Twin engine installations question

Discussion in 'Diesel Engines' started by Mik the stick, Dec 11, 2012.

  1. Mik the stick
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    Mik the stick Senior Member

    I know twin engined installations often have separate shafts and props. I know one big engine is cheaper than two small ones. I know two to six or more engines can be geared to run together on one or two shafts. I know that gearing costs more (but engines are better geared anyway) but for a twin installation one shaft and prop would save weight. it would have the advantage that if one engine packed up you wouldn't have to do anything, the other engine would continue without drag from a dead prop which was never fitted. The Nordhaven 46 has a 27hp backup engine. Under normal circumstances that engine and prop is deadweight (and drag). I think two 75 - 80hp engines on one shaft would be better. Why is this not more common on little ships.

    I love this site I have found out a lot of info in such a short time. I am a retired engineer and the theory of boat design is becoming a hobby for me. Knowledge seems to add more questions for me at the moment.;)
    mik the stick.
     
  2. El_Guero

    El_Guero Previous Member

    Well from an engineering perspective, it would really bite to go to all that work and damage the prop, bearings, shaft, gears .....

    :)

    Other than that, it sounds COOL!
     
  3. gonzo
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    gonzo Senior Member

    Damage to the underbody gear is more likely and more difficult to repair. You can rebuild an engine in the middle of the ocean, but will be hard put to take a bent or broken shaft and replace it. From the reliability point of view, two independent systems are the best. Also, it is not more expensive to install a second shaft and propeller than running two engines together. If the engines are not perfectly synchronized, one has not enough load and it causes all kinds of other problems.
     
  4. Mik the stick
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    Mik the stick Senior Member

    I didn't think of that. when I thought about the 27hp back up I thought scrap the shaft and prop fit a clutch and electric motor in front of the engine then you could have a big generator to run in port or supply the electric motor for emergencies a bit WW2 submarine style. In the end I decided that would replace one set of problems with another.
     
  5. El_Guero

    El_Guero Previous Member

    Now, if you loaded your cat up with two pairs of engines ....

    ;)
     
  6. keysdisease
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    keysdisease Senior Member

    Here's the reverse of what you're asking:

    http://mastry.com/mec/new_products/gearedup/index.html

    Modern diesels are very reliable and given proper maintenance and clean fuel will provide a very high degree of reliability. This translates into lower initial cost, lower maintenance cost, more efficiency due to less drag than dual engine running gear, more room in the engine space, etc.

    In the Nordhaven you reference the wing engine running gear with a single simple strut and a folding prop is a very small penalty in drag on this full displacement hull. Deadweight really doesn't matter in full displacement hulls.

    Many larger ships are now launching with single engines. With these type engines you can often take pairs of cylinders off line for maintenance while underway. These are starting to gain acceptance as operators crunch the numbers and see the benefits.

    Steve
     
  7. Mik the stick
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    Mik the stick Senior Member

    The pictures on site look like a car differential driving props instead of wheels , a great idea. The gearboxes might cost a bit but nothing like a second engine.

    A problem though I could see no way to reverse one or more props. The center gearbox probably doesn't have to be a differential.
     
  8. FAST FRED
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    FAST FRED Senior Member

    To me the biggest advantage of 2 engines hooked to one shaft would be the ability to optimize the engine selection for cruising or higher speed.

    This would help engine longevity and produce the best fuel use.

    Many LST and similar come with fine heavy duty trannys that could be used, as they have SAE bellhousings , so you could mount most any combination.

    The last package I saw was 2 gov rebuilt 6-71 engines with the tranny on a pallet for $6K.

    Sell one of the 6-71 and install a 3-71 in its place. 40- 60HP forever at LRC , about 180HP at a modest 1800rpm for speedy yachting.

    With the low cost of this package , a CPP might be affordable.

    FF
     
  9. keysdisease
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    keysdisease Senior Member

    The gearboxes for each drive are shift boxes, so you can even put one in Forward and one in Reverse, although you could not change the RPM in one without also the other.

    The center gear box is just gears, in this installation with outdrives the reduction is done in the outdrive leg.

    Steve


     
  10. Mik the stick
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    Mik the stick Senior Member

    This seems the most sensible idea I have come across with reverse done by a simple gear change. how is reverse achieved, I mean is the gearbox synchromesh and reverse means pulling a lever, or do you have to stop the engine to change the gears. Do other twin engined boats use a similar reverse gearbox or do they stop engines and start in the other direction. Sorry but I don't know these things.
     
  11. El_Guero

    El_Guero Previous Member

    You could set up two diesels on one side and two electric on the other hull.

    Then place most water and diesel on that second hull to balance.

    Now you have redundant electric and diesel ....

    Add bow and stern thrusters.

    Sport boating ....
     
  12. Mik the stick
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    Mik the stick Senior Member

    I discovered diesels can be reversed but they have to be retimed, what I managed to find on the net seem like more bother and perhaps expense than it was worth. Perhaps a better idea would be to have 2 CPP linked to one control and reverse that way. The reversible wing gearboxes could then be replaced by cheaper one way boxes.
     
  13. El_Guero

    El_Guero Previous Member

    Diesel-electric ... or even better .... TURBINE electric .... buy an old Navy Frigate and take the power train ....

    HOLD ON MAMA! We are going for a test drive.

    :)

    :)

    Who needs reverse? "Make a hole! And make it wide."
     
  14. El_Guero

    El_Guero Previous Member

    PS, you can reverse engines IF they were designed to do so. Otherwise, it is problematic.

    :)

    Oh, the joys of LOVING to fly. Some engines are better designed than others for reversing them. Learned a lot about them studying WW2 planes.
     

  15. keysdisease
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    keysdisease Senior Member

    In the installation shown they are using outdrive legs which carry the reduction as well as shift function.

    Steve


     
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