Single engine / dual shafts?

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

  1. apex1

    apex1 Guest

    What are we talking about here?
  2. Jim_Hbar
    Joined: Sep 2006
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    Jim_Hbar Junior Member


    I believe the initial point we were discussing were CVT's and and the possibility/merits of using them to drive two fixed pitch propellers.

    BTW, I am not trying to be argumentative, but I respectfully suggest that you are not reading the intended meaning of my words. I'll try to get my point across again.

    In my initial post, I stated, (with emphasis added):
    In my second post I stated (emphasis added):
    Furthermore, I've mentioned the Yellow Fin drive system, and provided a link to video of the FastTrack amphibian.. Both of which I believe to be rudderless, differential steering drive systems with one engine, and two final drives, in marine applications (which I believe we can agree on are not "mainstream"). I additionally provided other examples of one engine with two final drives, and you, a few more, which are not "mainstream marine propulsion". The point I'm attempting to make is that the technology exists, but is not currently used by the mainstream marine industry.

    I would define "differential" steering to be where the thrust of the two outputs are varied in a precise, controlled fashion, to effect steering of the craft.

    The one question you have not asked is where I believe there is a possible (note I used the word "POSSIBLE", not "probable", not "certain", not "future") application of such a drive system? I can see it having theoretical technical merit in a high speed surface-type drive system, in a relatively small light craft.

    I would suggest that having precise control of the relative thrust from the two prop shafts is necessary for differential steering - this can be accomplished by several means, one of which could be twin CVTs, another is twin CPPs, a third way would be a twin engine set-up with closed loop control of the engine(s) speed. There are other ways to accomplish this also.

    The FastTrack video shows differential steering - it could be done in a similar fashion if the final drives were propellers instead of tracks..

    The benefits of the single engine, twin prop approach, as I see them, listed in approx. order:
    1) The ability to swing two props, with twice the "disc area" of a single, with similar or reduced draft, and the resultant prop efficiency gains - see Rick's post #14 above.
    2) With any variable drive system, it then becomes possible to load the engine for better engine efficiency in normal operating ranges.
    3) With two CVTs, (or CPPs), it then becomes possible to differential steer the craft at speed. And with differential steering, rudders are possibly not required, particularly at speed.
    4) With CVTs and fixed props (compared to CPPs), the propeller efficiency is potentially better across the full range of speed, and surfacing drives are a possibility. (I don't see surfacing CPPs as being currently economically viable, and I understand that CPP blade geometry is optimized for one particular pitch)
    5) Possible overall reduction in drive weight, when compared to twin installations.

    The disadvantages of twin CVTs:
    1) Most designs being developed for automobile use do not use the mechanism to reverse - a separate gear change occurs. CPPs are superior in this regard.
    2) The CVT technology is currently not mature, and has not been applied to marine applications.
    3) Mechanical complexity, (and thus potential reliability issues), and cost.

    The disadvantages of single engine, two props as a concept:
    1) Mechanical complexity, (and thus potential reliability issues), and cost.

    The advantages listed above would most likely result in a more "fuel efficient" craft, when cruising at speed, as compared to conventional twin engine, twin fixed pitch prop, rudder system. It is doubtful that it would be "capital efficient".

    As an additional note, I suspect that the torque impulses from surfacing propellers would quickly destroy existing CVTs.

    Are these drive schemes appropriate for displacement craft? I rather doubt it.

    But, in the interest of moving the thread along, let's say the application has two Power-Vent like surface drives, counter-rotating props with a single engine, and a CVT driving each prop shaft. The objective of the craft is to plane with best possible fuel efficiency, and ultimate top speed is not a design objective. The craft would be in the 8 to 10 metre range, and weight is to be minimized.

    I suspect that there may be challenges steering such craft when in displacement mode, without rudders, but could you please explain why such a "differential steering" drive system, as I've spelled out above, cannot work in such a craft at cruising speed?

    Seriously, I do not see why it won't work at speed.

  3. apex1

    apex1 Guest

    Sorry, now I see much clearer what you wanted to express! And I have to agree and excuse!

    No, I cannot explain that, it should be possible. A boat that is not (or nearly not) steerable at displ. speed remains undesirable though.

    But apart from all those, partly sophisticed, partly more theoretical solutions, if we brake it down to the first question, then I still see absolutely no benefit in such a installation. A twin engine CPP arrangement is for shure not more expensive but very efficient, reliable and off the shelve. The minor disadvantage of the wider weight distribution (dreamer made that a issue) one can almost forget, I think.

  4. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    I seldom see a boat with a rudder that is too big. A intelligent computerized electric drive twin prop could work. But then the computer has to be constantly adjusting for yaw. People forget water is a constantly moving thing. Even in a heavy ship - it moves sideways and need constant rudder adjustment to keep bearing. It seems to me a lot of people in this board with great ideas have never been on a real boat...
  5. john.G
    Joined: May 2006
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    john.G Junior Member

    Think outside the square.

    Without reading the entire thread through consider this:

    Have your main run a hydraulic pump and have this in turn driving a hydraulic motor on each shaft.

