Single engine / dual shafts?

Discussion in 'Boat Design' started by dreamer, Apr 26, 2009.

  1. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    Title pretty much says it all. Would this configuration make any sense?

    Advantage:

    Maneuverability and weight distribution.

    Disadvantage:

    Only one engine! ;)
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    If the engine block is an inline six and you lay it on its side and stick six pistons in from both sides with porting in the middle you can do pretty well.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There's a number of ways you could do this, but it's not practical. Frictional lose increases, additional running gear drag, power separation loses, etc. would kill any advantages. You could spin a couple of shafts off the same transmission, but you'd lose all the benefits of having twin wheels.
     
  4. apex1

    apex1 Guest

    There is no advantage doing that. Spinning two props means cranking two engines.
    And, (not to offend) the cheapest and the best way to increase a vessels maneuverability is exercise! Learn to handle a single prop boat like all the fishermen worldwide had to learn.
    Regards
    Richard
     
  5. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    I was actually thinking two CV (constantly variable) transmissions. You could vary the thrust of each output shaft or reverse one for turning.

    Agreed the frictional losses would be an issue. Perhaps there'd be a way to do this with some sort of torque converter. Maybe that would help reduce friction?I suppose such a setup would not save any weight over two engines either.

    Probably the only advantage is that it might be a little cheaper.

    Just thinking out loud. This place is good for that.
     
  6. apex1

    apex1 Guest

    Nothing wrong with thinking loud. But that sort of arrangement has no advantage you would otherwise find it in one or the other commercial installation. And bringing more components into a system is usually counterproductive due to higher failure rate, higher service efforts and cost and the like. There is a reason why commercial ships run one engine only (not the cargo ships they do just to save money), the less stuff to service, the more likely it will be done, the more reliable the installation will run.
    Regards
    Richard
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Going through all the trouble and cost of installing two engines is rewarding only in case of an engine failure. The added manoevrability is a nice extra, but there are cheaper ways to achieve that.
     
  8. apex1

    apex1 Guest

    I mentioned the cheapest.;)
     
  9. StianM
    Joined: May 2006
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    StianM Senior Member

    Two props and one engine would be quite complex with gears.
    I think if this should be done it should be solved diesel electric.
    1 generator and two frequency converters and two motors driving fixed pitch propellers.

    A solution like that would make it quite easy to later expand it with a second diesel for reliability and adding AC driven bowtrusters.
     
  10. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    I rather have two engines (one large, one small) with one shaft... Still hard to do..
     
  11. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    let me step back a little.

    What I would like to do is keep as much of the weight near LCB as possible and as low as possible. This is easier to achieve with a single engine than two engines, however one can't deny the advantages of dual props in terms of maneuverability. A continuosly variable transmission on each shaft would not be too complex for this type of application and would allow for variability between the two props for tight maneuvering. The setup would be similar to a transfer case on a 4 wheel drive vehicle.

    the best solution would be as Stian mentioned and have two motors, each attached to rotating prop housings (as seen on modern tugs), but that puts a lot of weight in the ends where I don't want it.

    Most likely I'll fall back to dual electric motors in a traditional, shaft driven setup.

    At this point, not so interested in 'cheapest' in terms of initial investment. More interested in forward-thinking means of propulsion. Diesel electric isn't it either, though diesel generator + batteries would work in the short term.
     
  12. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    The number of shafts, props and engines have a much to do about the hull design and it uses. Not all hulls can be made single or dual. Single allows for narrower hulls, and greater efficiency in keel and rudder. But duals are better for shallower drafts and higher speeds. Form follows functions.
     
  13. apex1

    apex1 Guest

    Due to the fact that there is no such gear available "off the shelve" and that there are a lot of good reasons why that is so, I would give up the idea.
    The "rotating prop housings" are named "POD" drives or "Azipod", they must not be installed under the transom or at the aft end, but one of them is as much money as the whole conventional engine installation. And they add notecible draft.
    The "forward thinking methods of propulsion" are interestingly the steps back to good old technology as a single Diesel and a CPP for example, combined with a articulated rudder, you have a very sophisticated and economical driven propulsion that lives forever. The Diesel Electric way is not the way to go at present and bringing batteries and inverters into the game makes it even worse.
    Regards
    Richard
     
  14. Guest625101138

    Guest625101138 Previous Member

    The majority of boats are draft constrained meaning the props are smaller diameter than that required to give best efficiency. The possible exception is tugs where the prop is the most important part and the hull is designed to accommodate whatever is required.

    When prop diameters are constrained by draft there is some benefit in splitting the installed power between two props of similar or larger diameter than a single prop installed on the centreline.


    Taking an example of a 26 x 16 prop running at 1500rpm and pushing 10000N at 10kts the efficiency is 60%. Splitting the thrust between two 26 X 16 props drops the required engine speed to 1200rpm and efficiency lifts to 70%. So the result gives a 16% improvement at the prop. This gives quite a lot of room to play with extra losses from the gearing and additional appendages. There is a reasonable likelihood of overall performance benefit in certain circumstances.

    Rick W
     

  15. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Both Apex and Rick are right. After much consideration. I went with dual props, for draft reasons. Cost was similar between two smaller props/shafts and one single big one. Also get redundant engines, tranny and props. Unfortunately efficiency drops. But for coastal cruiser I think it is better. For max efficiency go with single prop. Diesel/electric good for cruiseships casino ships and military that need lots of power.

    On another note, I saw a Lazzara 80 something at Miami Boat Show. After a impressive maneuvering demonstration, I ask salesperson, "What happens if you hit something with pod".... He answered,"That is what insurance is for...." We are going to see many of these guys stuck on the rocks when they become plentiful.
     
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