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Hydraulic PTO versus Electro-Hydraulic Units

Discussion in 'Technical Discussion' started by goplay, Mar 7, 2008.

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  1. goplay

    goplay Senior Member

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    On a larger boat I am considering, the hydraulics are run from multiple individual electro-hydraulic units (one for rudders, one for thrusters, one for winches and one for stabilizers) instead of the more common power-takeoff from the main engines. Does anyone have a point-of-view on this?

    At first blush, it would appear multiple units provide greater redundancy albeit at the cost of greater maintenance. In addition, they can be operated with the generator running, instead of needing to have the motors running (worthwhile for the winches and stabilizers).

    I don't have info yet on the specific electro-hydraulic units but I assume they can be made at least as robust as a PTO from the main engines? Are they known to have similar MTBF?

    Thanks!
  2. Loren Schweizer

    Loren Schweizer YF Associate Writer

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    It came as a small shock to me that, a few years ago while working with a builder of 70'-80' Expedition vessels, the horsepower requirements for stabilizers (Wesmars, to be exact) are rather minimal, and that at least one of these boats is running around with an electro-hydraulic pump as a back-up to the engine-driven prime source of power.
    I'd imagine that if the electric motor was a reliable one--aren't most?--that this ought to be kind of bullet-proof.
  3. JAG1

    JAG1 New Member

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    PTO or self Contained

    Its an interesting topic - with as many different opinions as could be wished for. Personally I always liked the ring main setup; one circuit fed by PTOs and an electric pump. The MEs with their generally large power reserve take the load when they're on and the electric pack for when they're not. MTBF does not seem to be an issue for a decently maintained and well set up system. The several separate packs idea is a great consumer of space and when different suppliers are involved and a great way of spending $$$ on different spare parts. What size of boat are you thinking of?:)
  4. goplay

    goplay Senior Member

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    This is an 85'. You can never have enough space in a boat, but there is sufficient space in the ER for multiple hydraulic packs.

  5. JAG1

    JAG1 New Member

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    Your best option is going to depend on what you're trying to achieve. My current project is 70m designed to run in a tightly defined rev range so our mains don't have much spare capacity, combine that with our high potential hydraulic loads and the only conclusion is that separate packs are the best option for us. A previous high speed 45m had bags of spare capacity at manoeuvring speed to power all the thrusters etc from the MEs - our harbour loads were very low so a small electric pack could be used there - this system was very compact. The several control cabinets for the separate systems on the 70m would probably fill your engine room to say nothing of the different spare parts I will need to carry as they're all different sub contractors.

    Still think hydraulics is far superior to having electric thrusters and deck gear - more compact and reliable.
  6. AMG

    AMG YF Moderator

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    On the 85-footer I was, we had a compact hydraulic system from the ME and with an electric pump on the tank for backup. It was supplying the stabilizers, bow thruster, deck crane, anchor and mooring winches and the gangway. The only problem I had was to calibrate the manifolds so each user got the right pressure. After I managed to do this, everything worked just fine. The remote controlled gangway could use the electric pump only, same with the crane, but the rest was mainly in use when the main engines was running. (Steering and engine controls were pneumatic).
  7. nas130

    nas130 Member

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    I am on a 85' now.

    Our stabilizers and winches are run off of the stbd main engine and the steering system is run off of a separate electro-hydraulic unit.

    If I were building a boat. I would have PTOs on both main engines, and a separate electro-hydraulic pump for at anchor stabilizers.

    Best Regards,

    nas
  8. goplay

    goplay Senior Member

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    Thanks everyone for your inputs.

    Something I need to check into, but does anyone know if electro-hydraulic pumps suffer from "overheating" under continuous use like electric motors for thrusters do? If not, why not?
  9. AMG

    AMG YF Moderator

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    I guess it has to be the right size for your system, but I know that our pump was on a timer so when no hydraulic device was used, it shut down after a minute or two...
  10. Garry Hartshorn

    Garry Hartshorn Senior Member

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    I have been pondering on this question myself lately, I am leaning towards a PTO on each main a PTO on one generator and an electric pump all conected to a central manifold. Each PTO would be fitted with a magnetic clutch so that one could manage the flow to suit what was required at the time.

    I should also mention that the engine driven pumps would all be the same and sized so that under normal operating conditions only one pump would be needed to be engaged. ( normal operation would be underway with stabilizers and steering )
  11. goplay

    goplay Senior Member

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    That seems to make a lot of sense.