    Engine can be located anywhere you like, and can be mounted at any angle to the longtitudinal plane.

    Engine can be run under constant load at all times.. prop speed is governed by regulating the oil flow to the drive motors not by engine RPM. This makes the engine operate at best fuel efficiency.

    Compact hydraulic drive motors can be fitted in just about anywhere... means short shafts can be set low down to provide flat shaft angles and increases prop efficiency. All you have to do is run the lines from the engine/pump unit to them... one in and one out.

    The technology is already out there, is cheap, and is operating under severe service conditions on extended service intervals. Conpanies like Hydash, Cat and Hitachi build and fit these motors to excavators every day. The life of a walk motor between overhauls is about 6000 hours, and they aren't hard to overhaul.

    Weight is light, particularly if you water cool your oil. You don't need 40 gallons of oil if you can cool it quick.

    Reliability is very high. If you don't check your hoses and replace them when required and one blows.. you can shut it off and come home on the other because they are on seperate circuits of the same pump.


    Noise... go stand next to a 20 t excavator when its walking.
  6. john.G
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    john.G Junior Member

    oops.. I see someone has already raised this as an option.

    Regarding hydraulics.... if you worry about them on a boat, never ever go aboard a working trawler. Or any vessel with a winch thats not electric... hydraulics only leak if you don't maintain them. Same as with engines and transmisssions.
  7. john.G
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    john.G Junior Member

    And on rudders..

    Propellors push water, rudders only direct it a little.

    Thats why on a game boat when you need manoverability you face backwards.. sit your *** on the wheel to hold it amidships, and drive her off the throttles.

    BUT without a rudder on a long course you would have to constantly adjust heading via the throttles... tedious at best ( I brought a twin engine barge home 600 miles once after the rudders were damaged)... and you can't slave the throttles to an autopilot.
  8. peter radclyffe
    Joined: Mar 2009
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    peter radclyffe Senior Member

    your saying the majority of boats, dont have the right props,how many types of boats have you studied , i challenge you to take this up with prop makers,
  9. Boston

    Boston Previous Member

    your barking up the wrong tree on that one Pete

    Rick and I may not have agreed on more than a few things in the past but one thing this guy does know is props

    Ild bust out the eraser and modify that last if I were you

  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Actually Rick is quite correct. Not only are draft restrictions an issue but most boats aren't prop matched to their employment.
  11. Guest625101138

    Guest625101138 Previous Member

    Until I spent the effort to understand propeller physics I thought there were fundamental efficiency based reasons for the difference in shape between airplane props and boat props.

    When you understand the physics you would always go for high aspect blades and low velocity ratios if you wanted efficiency and this was the only requirement. The only way you can do this in most boats is to go to much larger diameter than you actually see and much lower area ratios than you actually see.

    You do not need to take my word for it. Just take a look at the Wageningen "B" series data as per attached:
    This is a standard design but only two blade and 0.3 area ratio. Not like most boat props but a tested design. So it is not unknown by prop designers, rather simply not widely known as your belief is not unusual.

    I even have higher aspect, around 15% area ratio, and can get high eighties efficiency with low speed operation around 6 to 7 knots. At higher speed, say 10 to 20 knots you can get efficiencies in the low nineties.

    From what I have seen there are not many people building boats versed in prop design from fundamental physics.

    The pleasure boat market has not been overly concerned about efficiency up till a couple of years ago. Efficiencies of the order of 50 to 60% has been acceptable and most boat designers/builders believe this is just the way it is.

    The practical problem with using such propellers is the draft required to swing the required prop diameter.

    If you want to get a grasp of general prop design take a look at JavaProp. It is based on the fundamental physics, not extrapolated output from limited test data.

    Rick W
  12. kapnD
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    kapnD Senior Member

    The concept of two shafts from one engine makes sense on boats where draft and manueveribility are issues. Mr. Willoughby has run the numbers on it for us, and it looks good, though one must understand the constraints of the system and match them to its intended use.
    A west Florida builder (was it Dolphin?) turned out plenty of these on a nice 40' lobster hull. The single 500 hp diesel was low and midship, and the in-deck fishhold was cavernous. I talked to several owners and visited their shop, and was very impressed. Nobody was complaining about fuel hogging or rudder drag! They accomplised the "split" with a tooth belt drive in a box beam off the bobtail, running seperate transmissions on each shaft. It looked and felt stout, but was much lighter than twin 250's.
    Kiko Villalon's IBEX Pro Boat 34 is set up this way, and I'm sure he's not just blowing smoke at us.
    The article is in the Feb/Mar 2009 issue of Professional Boatbuilder if you'd like to see particluars about his design focus.
  13. apex1

    apex1 Guest

    Interesting comment, thank you.
  14. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    OK, the coffee's kickin in, the builder's name was Dorado, not Dolphin.
    They were really into "multi" configurations, and produced some very interesting combinations of engines/drives. One boat I saw in their shop had split the power of two big diesels into four Volvo sterndrives, presumably to absorb the massive torque related problems of matching diesels to outdrives, and to better match propellors.

  15. Boston

    Boston Previous Member

    well who would have thought

    two propellers just might be better than one
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