Gear box question

Discussion in 'Inboards' started by Steve in SoCal, May 17, 2007.

  1. Steve in SoCal
    Joined: May 2007
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    Location: Woodland Hills, California

    Steve in SoCal Junior Member

    Hi all,

    I am thinking about a concept I have to drive two props from one engine only in a twin engine design. I have a good idea of what I need to do but, I need some information on marine gear boxes. Typically you have a forward neutral and reverse and one gear box is set to counter rotate. When the gear box is in neutral what effect would providing power from the other engine to the dead engines shaft have on the gear box.

    Instead of using diesel electric I am thinking of splitting power between shafts from one engine at low power cruise speeds. This keeps the engine running under a good load and in a range where it is near minimum BSFC. This would not be a hydraulic or electric load sharing but a mechanical connection between the driven shaft below the gear box and the driving shaft.

    Steve
     
  2. StianM
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    StianM Senior Member

    I think you have to order a custom made gearbox for this. I know there is gearboxes made for two engines, but I newer heard about one for two proppelers.

    So you want a gearbox with two engines in and two proppeler out am I right?
     
  3. Pericles
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    Pericles Senior Member

    Steve,

    Yellowfin have plans to offer this at some stage. A single engine drives two contrarotating Yellofin VSDs. The pitch and rake of the blades are changed using fly by wire controls to deliver the benefits seen on the videos. However it's not available yet. They are seeking boatbuilder partners, to work with them and the selected engine suppliers who are already on board with the project, for twin engine boats.

    http://www.yellowfin.com/pr_sportsboat_1.html

    All in all, a very exciting prospect. Of course, Duoprops are an option.

    Pericles
     
  4. Steve in SoCal
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    Steve in SoCal Junior Member

    I am talking about the other way around, I am drawing up an idea to use one engine to drive both propellers at the same time. Run one engine at say 700 HP and split the power between to shafts.

    Steve
     
  5. Steve in SoCal
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    Steve in SoCal Junior Member

    To phrase this a different way; I want to drive a prop shaft while the engine is not running. What inside the gear box is going to rotate and will it be damaged. Like coasting in neutral in a car or having a car towed with the back wheels on the road. In the case of towing the car, the transmission is turning and needs oil circulation. This is ok in a standard transmission but an automatic needs the engine running to provide lubrication. I am not clear if the out-put shaft will sling oil to the gears and shafts that are turning in the gear box.

    I have been thinking about the idea of diesel electric and Stian has given me the idea that two lighter diesel may be the way to go. I am still faced with the prospect of long segments of low power cruising and running two 800 HP engines at 12-1300 RPM is not a great option, so I have been doodling with a way to drive both shafts from one engine at a time. Instead of running two motors at 250-300HP run one motor at a much more efficient 5-600 HP.

    Steve
     
  6. StianM
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    StianM Senior Member

    Using 1 proppeler and one engine on low speed and 1 proppeler and 2 engines on high speed would have some benefits. There is alot off manufacturers making 2-1 gearboxes and 2 proppeler safts is extra cost and a single proppeler would be more economical fuel vise.

    I can't think off anny manufacturers making a gearbox with output to two proppelers so it would require a custom made gearbox, but then also all your truble is solved because you can have it made just the way you want.

    A custom made gearbox would cost a litle more, but I don't think too mutch. If the vorkshop doing the work for you have gear ratios in stock that fitt your aplication it should be no big deal, and I don't think velding a custom gearbox is mutch more labour intensive than casting. I think the moust expensive part would be the design. Drawing and calculations and all that ****.
     
  7. lazeyjack

    lazeyjack Guest

    can we start at the very beginning please
    Describe your vessel, disp, type, speed reqd, range reqd
     
  8. Frosty

    Frosty Previous Member

    Oh Oh i think I got it.

    He wants do drive 2 props with one engine.

    He wants to drive the non running engines shaft with the running engine.
     
  9. Steve in SoCal
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    Steve in SoCal Junior Member

    Give Mr. Frost a cigar, I am thinking about a way to tie both shafts to one power source. This an idea only but it would be say a 40-120 ton boat with 500-1500 horse power motors installed. I am looking at this from a view to run the engines at power settings where the load is high enough on one engine to drive both props but not at a max power say where both enegines would be at 1300 RPM, use one engine to drive both props at the same speed using perhaps 1800 RPM on one engine. You could get over load on the engine at high settings but in mid power ranges it looks to be OK.


    Steve
     
  10. Steve in SoCal
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    Steve in SoCal Junior Member

    Here is a bit more on the idea using a Cat 3406E engine as an example. You have a 50 ton boat that is on a harbor cruise and with both engines turning you are pulling 1200 RPM. The engine data from Cat shows that at 1200 RPM each engine is developing 329 HP and the fuel flow is 17.1 GPH with a prop demand of 114 HP. Using a single engine to drive both props is not going to save any fuel but it will save wear and tear on engines and keep the running engine in a desirable load range. There is a point on some engines where the BSFC is better at higher load factors and a small increase in fuel mileage could occur.

    The example shows that the engine could easily power both shafts and looking at the numbers this would work all the way up to torque peak at 1900 RPM . There would be some loss in power from transmission but the design I have in mind has no angle gears or changes in direction so the loss should be minimal.

    Steve
     
  11. StianM
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    StianM Senior Member

    Maybe you should draw it up in paint or photoshop since it would be easier to understand?
     
  12. lazeyjack

    lazeyjack Guest

    Iunderstand you perfectly now, but to keep the idle gear, from turning you would need a complex system of clutching.
    i KNOW one big engine is more than twice the cost of 2 smaller,. but really, if I were doing this and I had ROOM to swing the bigger screw, I would go for one big engine and perhaps a get me home engine that wouild also generate the AC plus a stern or bow thruster,
     
  13. StianM
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    StianM Senior Member

    [​IMG]

    You want this right, but also with two proppeler shafts?
     
  14. Steve in SoCal
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    Steve in SoCal Junior Member

    Jack,

    It is more of a way to use a twin engine boat with large engines at low speed where low load factors on the engines are a concern. The Idea is to keep it simple by de-clutching the combining transmission and letting the drive run free. Using electric motor bearings on the free running pulleys. The RPM range and axial loading are well within the safe region for these bearings with 20000 hour life. The only load would be the parasitic drag on the pulleys and bearings not more than a few horsepower. I am not so much adding a get home mode, more like a choice of using one engine at a time under low power use to get the single engine into a better load range. Over a long period of time by switching engines you save run time and idle time on both motors.

    The nice thing about the electric engine controls in the newer engines is you can limit the engine to a set RPM when the gear is engaged and I would also lock out the dead engines transmission.

    The unknown is, do the output shafts on the transmissions need the engine running to lube the output stage?
     

  15. lazeyjack

    lazeyjack Guest

    well thats easily answered call ZF OR TWIN DISC or whoever builds em, sound like you know what your doing anyways:))
     
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