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GM 6.2 diesels for Roamers

Discussion in 'Chris Craft Roamer Yacht' started by captdirk, Apr 18, 2009.

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  1. captdirk

    captdirk New Member

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    Has anyone ever put a GM 6.2 or 6.5 diesel in a Roamer? I have a 35' steel hull Roamer sedan, 1961. She has no engines. Since the block of the 6.2 is the same dimension as a 350 it should mount right up - but I am concerned with the height. There is only 22" from the top of the bed to the floorboards.
    I realize these engines are notorious for snapping cranks, but if properly balanced should work well, with more torque and at decent cost.
    Thanks
  2. Capt J

    Capt J Senior Member

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    I don't know. But If it were me, I would much prefer a current motor like the Cummins 6BT. The GM 6.2's haven't been made or sold for a long time and getting parts for those are becoming hard to find. The Cummins 6BT is a great motor and parts are easy to come by.
  3. q240z

    q240z New Member

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    hmmm, "notorious for snapping cranks" and "parts for those are becoming hard to find" makes me think there are better options out there. I like Cummins but they're not cheap. For an old 35' Roamer, you'd have to really be in love with the boat to justify >$30k for propulsion. Also, since I'm doing a gas to diesel conversion myself, you should keep in mind the different fuel system needs that diesels have. In general, gas tanks won't work for diesel engines because there's no return line.

    Why not just repower with gas engines?
  4. K1W1

    K1W1 Senior Member

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    Hi,

    When re powering I would have thought that to add a return line to the fuel tank would be a minor bit of work compared to everything else.
  5. q240z

    q240z New Member

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    Sure! If, that is, you can find somebody who will weld on an old gas tank and then certify it. Then again, I've heard all sorts of creative work-arounds for gas tank conversions, including running the return line to a T in the fuel inlet. There's all kinds of ways to skin a cat, though I'm not sure how an insurance company might view that.

    Though my tanks were in great shape, I wasn't willing to take a chance on an old tank conversion. If I had gone that way it would be just my luck that the tanks would have developed pinhole leaks shortly after the last work was done on the interior. lol
  6. Shangri-La

    Shangri-La Senior Member

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    I believe you would have to replace the fuel tanks on the old 61 Roamer because if I remember right you can't use a galvanized fuel tank with diesel.
  7. K1W1

    K1W1 Senior Member

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    Hi,

    You are correct about Gal and Diesel. John Deere have something to say about it too.

    http://www.deere.com/en_GB/manuals/cce/OMTCU17462_I5e/OMTCU17462_I5e/Output/OMTCU17462_I5e14.html

    Teeing the return line back into the fuel filter etc works only as long as your return flow isn't greater than your burn rate. There is also the issue of hot fuel getting sent straight back to the engine.
  8. biodon

    biodon New Member

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    The 6bt is longer and taller than the V-8s. I would vastly prefer the Cummins to the GM. If height is the most precious commodity, the 6.2 would be easier to fit. I believe the GM diesel is wider than a 350, but shouldn't cause a mahor problem.

    What transmissions would you propose to use? I do not believe you can make a 6bt run backwards like a gas engine or 2 -cycle diesel. I assume you can't run the original transmission in reverse, or they would have done that originally instead of a counter rotating engine?
  9. K1W1

    K1W1 Senior Member

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    Hi,

    Here is a pretty interesting article on reverse rotation engines.

    http://www.boatpartsinfo.com/engine-rotation.html

    We are going to use reverse rotation engines on a new 80m boat, on a previous one in the series both engines turned the same way so ( Cat "mistakenly" told us the new engines were not available in a Reverse Rotation configuration- but that's a whole other story)one Gearbox ran ahead and one ran astern when going ahead, both shafts turned inboard when viewed from astern when going ahead.

    If so desired it is a simple matter to swap the props and change the solenoid plugs on the gearbox to get them going the other way, the shaft tachos show the sense of direction so will work ok either way.

    This time we are going for outboard turning when going ahead- the build Captains preference.
  10. biodon

    biodon New Member

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    So, do you think you can replace a counter rotating engine with a regular-rotating engine and just drive with the transmission in reverse?

    My 1958 28-footer has Velvet-Drive transmissions, for instance. I haven't studied the internal working of the gearbox yet, but I assumed it would not be built for sustained load in reverse.

    What would be the advantage of outboard turning props?
  11. q240z

    q240z New Member

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    Driving the gear in reverse works, but only if the gear is designed for that operation. The Twin Disk 502s on my Roamer are speced to run either way without trouble, but the Allisons on my Connie aren't. If I remember correctly, I believe Velvet Drives can be modified to run counter-rotating, but it's not a simple swap. I considered using the VDs that came with the Roamer, but opted not to go that way after checking with the manufacturer and a gear shop that could do the work.

