Is 1" propsshaft enough for 90hp diesel in 23' alumium CC

Discussion in 'Props' started by Northeaster, Aug 25, 2013.

  1. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Hi Folks,

    I realize that I ideally one would figure out prop size/ boat design/engine and gear choice all togteher. But, I am now considering an older Volvo Penta, in good condition, with a Hurth 1.65:1 gear and some extras thrown in like 1" shaft, stuffing box, and prop, of which I don't know the size yet.

    Main questions

    - Is 1" propsshaft enough for 90hp diesel in 23' alumium CC

    - are wide range of sized props available in 1" shaft - or do they tend to use large shafts like 1 3/8" etc, when they start getting larger.

    A guy on the glen L site runs his similar 25' boat with a 130hp Mercruiser and 2:1 Velvet Drive and has a 16" prop (not sure of pitch)

    May buy the engine / trannie anyway, but wanted to know if I would just sell the shaft and look for larger diameter, or if it would be OK.

    Any help appreciated!
     
  2. gonzo
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    gonzo Senior Member

    Do you know what the shaft material is?
     
  3. Northeaster
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    Northeaster Senior Member

    just emailed the seller to find out. The pic of it is small but it looks shiny - so thinking more likely stainelss than bronze.
    If it is stainless, would that be OK?
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I've used 1" shafts, 316 stainless, for VW 1.9 TD diesels with 2:1 reduction. It never crossed my mind to take a larger size.
     
  5. Northeaster
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    Northeaster Senior Member

    thanks for the info. It is not a deal breaker, but would be a bonus if the shaft, stuffing box and shaft coupler were all usable.
     
  6. Northeaster
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    Northeaster Senior Member

    confirmed - yes, the 1" sgaft is stainless. Has a couple of bronze props, 13" and 14".

    So, do you guys think the shaft will work?
     
  7. johneck
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    johneck Senior Member

    Torque is what matters; what is the RPM?
     
  8. jonr
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    jonr Senior Member

    > Torque is what matters; what is the RPM?

    While I agree, diesel vs gas also makes a difference. Diesels have more torsional vibration. I suppose that even number of cylinders may have an effect (a 25 HP 1 cyl engine may have instantaneous torque values equal to a 200 HP 8 cylinder engine (not that either are common in boats)).
     
  9. TANSL
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    TANSL Senior Member

    In order to help, I attached the Bureau Veritas formulas for calculating the propeller shafts.
     

    Attached Files:

  10. Northeaster
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    Northeaster Senior Member

    Last edited: Aug 27, 2013
  11. johneck
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    johneck Senior Member

    Using the formula provided (which is the same for ABS) a 1" shaft does not meet the requirement for that power and RPM. If the shaft is some very high strength 17-4PH material (~120 ksi tensile), the required diameter would be 1.1". If the material is of lower strength ~(90 ksi), then the required diameter is 1.2"

    Now the rules do provide for a reasonably high factor of safety; so if you have a good situation regarding torsional vibrations and overhung momoent, or don't run very often at full power, it may be OK. But you will be in a situation where the stresses in the shaft are higher than recommended and the probability of fatigue failure exists.
     
  12. Northeaster
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    Northeaster Senior Member

    Thanks very much for the reply, and thorough explanation! If I decide to buy that engine/trannie and shaft material, I will still look around for a larger shaft and prop, as I will not require them for another few months.

    Thanks again!
     
  13. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yes, a 1" shaft is adequate for your application, while still having a reasonable safety margin.
     

  14. Northeaster
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    Northeaster Senior Member

    Thanks for all of the help!
     
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