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Thread: backup power for old sailboat

  1. #1
    Associate Engineer
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    backup power for old sailboat

    hello.

    i am posting this here first to see how it works, and get comments, if possible..

    new to all this, not an engineer, only wish i was - had planned to study for a degree, things did not work out that way, now stuck with a head full of probably impractical ideas, and no way to assess these.

    thought i might try here..

    my first problem is that i have a old 31 foot glassfiber sailboat hull, 8 foot beam, draws 4 1/2 feet on a concrete ballasted keel..

    this is powered by an equally old Yanmar YSE12 inboard diesel engine, originally rated at 12HP. the engine is good shape, however needs care and attention, and i must have a backup plan.

    i do not want to carry a petrol/gasoline engine - it is bad enough i must carry diesel. diesel outboards exist but are very heavy and usually very noisy. electric is probably my answer, but i would prefer an hydraulic solution.

    my hope would be that i could attach an hydraulic pump to the power take off on my engine, and use this to fill an accumulator or more. the idea being that i could then use this accumulated power to run an hydraulic outboard leg for a given distance, should i need power in an emergency, and also for maneuvering in tight spots.

    given that i can find or construct an hydraulic outboard leg to suit, i am told that about 2Kw average is what i will need to drive my boat effectively.

    is there a way that i might calculate, on the basis of Kw consumption, what sort of range i might expect to get from a given accumulator?

    i understand that a lot will depend on how wide and how long the hydraulic lines are, as well as the efficiency of the motor - at this point i just want to get an idea how impractical this plan probably is, in terms of how large the necessary accumlator(s) would be, to provide any sort of useful range.

    could anyone suggest where i might find these answers, and/or if this has been tried before?
    Last edited by james_knight; 04-04-2019 at 09:02 PM.

  2. #2
    An electric motor needs power - which would be batteries. You would need a lot of weight and money invested in batteries have a limited range where you could just installing a small out gas outboard on a swivel bracket.

    Google "sailboard backup motor"

  3. #3
    Associate Engineer
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    Quote Originally Posted by administrator View Post
    An electric motor needs power - which would be batteries. You would need a lot of weight and money invested in batteries have a limited range where you could just installing a small out gas outboard on a swivel bracket.

    Google "sailboard backup motor"
    electric is too expensive at this time - however, when i get that far, i plan to run hybrid - 2Kw propane (probably) genset, and a small lead-acid battery bank.

    please understand - gasoline is not an option, i despise the stuff, and pretty much everyone still using it.

    having to carry and burn diesel is bad enough, there is no way i am carrying petrol also.

    hydraulic is much cheaper than electric, by an order of magnitude, and can be at least as efficient, in some circumstances - this has been demonstrated, for example, in the "eaton drive" and similar - and hydraulic boat drives are not uncommon.

    from these i can deduce that my proposal will work, however the problem is a question of the space that will be required for the accumulator(s) it will take to provide the necessary range.

    this is what i need to be able to estimate, and why i am posting.

    is anyone likely to answer my question, if i post this in the main forum?

  4. #4
    Principle Engineer
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    Regarding hydraulics, you cannot compress a liquid so your accumulator piston must work against some sort of spring like compressed air or steel. So the real energy storage is in the spring.


    2Kw is approximately 2.7 hp. A horsepower is 33000 ft-lbs/minute so you need 2.7 X 33000 ft-lbs or 89100 ft-lbs to operate you boat for one minute assuming no losses.


    So how long a spring do you plan to use? How about 3 foot? If your spring puts out a uniform force over the 3 foot length you would need to force it back with 29700 lbs of force. But springs push harder when fully compressed than when expanded so that figure has got to be an average. We will allow you some preload if you like but, let’s not get into the details of spring design right now. For now, just compare the 29700 lbs to your 3500 lb car that sits on four springs.


    So say you have an average hydraulic pressure of 3000 psi you will need almost 10 square inches of piston area or a cylinder diameter of a little over 3.5 inches to produce the 29700 lbs of force.



    You’ll need to figure out if a 3 foot cylinder that diameter (or whatever you choose) plus extra spring length is worth packaging in your boat for one minute of propulsion.


    No? Then please keep the diesel in running order and appreciate petroleum fuels as handy energy storage.

  5. #5
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    Quote Originally Posted by Hudson View Post
    So how long a spring do you plan to use?
    was thinking more in terms of something like what Parker calls a "global bladder" accumulator, however not even sure i am understanding their literature correctly. i think the largest will store up to 57 gallons of fluid at up to 3000 psi, unfortunately i have no idea what the initial first gallon pressure would be - if any - nor the equation for fluid/pressure. i presume these change depending on the N2 charge.

    Quote Originally Posted by Hudson View Post
    No? Then please keep the diesel in running order and appreciate petroleum fuels as handy energy storage.
    should have left my politics out of this, i apologise.

    thank you for taking the time to explain all that to me, very much appreciated.
    Last edited by james_knight; 04-06-2019 at 11:47 AM.

  6. #6
    Principle Engineer
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    Take another look at the Parker catalog (fine folks they are) and see if you haven’t misread the volume as gallons when the actual figure is in liters (57).


    57L is more like 15 gallons or 2 cubic feet. On a good day you might store a little over 200 ft-lbs of energy in such an accumulator. (Based on raising 2 cuft of air to 3000 psi)



    You will use more energy in pumping it up and loose more energy trying to convert it to motion so in the end your idea wastes a considerable amount of energy.



  7. #7
    Associate Engineer
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    yes, educated in metric, got the units tangled. i take your point, although i cannot quite follow the calculation. some bladder accumulators are able to carry 6000 psi and more. parker also makes gas piston accumulators that can go to 20 000 psi and more, if that makes any difference.

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