Only one steerable engine on small power multihull?

Discussion in 'Boat Design' started by massandspace, Nov 16, 2018.

  1. massandspace
    Joined: Sep 2017
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    massandspace Junior Member

    Designing and building a custom 28' power cat...dual engines...hydraulically steered...

    It occurred to me...why go through the expense and labour of having both engines steerable (a bit tricky as there can be no direct tie bar)....my argument:

    -On any "slower speed" boat I have ever owned (mostly sail) I have only really hand-steered the boat less than 1% of the total time on water. The other 99% is autopilot...straight ahead, point A to point B, with only super small course adjustments along the way....and that will certainly be the case on this boat. I think there will be no noticeable difference to the occupants of the boat if only one engine is constantly making small course corrections via autopilot during those long periods of travel between turning or destination waypoints.

    -The other 1% (approach to and arrival at dock, basically) I think could be done quite easily by not only using the one steerable motor but also by using the dual throttle/shift controls to add and subtract power (and use reverse) to each engine independently. I would actually argue that I would have even MORE maneuverability than a single hull, single prop boat by using the controls and one steerable engine carefully when docking.

    I can imagine some type of rare emergency maneauver (a big log appears right in front of boat, for example) could be an issue....but the boat is only going to cruise at "trawling speeds" (about 8-12 knots), and of course there will be someone at the helm at all times, so at the very least both engines could be thrown into neutral immediately to minimize any potential impact, or maybe the unsteerable side can be thrown into reverse to "force" a turn...and, in the end, I think even a cat with both engines steerable might be in trouble as well in this type of emergency.

    Ideas?

    Thanks!
     
    Last edited: Nov 16, 2018
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    What kind of engines/drive ?
     
  3. fallguy
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    fallguy Senior Member

    Well, it is not ideal, of course.

    For example, consider the steerable motor failing. A battery or fuel or something. Now you try to get home on almost a rudder.

    Or, hours. The meter on the steerable is going to quickly double the other one.

    Just get a liquid tie bar and another cylinder and get over the silliness. And gain back the redundancy of two engines.
     
  4. massandspace
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    massandspace Junior Member

    To reply...

    The steerable motor will have the same hours as the non-steerable...both on at all times, just one steers and one not. Not sure I understand why one would have more? And will still have the redundancy of 2 motors...that is crucial and will not change.

    But a good point about the steerable motor failing. Would be hard to steer with a broken motor acting as a rudder. Maybe a simple wood/epoxy 2-part “rudder” that could be used in am emergency....would quickly clamp onto the leg of the steering motor if that died.....or maybe a very simple collapsible fiberglass connecting rod that could be used as a temporary steering arm?

    Keep in mind these will be brand new 4-stroke motors....and I cruise well traveled routes....not offshore...just coastal and harbor to harbor.

    Cost of adding extra cylinder/liquid tie bar/hoses/through hull fittings is quite a bit.....not to mention adding weight to a weight-sensitive cat.

    Forgot to mention....these will be outboards....one 20 hp per side. Total 40hp.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    So the un-steered engine will be locked into position ? I can see potential cavitation problems in turns, if it is, and other potential problems if it isn't.
     
  6. massandspace
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    massandspace Junior Member

    Yes, the unsteerable engine locked straight ahead.

    But....there will be no cavitation near the fuel dock when traveling at 1 knot or less of speed.

    At speed (8-12 knots) I also don’t think there will be any cavitation as all that will happen is very minor course corrections (unless emergency....then, yes, all bets off....but only in hard hard turn due to emergency move).
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    This is a silly idea, you don't want nasty surprises in any situation, and fallguy is correct in pointing out that the whole steering dynamic goes into "uncharted waters" if the steer engine fails, which might make steering in one turn direction very difficult. I'd much rather have a mechanical steering set up that steers both, than a hydraulic working on one. I understand budgets can be a problem, but I don't think this is a good idea.
     
  8. massandspace
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    massandspace Junior Member

    Mechanical not an option, as stated. Hydraulic. Already purchased system and semi installed....intergrated with the autopilot system.

    Just trying to keep the boat light and simple! I am sure the 99% part I mentioned will be fine....the more I think about it...minor course corrections I think no issue with one motor steering...just looking at the docking and extreme circumstances.....
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    If these engines are widely spaced, and the wind conditions are disadvantageous, you could find you don't have the full range of steering, if the steer engine fails. You might be able to go to starboard, but not port !
     
  10. massandspace
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    massandspace Junior Member

    Yes, that is a good point. And yes, the engines are widely spaced...
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You might be able to fit a push-pull cable link (not a tie-rod) between the two engines, or a simple hydraulic link of some kind, which is what fallguy was suggesting, I think. You really don't want to be trying to fit add-on rudders to an outboard leg, out on the water.
     
  12. BlueBell
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    BlueBell Ahhhhh...

    One, centre mounted engine would have better met that objective than omitting one steering system.
     
  13. massandspace
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    massandspace Junior Member

    I have owned about 8 sailboats in my life...all with “emergency tillers”....and never used any of them once, ever. True, these boats had diesels....but I am installing modern 4-stroke fuel injected motors....a close second.

    One motor in center not an option as I fully agree with the argument of the redundancy 2 motors ...for safety


    The boat will be used in close coastal conditions....not offshore...
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I like the idea of twins for reliability, but you don't want to compromise reliability with the steering.
     

  15. massandspace
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    massandspace Junior Member

    I also need the other engine for output....to get to speed....so yes, redundancy, but also to get to design speed.
     
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