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Prop Shaft Diameter

Discussion in 'Chris Craft Roamer Yacht' started by biodon, Nov 12, 2009.

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  1. biodon

    biodon New Member

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    When installing more powerful engines and/or bigger props, how do you determine when it is time to go to larger larger diameter prop shafts?

    Can you get modern alloys shafts to provide greater strength in the original shaft diameter as an alternative to refitting the hull?
  2. Fishtigua

    Fishtigua Senior Member

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    Call a respected profressional who does it everyday. There is no cheap short-cut or e-bay guru. Talk to them (some may talk like Yoda) but they do know what they are talking about.

    As an aside, funny(ish) thing happened yesterday. Just fitted new props, shafts, seals, bonding brushes, coupleings; you name it, all new.

    We went out on sea trails to check everything was OK. The engine cover was off in the saloon so that we could watch the G/B's and shafts. The weather was a bit rough and just as we came off a big swell, the stainless kettle descided to jump off the stove and in a perfect arc, shot into the engine bay.

    The kettle bounced and thrashed between both shafts and as it grow progessively smaller and more compact, we looked at each other.

    "Are you going to pull it out?"

    " 'You?"

    I think you know the answer.

    When we got back in, luckly no damage was done, though I do know what Mark, the owner, is getting for Christmas.

    It whistles.
  3. Capt Fred

    Capt Fred Senior Member

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    The website boatdiesel.com has a calculator for engine power, propeller size and shaft diameter for its members. Membership is very inexpensive and contains alot of engine mechanical information. I used the calculators when I repowered and got the expected results predicted by the calculators.
  4. biodon

    biodon New Member

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    Thanks for the tip! I am a proud new member of boatdiesel.com. I look forward to learning more from the reference material there.

    I'd like some real world dimensions/applications to go with the output from their calculator.

    My 28-foot, 10,000 lb Roamer has 1.25" shafts with 225 hp V-8s. I don't really want to change shaft diameters, but I do intend to repower it with diesels and now would be the time to do everything right.

    What's the biggest hp out there running 1.25" shafts?

    Who has ever broken a prop shaft?
  5. homer1958

    homer1958 Member

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    Shafts

    Aquamet 22 is the hardest of shaft materials, more corrosion resistant as well howver Aqumet 20 is probaly fine. Origianl CC Romaer shafts (at least from the late 60's) are very soft and suseptible to whipping.

    I have 370HP diesels 2.45 gear ratio tranny and use a 2 inch shaft on a 38 footer. Slight overkill, 1.75 is fine with most applications or you can keep 1.5 which is the standard on the 37-41's. You will need to put in larger shaft logs if you do this. Shafts that are not perfect will vibrate as well.

    HOMER
  6. homer1958

    homer1958 Member

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    1.25 it is not enough and also it depends on the transmission ratio. Higher ratios turn bigger wheels. In your application probably depending on the engine you choose will want at least 1.5" and Aquamet 22 shafts.
  7. biodon

    biodon New Member

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    My new engines may not exceed the original horsepower, but will exceed the original torque.
  8. Capt J

    Capt J Senior Member

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    Actually the answer is yes. If you change to a splined shaft. I once worked on a custom 75' Jim Smith SF with 16v2000's 2000hp DD, the boat had 2 1/2" shafts and they were splined like an outboard shaft and the propellors were splined also. Normally a boat with this HP and engines would have 4" shafts with keyways. The boat cruised at 35 knots and topped out at 42 knots and to date has not had any shaft issues (6 years).
  9. dsharp

    dsharp Senior Member

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    Prop shaft daimeter

    As long as you use a quality material such as Aquamet 22, as has been suggested, you should be fine. I replaced a pair of 318 chryslers with a pair of 6-53 turbos that, I believe were rated at 300 hp in a 31' Bertram. I want to say the shafts were 1-1/8". We used Aquamet 22 for the new shafts. I never heard any complaints from the owner. Based on my experiance broken shafts are pretty rare.
  10. biodon

    biodon New Member

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    Is whipping a factor of shaft length as well as diameter? My shafts are only 5 feet long; shorter, I expect, than the larger boats.

    I am getting ready to replace the shaft logs in the hull due to corrosion. Could I install oversized logs in the event I wanted to upgrade shafts at a later date? Would that introduce drag or turbulence that would make the packing boxes harder to do their job sealing out the water? Maybe I could engineer some removable sleeves to give me the option of different shaft sizes.

    The strut would have to be addressed later, but changing that seems like a less invasive survery that could be done separately.

