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HP Rating of Shafts

Discussion in 'Props, Shafts & Seals' started by DOCKMASTER, Mar 2, 2015.

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  1. DOCKMASTER

    DOCKMASTER Senior Member

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    I'm researching repower options and a little conflicted on shaft rating. I have 2.5" Aquamet 22 shafts. I was previously under the impression they were good for up to 1,450 hp. I used the shaft calculator on boatdiesel and with a safety factor of 2 it suggests I'm OK up to 2,000+ hp. Is a safety factor of 2 enough? if I go to a safety factor of 3 it drops to just under 1400 hp rating. Seems like a factor of 2 is enough? I'm looking at engines in the 1400 - 1600 hp range. So even with a safety factor of 2 I would be conservatively under max allowable. Am I looking at this wrong?

    Side note: I pulled shafts two seasons ago and did a TIR and full NDE and there were no indications of any issues.
  2. Bill106

    Bill106 Senior Member

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    A safety factor of 4 is considered minimum for pleasure craft use using diesels, commercial is more like 8. For 1450 hp. everyone I know is using 3" 22HS. Horsepower along is not enough information to determine shaft size though. Wheel diameter and reduction ratio also play a big part. I have seen a few successful installations using splines instead of tapers/keyways that pushed the limits.
  3. DOCKMASTER

    DOCKMASTER Senior Member

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    Thank you for the info. That helps a lot.
  4. Capt Ralph

    Capt Ralph Senior Member

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    Are you writing on pleasure (E+ rated) or more severe duty?
  5. Capt J

    Capt J Senior Member

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    I worked on a custom SF that used 2.5" splined shafts behind 2,000 HP diesels.
  6. DOCKMASTER

    DOCKMASTER Senior Member

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    Yes- E rating or recreational/pleasure
  7. DOCKMASTER

    DOCKMASTER Senior Member

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    So the splines replace the taper?
  8. PacBlue

    PacBlue Senior Member

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    The shaft calculation takes into account hp, engine rpm, gear ratio, the shaft material strength, and a safety factor. Have done this many times over the last 25 years and you must have a safety factor greater than 5.0 for diesels if you do not want any problems in a conventional shaft set-up. Observed breakages occur with safety factors in the 3.25 to 4 range, especially with deeper gear rations (greater than 3). High Strength Aquamet 22 helps keep shaft diameter sizes manageable. If you provide the engine data and gear ratio, I can confirm your safety factor, it is a very basic calculation.

    There are some additional factors to consider, including shaft length, distance between supports, and the ratio between shaft diameter and propeller diameter.
  9. Capt J

    Capt J Senior Member

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    Yes and the key way, and the shaft is stronger that way.
  10. PacBlue

    PacBlue Senior Member

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    The keyway fit and design is usually the weak link. This is where failures usually occur and the spline design is a way to get around this issue. The propeller should be lapped onto a standard taper to ensure the contact patch between the two tapers is greater than 90% and that it is not riding upon the key.

    There are other issues at play as well, like using full length SAE tapers for propeller hubs and strut barrels. The distance from the aft face of the Strut Barrel should be less than one shaft diameter to the forward face of the Propeller hub.

    Western Branch Metals has plenty of data/specs you need: http://www.wbmetals.com/
  11. DOCKMASTER

    DOCKMASTER Senior Member

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    Thanks for all the great info. I am aware the taper is the weak spot so that's why I assumed the splines would be here.

    I'm looking at diesels in the 1400-1600 hp range. Gear ratio will likely be in the 1.6 -2 range . My current ratio is 1.6: and existing diesels are 1150 hp. Sounds like I would need to go up to 3" shafts to be safe.
  12. Donzi 54

    Donzi 54 Member

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    Adam,
    when we repowered to 1485hp, we went with the 3" shaft. I would think thats what your going to have to do.
  13. DOCKMASTER

    DOCKMASTER Senior Member

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    That's what I figured, I'm just confirming.

    Did you have 2.5" originally and went up to 3" ? If so, do you recall if you were able to use the same struts? It looks like mine can accomodate 3" without any problem and I know 3" was an option from the factory so I'm guessing it is the same strut.
  14. PacBlue

    PacBlue Senior Member

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    Check the shaft material as well. It looks like you will need AQ 22 High Strength (70,000 psi Yield Strength in Torsion) to get a safety factor of at least 5.0 with a shaft diameter of 3.0" for 1485 bhp.

    Note that AQ 22 has only 50,000 psi Yield Strength in Torsion from 2-1/2 up to 3" shafts.

    A normal cutlass bearing for a 2-1/2" shaft is 10" long, while a cutlass bearing for a 3" shaft is typically 12" long. This is where the compromises come into play, my own preference is to keep the 4 to 1 ratio for cutlass bearing length.

    The Outside Diameters are different requiring a larger bore for the cutlass bearing in a 3" shaft set-up, the struts may have to come off to accomplish this. At this point, you can make a decision on cutlass bearing length, original vs. new struts.
  15. Donzi 54

    Donzi 54 Member

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    3" shafts were installed and the struts were remove and bored to fit new bearings. The shafts like PacBlue states are AQ 22 High Strength with a safety factor of 5.735 if memory serves me right. Another thing that had to be done also was engine seacocks and though hole had to be made larger for my engines.
  16. DOCKMASTER

    DOCKMASTER Senior Member

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    Thanks Lew. That helps. I was thinking the strut could be bored for a 3" so thanks for confirming. I hadn't thought about larger raw water intakes but makes sense. Although existing are pretty large. Were you able to stay with original 12" exhaust on each side?
  17. Donzi 54

    Donzi 54 Member

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    Exhaust system was fine, just fabricating new stainless elbows and adding bypass water pipes.