dual main engine a single output

Discussion in 'Propulsion' started by Brian.Lin, Mar 30, 2016.

  1. Brian.Lin
    Joined: Aug 2006
    Posts: 56
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: TW

    Brian.Lin Junior Member

    Dear all,
    there is a propulsion system with dual main engine input and single shaft output. there are two gearbox with different gear ratio. But, I don't know how to calculate the propeller shaft rpm. should I use ration A or ratio B to calculate the propeller shaft rpm.

    l
     

    Attached Files:

  2. baeckmo
    Joined: Jun 2009
    Posts: 1,301
    Likes: 273, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Use the following logic: The Engine that first reaches its rpm max limit constitutes the limiting factor; it is the "leading Engine".

    At that propeller shaft speed, you have to check the actual speed and Power of the "slave" Engine in order to find the total Power for propeller calculation.
     
  3. Brian.Lin
    Joined: Aug 2006
    Posts: 56
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: TW

    Brian.Lin Junior Member

    Dear Baeckmo,

    thx for your prompt.
    but, I couldn't really understand what you mean.

    if the two engine is same type, how to know which is leading engine.
     
  4. baeckmo
    Joined: Jun 2009
    Posts: 1,301
    Likes: 273, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    .....now Think: with different gear ratios to the same output (the propeller shaft), which Engine has the highest rpms at a given prop shaft speed?
     
  5. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    That is very strange , what is the purpose of the design?

    A military boat that needs the second engine for a high speed sprint?
     
  6. Barry
    Joined: Mar 2002
    Posts: 1,498
    Likes: 283, Points: 83, Legacy Rep: 158

    Barry Senior Member

    Are you suggesting that the two engines are running at the same time and at the same ENGINE rpm and connected to the same shaft???

    Without a viscous drive in the "gear box", this will not work
     
  7. gonzo
    Joined: Aug 2002
    Posts: 15,511
    Likes: 1,045, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If they were identical engines, the gearbox could be run with one or two inputs. In that case, you could treat it like a system which has an input of A HP or 2A HP. That could result in a two speed gearbox.
     
  8. Barry
    Joined: Mar 2002
    Posts: 1,498
    Likes: 283, Points: 83, Legacy Rep: 158

    Barry Senior Member

    The OP's diagram shows that the engine rpms are the same, "c"
    Say 2000 rpm.
    If the gear box on one engine is 2:1 and the other is 1.5:1,
    the output shaft would then turn at 1000 rpm and the output shaft would also turn at 1333 rpm at the same time.

    The shaft cannot turn at two different rpms at the same time
     
  9. gonzo
    Joined: Aug 2002
    Posts: 15,511
    Likes: 1,045, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I was thinking the diagram may be wrong. Otherwise, it makes no sense to run two identical engines at different RPMs.
     
  10. baeckmo
    Joined: Jun 2009
    Posts: 1,301
    Likes: 273, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Jupp, it was my intention with the leading questions to make the OP aware of that.

    Using Barry's example, with a maximum rpm of 2000 on the leading Engine, the propshaft will make 1000 rpm and the slave 1500 rpm. In this combination, the leading Engine is delivering maximum Power, while the slave is operating with a lower output (but still on its max Power curve).

    The risk with this kind of setup is, of course, that in the event of light propeller load (fe ventilating in high sea state), the slave will strive towards its limit rpm, thereby forcing the leading Engine to operate on the downregulating curve, ie overspeeding. So, the context as such is questionable, but as an example it seems to have the value of stimulating some new synapses!
     
  11. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    I think the OP may have been a student Skool problem.

    For anyone wanting to operate 2 engines with a single shaft , its cheap & OTS .

    Landing craft have been using 2 engines , side by side since WWII.

    Usually 6-71 size engines and on up.

    A lever will disengage either engine , even underway , but both must be stopped to reengage.

    A cruisers dream small engine for economy , big fuelish monster for making wakes to surf from.
     
  12. slow fred
    Joined: Feb 2011
    Posts: 88
    Likes: 6, Points: 8, Legacy Rep: 61
    Location: florida

    slow fred Junior Member

    We had a work boat with a single 4-71 with a gear box with 2 output shafts and 2 IO drives. It worked great.
     
  13. powerabout
    Joined: Nov 2007
    Posts: 2,925
    Likes: 66, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    I had a look at a cruise ship15 years ago that had 4 mains 2 props in one large gear box.
    There was a megayacht with 2 and 2
    as we know with displacement hulls it could cruise on one main driving 2 props far more fuel efficient than running both
     
  14. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "as we know with displacement hulls it could cruise on one main driving 2 props far more fuel efficient than running both"

    If the prop diameter is not a limit a single large prop would be even more efficient.
     

  15. powerabout
    Joined: Nov 2007
    Posts: 2,925
    Likes: 66, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    yep thats true
    but possibly not one engine driving that
     
Loading...
Similar Threads
  1. gauravdutta
    Replies:
    4
    Views:
    2,463
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.