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Old 15-03-2009, 14:19   #1
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Hydraulic drives for multihulls?

Hi Everybody Why do I never see hydraulic drives in catamarans? I know that there are basic pumping losses associated with hydraulics, but it's my unerstanding that you can keep it to 85% efficiency with a well enginered system. Considering the weight (right where you don't want it), cost and added maintenance of typical twin diesels, not to mention the gen set that most people stick in as well there ought to be an appeal to a central diesel installation running all systems, with the exception of A/C, you need a gen set for that. I'm sure theres something I'm overlooking and if someone could point it out to me I'd appreciate it, because on the surface it sounds like a great idea. Fifty hp in a 40' cat ought to give you close to 40 hp at the props. No? Thanks George
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Old 15-03-2009, 15:00   #2
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I am sure it has been thought out before.

Where would you put the engine, the hydraulic fluid reservoir, would you have room for all the hoses?
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Old 15-03-2009, 15:36   #3
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hydraulic drives

It seems to me that there would be sufficent room where the gen set is typically mounted, on the bridge deck, usually ahead of the salon, the resevoir tank wouldn't have to be that big if you used a heat exchanger to cool the return oil and the hoses could easily be routed under the floor of the hulls. If you gave up on A/C at the outset (not a difficult decision) then a couple of alternators, a good battery bank and a 3000w inverter would take care of your electrical requirements. I guess my question is why isn't it being done? I'm not the brightest cat out there, so I assumed there is a sound engineering reason. But when you look at hydraulic drives on all of the construction equipment around it makes you wonder. George
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Old 15-03-2009, 16:05   #4
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It was never very common, but several British Cats tried this in the early days. I'm sure that modern hydraulic systems and proportional controllers could be successful, but the damage was done. High pressure leaks, drips in the bilges, blown seals, and other typical operating issues ended the future of hydraulics in cats. Sailors are a tight-wad conservative bunch, and are death on past failures.

I'd like to hear some new ideas, though. Pick some specifics and tell us about delivering infinitely variable, ultra quite 20 to 50 shaft hp at 300 rpm to each hull from a central diesel engine! What would the individual components be, how much would they weight, how much noise would they make, and how much would they cost?
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Old 15-03-2009, 16:21   #5
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hydraulic drives

Damn, I was hoping to pick the collective intelligence of the cruising community and find out why it wouldn't work. I believe that you know more about them than I do. I spent 8 mths here in Pago Pago repairing the self propelled man lifts, cranes, etc at the shipyard, they all use hydraulic drives and keeping them going and drip free isn't much different than a regular diesel/hydraulic tranny. I've not spent any time specing or pricing them, but that wouldn't be tough, there are lots of hydraulic outfits that'll do that for you. Sorry, it was just curiousity on my part, surely there's someone out there with more experience. This is the time to step forward. Thanks George
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Old 15-03-2009, 17:35   #6
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In a cat I would do 2 30HP generators with twin electric shaft drives under the floor in the middle of the hulls. The generators would be in similar locations to where they are in larger cats (under/behind seats on the bridge). This would allow for backup power should a engine fail and if they were both working you could have the extra power when needed. The electric drives could be alternators that charge the battery bank or in the fluke instance the engines do fail you could have auxillary power from wind and solar. This is essentially a hybrid. Sorry this Isn't hydraulic but Thats messy and only useful for one thing, propulsion, and maybe winches.
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Old 15-03-2009, 18:02   #7
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We have a boat near here that does use a hydraulic drive as well as everything else. Windlass and about anything you could think of. It's an old torpedo retriever all steel. It has about 6 engines aboard. The whole boat is hydraulic. I think you really need a bigger boat to make such a system really effective. This is a 80+ ft boat with a 24+ beam. They have a below deck engine room you could walk around in. On larger boats there are a lot more options. Making a primary drive on a Cat with the hydraulics just does not seem practical to me. The methodology really does not change the energy requirement. Hydraulics seem to me to work better in the case where engines are always on - like the boat I am referring to.
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Old 15-03-2009, 18:43   #8
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Hydraulic drives are really just like diesel electric, just a different method of transferring the energy, and with much the same energy losses.
Bob Harris used to specify them for his big cats in the 70's and they have been quite popular in UK. Volvo used to sell a system in the 70's.
For current state of play check
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Old 15-03-2009, 20:29   #9
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As far as speed...you might want to Google TRASHCATS...Their top speed is about 5 knots or so.....Keep in mind, with a diesel running pumps....it will be like a genset running all the time...same noise whether you are going fast or slow.
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Old 15-03-2009, 22:19   #10
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hydro drives on cats

Cost,
When you add up all the cost for the project I think you will go a different way. I like the electric power idea with drives you can get out of the water

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Old 15-03-2009, 22:30   #11
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I know a fellow with a cat that has hydraulic drives powered with a single diesel. I don't have a whole lot of details, but he loves it and it works well for him...
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Old 16-03-2009, 04:35   #12
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Sure there has been a previous thread on this.

The Catfisher 28 uses a single engine and hydraulic drives as this maximises their available space

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Old 16-03-2009, 12:58   #13
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Remember that I can lift my engines out and work on them in my lap, so I'm not an expert on anything bigger. I'm just curious about how hydraulics could be done using state-of-the-art equipment. I watch Diesel-Electric developments pretty closely, and would like to get an idea of side-by-side comparison. For starters, I would guess that the required hydraulic hoses and comparably electric cables would weigh the same. but there's no equivalent to batteries in a hydraulic system, so there's no possibility of regeneration.
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Old 16-03-2009, 14:43   #14
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The catfisher could not be described as state of the art , as it is about 30 years old.

As for weight. I would expect one 30 hp engine is lighter than 2 x 15 hp engines
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