft sizing..

Bill

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27 Terry Jason
Got my shaft out of my 25 t Jason and it's all scored where the packing was.. I don't have room to move it forward or aft so looks like I'll be finding a new one.. Right now it's 1.5" x8ft.. I'm running a 2:1 gear with a 240 yanmar .. From what I'm told ill prob be going with a 20x21 wheel.. Any input on this? Should I go with another 1.5" or is a 1 3/8 ok? I'm not sure if I'm going back with the old stuffing box with a new cutlass or going to frp tube with a cutlass machined into it .. I'm gonna run up to h&h tomorrow if I have time but figured id get some input on here..like everything else new to all this
 

Toolate

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1998 36' Sabre Express CAT 3116TA's, 2015 ROS Scotia 16 Etec 50
Why would you go smaller? Just curious why you are even thinking of downsizing?
 

BillD

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25 Terry Jason with Cummins 370 power
Bill,

When we "sized" the the shaft for the Cummins 370 ZF63A 1.54:1 gear 1.5" shaft was suitable. The shaft on my install is 102".

I decided to go OVERKILL with a 1.75" shaft. The boat will be swinging a 20"X20" prop.

I'd stay with a 1.5" shaft on your setup.

Bill D
 

Keelboater

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Bill, what is the rated rpm of the motor? Divide that by your reduction ratio to get shaft rpm. Using Aquamet 22 material? Then use 70,000 psi torsional shear.

[(321,000 x 240 hp x3) / (70,000 x shaft rpm)] ^ 1/3 power

You should find that 1 1/2" dia. is sufficient. I took a guess at 3400 motor rpm and came up with 1.247" dia. (say 1 1/4" dia.). This includes a safety factor of 3 which is commonly used. This is for strength only and does not take into account the length.


Max bearing spacing:

[(3.21 x 1.5")/(1700 rpm)]^1/2 power x [mod. of elasticity/shaft density]^1/4 power.

For aquamet 22 I get a max shaft bearing spacing of about 5.27 feet if you go with a 1 1/2" shaft. I'm sure there will be comments made on this number. It is what it is according to Dave Gerr.
 

Bill

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Thanks appreciate it.. Oh i wasn't looking to downsize. I had someone who had a 1 3/8 shaft sitting around... I'm gonna make a few calls today.. One thing I was told is there is no other way to go besides that aquamet 22..keelboater.. Thanks for that bearing spacing reminder. I have about 6ft of unsupported length so 1.5 it is
 
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