    Effectively though, you are favoring a "centralized" approach, with lots of redundancy, versus a distributed approach with units near their point of use.

    What were your reasons for going this way for your implementation? Cost, space, power requirements, MTBF, MTTR?

    Thanks!


  12. Garry Hartshorn

    Garry Hartshorn Senior Member

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    The redundancy is a big part of it, but the biggest advantage is the ability to be flexable with the primary power supply. For example with such a setup you can operate all the hydraulics dockside on shorepower : cranes, davits, swim platform, capstans etc... While at anchor you could use either the electric pump or the generator driven PTO for zero speed stabilization as well as all other systems. and while underway you can use a mains driven PTO but not be tied to having to use a the same engine al the time especially handy when doing long passages when you may depending on the vessel chose to run on one engine.

    I also tend to think that the whole system is neater and would ultimately be easier to maintain, it has been my experiance that when you have isolated and designated systems that you end up having to carry different fluids and you run the risk down the track of someone making the mistake adding the wrong fluid to the system which in tern create a bigger problem.
  13. JAG1

    JAG1 New Member

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    Yes you will most definitely need a heat exchanger for any system that is running constantly. Small units that are operated occasionally such as for cranes/passarelles generally use the tank to cool the oil - as there is little buildup of heat in such a system. Stabilisers etc will need to be cooled. The electric motors themselves (if chosen correctly) will not need to be cooled. The electric motors often used for small thrusters are not rated for constant running. As a general rule hydraulic oil should have an alarm point of 60 degrees celsius - over that the oil will oxidise at an unacceptable rate and cause you a world of pain.
  14. Crglmb

    Crglmb New Member

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    Flow

    The Wesmar Stabilizers require 8gpm, the steering system 4-8 gpm, Bow thruster for most 85' foot yacht's are about 30-40hp and require about 20-25 gpm at 3000psi load sense system. All but the steering system uses aw46 or 68. Most steering systems use a iso 15. Depending on the engines and generator you can run your steering and stabilizers on gear pumps off the engines. The thruster can be ran off the generator crank and or engine main crank pully. Well that's my 2 cents.....
  15. sbmar

    sbmar Guest

    Hydraulics

    Our thinking on Hydraulics and how to provide the needed power is based upon what you are trying to do and what is most practical for each application.

    One way to start making a decision that best fits your needs is to look at the duty cycle of the application and the power needed to accomplish the task.

    Bow Thruster: High HP needed ( 20-100++ hp)—very low intermittent duty cycle-- Depending upon the generator power you have available, many times the only solution is a PTO off a main engine or an auxiliary engine / generator. The conversion to hydraulic power thru a PTO is about 30-40% more efficient than doing it electrically. A 50 HP bow thruster would require about a 75 Hp electric motor, meaning that you may need over 100Kw for a generator. Practical ? But a 90Kw set w/ a 30- 40 Kw hotel load could deliver that 50HP hydraulically thru a PTO to a load sensing pump and not be brought to it’s knees.

    2) Stabilizers, Power Steering, & Autopilot Operation—Consider this a continuous duty application but the power requirements are relatively low. 2-10 Hp would be typical…The systems are small, but because of the duty cycle, you must think about cooling the oil (was mentioned earlier). Depending of the design of the system, many times the hydraulic reservoir may act as a cooling medium, but how you cool the oil is optional as long as you look at 140-150 F as the upper limit for continuous use. If your vessel is generator dependent for other reasons, then it makes sense that you should consider an electric/hydraulic system.. Typically they are 100% reliable (designed, sized, and applied right) and give no issues in long term continuous duty use.

    3) Anchor and Davit Winches: Intermittent duty and low power . Electric/hydraulic systems can be very practical here too because they are so flexible in how they are set-up.

    If you make long range trips or ocean crossings, had a minimum of electrical needs along the journey, and efficiency (fuel burn was a key consideration) then having the main engine do it all certainly wins in this case—PTO load sensing or multiple fixed displacement pumps specific to each application would easily be the most efficient that way. But if the vessel has high electric needs under most or all conditions (lots of creature comforts), then electrical and / or a mix of both PTO driven hydraulics may need to be looked at.

    It all comes down to looking at the needs of the vessel in it’s entirety, understanding what needs be done, having a through knowledge of what hydraulics can do & what they cannot do, and understanding the most practical way to go about accomplishing the task...