Horsepower limit for 1 3/8 SS shafts?

Discussion in 'Powerboats' started by sdowney717, Jul 11, 2015.

  1. sdowney717
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    sdowney717 Senior Member

    Currently have 1 3/8 shafts with 22 by 20 props three blade bronze.
    The shafts are some kind of good grade of SS shaft metal.
    How much HP - torque can be applied to the shafts?
    How many rpm?
     
  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Depends. How long are your shafts? How are they supported? What sort of motor/gear combination are you running? Does your boat run only in open water, or do you take it into a swamp every so often?

    Everybody's situation and set up is different.
     
  3. waikikin
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    waikikin Senior Member

    I think you have two questions that rely on knowing one or the other.
    there are tables available for recommendations like in here http://oa.upm.es/14340/2/Documentac...lar-Marine-Application-Installation-Guide.pdf

    Caterpillar installation guide link dont go...

    Looks like you can have around 3-4 hp for each 100 rpm off the table, but spacing of bearings & other factors affect choice also.
    Not my feild but interesting.

    Jeff.
     
  4. philSweet
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    philSweet Senior Member

    Somewhere between about 50 and 300 hp at 1400 rpm depending on the details per Naval Artichoke.

    Rpm depends on bearing spacing and is very sensitive to shaft material properties.

    You need blueprints and part numbers to answer your question with anything more than a guess.
     
  5. TANSL
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    TANSL Senior Member

    According to the attachment, your shaft can serve up to 57hp. (You can change the data in the formula according to the actual values of your axis and rpm)
     

    Attached Files:

  6. Deering
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    Deering Senior Member

    Big difference between shaft metals. Stainless 316L has a yield strength of about 70,000 psi while aquamet 19's is 130,000 psi. But the cost is commensurate with the strength.

    What kind of safety factor do you want? Assuming a conservative factor of 4, with an engine that runs at 3,500 rpm and a gear ratio of 2.5:1, you could run 57 hp with 316L, but if you jumped up to Aquamet 19 you could run something like 200 hp.

    Obviously if you have a long shaft and run in junk-infested water, you might want a higher SF, or a lower one for casual use in protected waters.
     
  7. sdowney717
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    sdowney717 Senior Member

    Shafts are about 8 feet long.
    There is only the strut at the end holding the shaft.
    The strut bushing is 6 inchs long.

    Boat is always in open water, never mud.

    I cant say what the shaft is made of, it is shiny SS looking, it is original to the boat from 1970 and has survived without any problems in salt water this whole time, so I figure it is a good metal.
    Boat is a 37 Egg Harbor. I wonder what the OEM typically used?

    I accidently stressed it one time jacking up the motor to change a front motor mount and I was looking at it and it was slightly curved under lots of pressure. It went right back perfectly straight. I know I should have disconnected the coupler.

    Trans are velvet drives with gear reduction of 2.57 or 2.75. Motors say they are 265 HP each.

    What I was wondering is if I could run around 350 HP, another 37 Egg, the owner replaced the motors with 340HP 454, and seems to be fine.
     
  8. gonzo
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    gonzo Senior Member

    Ski boats run small blocks with 300 HP and 320 ft/lb of torque. The shafts are 1 1/4"
     
  9. waikikin
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    waikikin Senior Member

    True, & even more power than that. The differences being prop diameter & rpm at the prop is significantly higher. That's what's interesting, change one parameter & everything else come out different.
    J.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    bummer...ignore this one.

    Old formulae never work at small end of limits.
     
  11. waikikin
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    waikikin Senior Member

    I cant answer if the 350hp will be ok, but to get the best out of it maybe the props need adjustment also. From your threads seem to be chasing previous speed available? Look first at the things that have changed or could change like the trim tabs or tune/fuel delivery on engines.
    Jeff.
     

  12. rxcomposite
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    rxcomposite Senior Member

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