    It may sound cheap, but my reluctance to upgrade is based moslty on not wanting to throw away perfectly good shafts and props I already have. Keep in mind this is only a 28-foot boat being operated on an inland lake.
  11. Capt J

    Capt J Senior Member

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    We could advise you much better if we knew the engines you currently have, and what you are planning on putting in. Both type (gas/diesel), HP, and torque
  12. homer1958

    homer1958 Member

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    Torque

    You need to know the gear ratio, the horsepower, the shat length and shaft material. STD CC shafts STINK from that era.. very soft and can whip..... you want a nice smooth ride, ad drive savers and use aqumet 22.. 20 is probably fine and way better than what CC used.. CC sometimes cut corners and sometimes did not depends on the year and model.. their great strength was their good looks and strong hulls.. by far.
  13. biodon

    biodon New Member

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    Current engines are 225 hp Nash/Gray marine V-8s with 2.59 reduction Velvet Drives.

    I want Cummins 6BTs. I haven't determined final horspower yet. I have one 160 hp, 400 lbft motor on hand. Haven't decided to find a mate for that one or source a pair that would at least match the hp of the original engines.

    I would like to go to direct drive transmissions. Eliminating the reduction unit will make space for the longer, taller diesel in the engine compartment. Less gear reduction will help match the RPMs of the slower turning diesel. however, 2.59 to direct is a pretty big jump, which leads me again to needing plenty of horsepower to keep the props swinging.

    Oh yeah, I haven't figured out if you can make B-series counter rotate or how to address that with transmissions. I would like to use 71 or 72 model Velvet Drives.
  14. dsharp

    dsharp Senior Member

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    I was going to mention that cutlass bearings can sometimes be bought with different wall thickness so you may be able to get by without changing the strut. I am not familiar with that particular boat so I don't know if it has a shaft log or a tube with a stuffing box. If you have to replace it anyway it won't hurt anything to go up to 1-1/2". Like the other guys said, some more info and a picture would be helpful. If the shafts are only 5' you could probably find some longer shafts that have worn at the stuffing box and cut them down. I don't know how your fixed for boatyards in the area but, check with them if available.
  15. homer1958

    homer1958 Member

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    1/2 inch aqumet 22 is the the right answer IMHO
  16. biodon

    biodon New Member

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    pictures are posted at http://imageevent.com/shaggy/roamer

    The struts are corroded some. I inted to weld up the pits. It wouldn't be too much trouble to cut them off and weld on new at this point.

    I figured I could weld in new logs with the existing strut and transmission output as alignment guides. Then weld on new strut with log and transmission as alignment guides. I'd be interested in better methods to make sure everything ends up in the right place.
  17. Corsair23

    Corsair23 New Member

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    Here's John C s Roamer. It has 2 , 6.2 Diesels in it. New Shafts. We were getting the bugs put of it. Sound isn't that great. 10 MPH on the GPS. The Boat has 12 " Thrust Tubes on it. I think all thats left to do ( other than getting them set up by a diesel Mech.).
    The second one is a running video from my 66 C/C Sea V 20 footer.

    http://www.youtube.com/watch?v=cu5uvnI3pLI

    Heres the other.

    http://www.youtube.com/watch?v=8MUSJTAIONg

    Scott
  18. dsharp

    dsharp Senior Member

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    We always used the high-tech method of stretching a string from the coupling through the strut. Actually that's the only way I've ever seen it done. You're going to need to get the engine dimensions. You may have to change the shaft angle to keep the flywheel housing from hitting the bottom of the hull. It doesn't look like you can move forward much and still stay under the deck. Time to start measuring.
  19. Capt Fred

    Capt Fred Senior Member

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    I have to examples to offer; I had a 34' Mainship with 1 1/4 shaft, a velvet drive 2.09 to 1 ratio turning approx 22x22 4 blade prop and all worked just fine except the trany, it distroyed the gears due particually to misalignment and a weak design. The engine was a 200HP @ 2800 Perkins 6 cylinder. I had the shaft out once and asked if I was under sized and I was told that I was not, I was told you could see stress/strain at the key slot which showed no signs of stress.

    Secondly, I currently have Cummins 6 bta, 250 HP at 2600, 1.5 to 1 ratio, ZF tranys and 1.5 inch shafts. I'm turn 22x22 3 blade props just fine. I have inspected the shafts at the prop and no stress indications. From what I have been told: diesels do not have a counter rotating versions as do gas engines. For diesels you change the direction by runing the tranny in opposite direction. Tranny like Twin Disc and ZF can run in either direction for ever, not sure about Velvet Drives, check with Borg Warner.

    Before you go direct drive, no ratio, verify that you will not lose efficiency. My understanding is that props of this size like to be turned at 1500 RPM and 2000 being the upper end. Running 6BTA without reduction may spin the props to fast to be safe and efficient.

    From my respective you will do fine with 1 1/4 for 200 HP diesels at 2 to 1 or less ratios. I would not recommend use the Velvet Drive trany or going w/o a reduction of at least 1.5 to 1.
  20. q240z

    q240z New Member

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    Strange...my Roamer came with 1.5" Aquamet 17 shafts, which many sources indicate are almost equivalent to 22 in strength. Musta been a lucky year/model. ;-)

    biodon, I'll be interested to hear how welding the pits goes. If this is an aluminum hull, it's been my experience that you just can't clean them out well enough to get a nice burn.