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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. John_C

    John_C New Member

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    The 5000V and the 5000A Velvet drives both come designed for use in boats using twins, while still using automotive rotation on the engines. My 6.2's are both running just as they would for automotive, the Velvet drives take care of the rotation. My Roamer also uses a closed cooling system that allows me leave the engines virtually identical to automotive use, even the exhaust manifolds.
  2. John_C

    John_C New Member

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    Captdirk....we are still working out the bugs, I picked the boat up with most of the conversion done, so a lot of the fabrication that went on to make them fit I am still getting familiar with. The mounts for the chevy small blocks are in a bucket on my floor though,so I know they did not work...lol
    My boat is a 1963 Rivera Express,32 feet,steel. The drives are 1.1(Velvet),and allow both engines to run the same, one of the trannies reverses the rotation.
    I am using the same props it had on it for the gas engines right now, and thats not working out so well, so I found a company in the states that can calculate what I need in props to make it perform better. If you want to see the boat running on the water,just search 1963 chris craft on youtube. The video on there is from test runs earlier, its running much better now,and huffing far less black smoke....lol
  3. Capt J

    Capt J Senior Member

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    You should be using water cooled exhaust manifolds (marine) and not automotive exhaust manifolds and risers as they throw way too much heat in a confined engine room and are a fire hazard.
  4. artwork

    artwork Member

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    It's all in the way you look at it

    We're dealing with symantics - ie. 'top of the prop or bottom of the prop' when saying turning inboard or outboard . No question that 'traditional counter-rotation' is outboard at the top of the prop. But the greater thrust is at the bottom, where the water is more dense (providing the advantage of 'inboard turning' at the bottom). My question to K1W1 was "what is the advantage of reversing the 'traditional counter rotation'?"
    I guess "the build Captains preference" could be the answer.

    Previously K1W1 said:
    "both shafts turned inboard when viewed from astern when going ahead.
    . . . . . . .
    This time we are going for outboard turning when going ahead- the build Captains preference."

    BTW - I agree with Capt J - get water cooled ex manifolds unless you have a keel-cooler and dry exhaust. But that's another story.
  5. K1W1

    K1W1 Senior Member

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    Hi,

    The use of Inboard turning Props when going ahead has shown in powering and propeller tank testing to be slightly more efficient than outboard turning and supposedly less prone to noise.

    The efficiency bit corresponded with my own experiences being on a 51m vessel with inboard turning and using 52 to 55,000 Lts of fuel to get from Antigua to Gib, a sistership with outboard turning wheels supposedly used 54-58,000 for the same journey.

    The big dis advantage of inboard turners for the average Yacht Captain comes in maneuvering in close quarters especially when against a face dock. The Captain who was involved in the project I mentioned previously had also driven an earlier one in the series with inboard turners and said he didn't like it so we have them the other way around now and a new Captain who says he doesn't care which way they go.
  6. Capt J

    Capt J Senior Member

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    I agree that they're more efficient.

    The big disadvantage is maneuverability in close quarters. Since they turn in opposite directions as what most Captains are used to. But the problem lies when one propellor is in foward and one is in reverse it washes the wash more into each propellor and the vessel does not respond nearly as quickly or as fast. The propellors will cavitate a bit from what I hear. I guess it's the difference of maneuvering a 35' outboard boat with both motors mounted fairly close together, or maneuvering an inboard boat. The inboard boat is easier to spin and such.
  7. John_C

    John_C New Member

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    I do have a keel-cooler and dry exhaust,the whole system runs through pipes along the keel,and back up through the engines using the automotive pumps.
  8. Charlie D

    Charlie D New Member

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    6.2 Diesels in a Roamer?

    The 6.2 Diesels quickly developed a bad reputation when installed in Pickups, Suburbans, etc. An engine in a road vehicle has a much easier life than one in a boat. So, with the 6.2's being bad in a road vehicle, I wonder why you would want to go to the expense & work of converting and installing lousy engines into your boat. Look for better engines.
  9. K1W1

    K1W1 Senior Member

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    Hi,

    Would you care to elaborate on this rather wide sweeping statement?
  10. Marmot

    Marmot Senior Member

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    That sounds like a reasonable debate, that might go either way depending on the road and the vehicle the poster had in mind.

    I would love to see "artwork" elaborate on this claim though:

    "... the greater thrust is at the bottom, where the water is more dense ..."

    The figures for density are very easy to obtain and it would be nice if the poster would tell us the difference in density of seawater between the top and bottom of a 1 meter diameter prop rather than expect readers to just accept this as fact.
  11. q240z

    q240z New Member

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    The analogy I like best is that every engine has an amount of power it can produce. The faster you extract that power, the sooner you'll be rebuilding the engine. This is why drag racing engines expire after one 1/4-mile run while F1 engines might last between 6-24 hours--the F1 engines are far less stressed than the nitro methane-burners. Less exotic applications, like passenger cars and OTR truck engines, last for years or even decades because the power extraction is less per unit time.

    In addition, on-road vehicles are basically only really working when accelerating and climbing hills. Their gearboxes allow them to operate at optimal RPMs in normal usage through a relatively large speed range. Get a car or large truck up to cruise speed, throw it into overdrive, and you continue at cruising speed with relatively low throttle input. Compare that to a boat, where there is no such thing as downhill (in any practical sense), most propellers are fixed, and cruising along requires somewhere around 3/4 throttle for every hour of operation (to say nothing of the salty, wet environment and long spans of disuse in most areas), and it seems clear that on-road engines have an easier life than marine ones.