    Of course, YMMV.
  12. artwork

    artwork Member

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    Running one gear in reverse

    Remember that load design is not the only consideration. Many older trans' had different ratios for fwd as opposed to rvs. So balancing prop speed causes an 'out of sync' on the engines, or Sync'ing the engines make you go in circles.

    But remember the old addage - "Blessed are those who go in circles, for they shall be known as Big Wheels".
  13. captdirk

    captdirk New Member

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    I don't know what transmissions to use.
    My mech. says the 6.2 can theoretically run 'backwards", but there is too much involved to even consider it as an option. He said there's such a thing as a "paradox drive" unit, which is an extra gear for the trans. which mounts in the bellhousing, making the output turn the other way. One eng would need to be equipped with one.
    It all boils down to the prop speed - I have 20X20 props, and haven't found any info on proper speed yet. Anyone?
    As for other posts, THANKS for the info on galvanized tanks. I think a return line could be done but the wrong tank material is something else to think about.
    As far as just re-engining with gas, I was hoping to get away from the thirsty fuel burn. It is undoubtabely the most straightforward plan however. I intend to run the boat as a charter on San Francisco Bay, and could get killed by gas bills. Of course one can buy a lot of fuel for the price of new diesels. The 6.2's are tempting because of the low initial cost.
  14. biodon

    biodon New Member

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    Somebody told me about this site: http://www.miwheel.com/PropItRightSizing.aspx . I haven't tried it yet.

    You should confirm that the GM diesel turns about the same RPMs as the original engines. The Cummins prefers to turn under 3,000 RPM, so bigger props would be necessary to plane the boat.

    San Franciso is a hot spot for biodiesel. You could drum up extra business by offering biodiesel charters. There are state-of-the-art, new production plants being built there. The city uses it extensively in buses and firetrucks. The Red and White ferries use it. See more at:
    http://www.youtube.com/watch?v=L-iQaAsBhAg

    The Bay is where Bryan Peterson started and ended his 2-year, round the world trip in a 24-foot biodiesel-powered boat. My organization gave Pete Bethune an award in Frisco this Feb for his record setting trip around the world in 60 days on biodiesel. http://www.earthrace.net/index.php?section=1
  15. John_C

    John_C New Member

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    My roamer is using 6.2 diesel's to power it. The transmissions are Velvet drives, and one of them turns opposite,so the engines both run as normal. The 6.2 is not a crank snapper,that would be the 5.7. The 5.7 was an old gas engine converted to diesel,and was notorious for breakdowns, the 6.2 is actually a Detroit designed engine, and is much stronger.
  16. Clansea

    Clansea New Member

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    350's with VD

    My 35 footer has a pair of 350's with velvet drives both engine rotate the same, the 2.91 to 1 changes the rotation of one engine and it is matched with a 2.10 to 1 that rotates the same as the other engine.
  17. captdirk

    captdirk New Member

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    Glad to hear 6.2s are working in a Roamer!
    What ratio are the Velvet drives, and what prop size are you using? How noisy would you say they are? How close in size to a 350 V-8 are they? (I'll probably build a mock up to see if they'll fit). Also, how is the performance? Would you do anything differently if you had the chance? I have a 35, what length is yours?
    I know - a lot of questions, but better dumb questions than dumb mistakes.
  18. K1W1

    K1W1 Senior Member

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    Hi,

    Do both your props turn the same way?

    If you have the same input speed from the engine and different gear ratios you should if there is no clutch slip get different shaft and prop speeds.
  19. artwork

    artwork Member

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    I'm with K1W1 on that. Those velvet-drive ratios sound like the difference between fwd and rvs. Clansea, are you SURE your 350's are not counter- rotating? They were typically installed as R and L turners. I owned a marina in the 80's and had a brand new Trojan deliverd with same rotation 350's. Due to the cost of pulling the eng, we did the conversion in house. That's a long time ago, but memory says it was just a cam and distributor change.

    K1W1 - tell us more about 'outward turning' props. What is the advantage - and does it outweigh the 'denser water' theory?
  20. Capt J

    Capt J Senior Member

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    Yeah, it is pretty simple to convert a gas motor to turn the other way. It is simply a cam change, distributor change, and a reverse rotation starter, and I think freshwater pump (although that might not need changing).

    The traditional counter rotation setup is that both engines turn outward. The stbd propellor turns clockwise and the port turns counter-clockwise. This is more efficient then both having clockwise turning propellors, because the power pushes the boat foward instead of slightly off to the side.

    I remember reggie fountain experimented with having both props turn inwards instead of outwards and I think on race boats it picked up a couple of knots. However, it never spread to anything else due to less manueverability (slow speed), and I am not sure if they still practice that.