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davidk 02-06-2002 04:30 AM

Hydraulic Drive
I''m about to join a new boat fitted with a hydraulic drive system. I could do with someone telling me what it is all about since I will be living with it for the next year. Hopefully we will spend most of our lives sailing, not flogging the iron horse.

I cannot get an objective judgement (at this stage) from the owner who has elected to pay $10k''s instead of a normal direct drive deisel, and so he is physically and mentally committed.

As far as I understand it a 40hp diesel will drive a hyraulic pump and that will feed to two separate shafts. The contol system will allow inmdependent control of the two propellers, so you can have full ahead port and full astern stbd on the two shafts simultaneously.

What In am not clear about is (apart from the fact we can run a two shaft/prop system on a long keel which would otherwise require a single shaft to be mounted asymmetrically) what are the advantages and disadvantages of hydraulics versus direct drive via a gearbox? Is one, intrinsically, more reliable than the other? Don''t tell me hydraulic systems are more expensive ... I have see that!!!!!! ... are they any better than the traditional system most of us have managed to live alongside happily for many a year?

Constantin 02-27-2002 12:15 PM

Hydraulic Drive
As the maintenance person and skipper of a multihull with the kind of drive system you are describing, I guess I have a couple of observations I can share:

1) There are no cost or weight savings - the whole rig tends to weigh more than two small engines. A true dual system is inherently more redundant than a single engine driving two props.

2)The hydraulic system is mybe 80% efficient at best (transmission losses due to the pumps, motors). If you go with expensive technology like adjustable pumps, etc. then you can have better control, torque even at low prop speeds. We have variable displacement pumps on our cat, work well enough as long as you don''t mind the $1500+ maintenance costs every 5 years or so when they get serviced.

3) If the bypass pressures are not set correctly, a snag in the prop will kill the engine. Our bypass valves were set to 4000 psi - 1500 psi above what the engine could develop without stalling. Damage to the pumps also results (see maintenance bill above).

4) Stock up on filters and oil. Your hydraulic system will invariably have parts not accessible anywhere you are going. Our metric stuff takes 3 weeks in the US from time of order to arrive. God help you if the pumps or the motors fail. The average turnaround time increases to two months in that case. Carrying spares isn''t really an option (unless you''re rich and don''t mind the wait).

5) Depending on how the hydraulics are mounted, they can cause more noise than the engine. The brain-dead designers at Diport AG in Switzerland specified that we use saildrives (mistake #1) and mounted them directly to the skin of the hull (mistake #2). Saildrives are fragile and ours now hangs lower than the hull, ensuring that whatever we hit, we hit first with the saildrive. As for mounting the saildrive to the hull, imagine the noise that a high-speed pitchy whine produces throughout the structure - it''s louder than the engine!

6) Have a look at the propulsion part of my web-site. I go through all the troubles we''ve had with our system in gory detail <a href=>at</a>.

7) Having said that, some people do benefit from such drives, monohull racers in particular. They never use their engine unless they have to, and the hydraulics allow them to mount the engine where it''s closest to the desired center of gravity.

I''d stay away from such systems. No matter how interesting and "good" they look on paper, they required skilled mechanics, not found everywhere. Small saildrive aux engines are a better bet IMHO.


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