    If I had to chose between engines of a given power rating, longevity and reliability would absolutely factor into the equation. If, in fact, the 6.2 had known issues on the road, it's unlikely that it would have fewer issues on the water.

    As to Artwork's density statement, I suspect near-hull turbulence and perhaps air bubbles have something to do with it. Let's see what he has to say.
  12. Capt J

    Capt J Senior Member

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    In general I would have to agree for a non commercial vehicle and a non commercial boat. IN GENERAL, Engines in most road situations are geared so that they run a lighter load factor most of the time instead of 80% load for hours on end. They're not injesting salty air. They're used much more often then a recreational boat. The fuel is generally fresher because of the use, algae in the fuel of a diesel road vehicle is almost unheard of. There generally isn't as much condensation on land as there is in an engine room on a boat. The exhaust on a road vehicle is dry and you don't get condensation when it sits from water in the risers.
  13. artwork

    artwork Member

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    out-turning propellors

    Geez, guys - I said I learned it a long time ago - like 40 years ago.................. But, I looked it up - in Dave's Gerr's 'Propellor Handbook' p.20 quote:
    "The water at the bottom of the propellor is a bit denser and freer to flow (there's no hull above it) than at the top of the propellor. This makes the lower blades a bit more effective, so the propellor and stern "walk" sideways in the direction of rotation."
    " On a twin screw craft, the propellors should be out-turning. The starboard or right propellor should be right-handed, and the port or left propellor should be left handed. This gives the best efficiency. Twin-screw vessels with propellors of the same hand can experience serious handling problems."
    Unquote
    Dave Gerr is a naval archetect (and author of 3 or 4 books) and now a director of Westlawn Institute of Marine Technology, the education arm of ABYC - really, I didn't make this up. I also didn't calculate the difference in density. The point is that it is a 'bit' more, causing 'walk'. And I can attest to it, having owned several single screw boats that I could turn around in little over a boat length using that 'walk'.

    Now, I was open minded asking why the reverse turning was done, looking for a scientific answer. 'Customer's preference' is an acceptable answer, just not scientific enough for me to build mine that way. So I can accept differences of opinion, I learned this stuff years ago and never questioned it - til now??. should I ??
  14. Capt J

    Capt J Senior Member

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    Reverse turning causes the wash of both propellors to combine, you're pulling water from the exposed side of the hull (not the center where there could be a keel and the v is deeper) and used to pull the boat foward and therefore a little less thrust is lost to the sides of the boat and creates a little more lift from what I've been told. Increased speed/efficiency was a small %. I think back when Reggie Fountain was messing with it....They gained something like 3-4mph on a 140mph boat, at the expense of turning radius and such, and he deemed it only worthwhile for setting speed records and not actual racing with turns.
  15. q240z

    q240z New Member

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    C'mon, Art...it's not like you're working on your Roamer. What else have you got to do with your time?!? lol
  16. artwork

    artwork Member

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    reverse turning props

    Capt J

    OK, I can see the theory in that - Deep V particularly, and at high planing speeds - Makes sense. Shows there is something new to learn every day.

    q, right-o. gotta take one day off a week. :cool:
  17. biodon

    biodon New Member

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    I want to make sure I am getting this.

    When repowering with enignes that both turn in the same direction, you can change the valve in one of the Velvet Drives (except the 1.9 ratio) so that it will work with the new engine. This just means the transmission operates in that direction, it does not reverse the output shaft direction relative to the engine. With no other adjustment, you will have both prop shafts turning in the same direction.

    To get the props counter-rotating, you actually have to run one transmission in reverse. This means the sun and planetray gears are always in motion in that transmission, in contrast to the forward gear which simply locks the input and output shafts together. This would seem to cause more moving parts, more friction, heat and wear in the transmission that is always running in reverse. It sounds like this may be an acceptable compromise providing the transmission can handle the stress? Is this fairly common practice?
  18. John_C

    John_C New Member

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    Charlie I did not notice this post at first,so sorry for the late reply. The engine that developed such a bad reputation was the 5.7, often confused with the 6.2. The 6.2 is a true diesel, not a converted gas engine, and is much tougher than the 5.7. I did a lot of research on these engines,and although they are not great pullers,compared to a 6.5,but they are reliable under continuous use.
  19. biodon

    biodon New Member

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    I'm deducing that instead of "2.91 to 1", this should read 1.91 to 1?

    In reading the Velvet Drive literature, it appears the 1.91 ratio is the reduction unit with output rotation opposite of the input rotation. All others spin the output the same direction as the engine.
  20. Henning

    Henning Senior Member

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    That's actually untrue in practice though. It's really an enhancement to handling, especially with no bow thruster. With inboards, you can give it 20* Port helm, put port in forward and starboard in revers and make the boat walk straight to starboard. If you give her full port helm, she'll actually turn to port in that configuration while walking to starboard. The thing is though that you have to use the rudder when backing. You can't just use starboard prop to pull the stern to port and port to pull it starboard because the prop walk counter acts the offset thrust, so you have to use rudder at speed and prop wash from the outside in Fwd at low speed. Once you understand it though, inboard turning screws (when viewed from the aft) is a much more precise handling setup. Lot's of the bigger oilfield boats in the days before bow thrusters were set up like this, many still are.