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Frogwatch

Why not hydraulic motors for cats?

12 posts in this topic

Most cruising cats seem to have two engines, one in each ama. This is absurd as it is twice as complex as it shoud be. So, why not a single engine driving a hydraulic pump. Then there could either be a simple hydraulic motor in each ama or a single hydraulic motor in the middle. You would have simpe hydraulic lines running from the pump to each motor. The motors could easily be driven separately.

For that matter, a single centered hydraulic motor could be on a system that can be raised from the water for less drag.

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A lot of cruising cats do suffer from having two of everything and symmetry for it's own sake. So, there's probably some of that going on. But, hydraulic transmissions, electric / hybrid, sonic drives, single engines and a fair number of other layouts and tech have been tried over the years. All of them have advantages and costs. Absurd as it may be to have duplicate engines / motors when people start looking at implementing complete systems and paying for them with money more often than not they choose twins.

 

So, you've pointed out the advantages of hydraulic but there are some costs that you haven't mentioned. In practice the big one will likely be engineering. It's just a lot cheaper and less risky to use off the shelf technology. Even if all the bits you need exist on a shelf somewhere, tying them into a working system isn't going to be free. So chances are it will cost a lot more for the same power. It will also weigh more for the same power. If you travel off the beaten path parts availability may be an issue. You'll probably have a smaller pool of buyers interested in a boat with a non-standard system installed when it comes time to sell. And so on. To be sure, some of those are market or sociological issues rather than engineering ones and those may well change over time. But for most folks, today, twins are a more attractive compromise.

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0n a lee shore fighting for your life the redundancy of a second motor would be good when the odd sheet goes around a prop.

 

same reason I like aeroplanes with more than one engine.

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A single engine driving twin hydraulic drives is a common way of powering a catamaran. However there are some disadvatages not mentioned yet, which tend to put people off.

 

Apartr from the engineering, reliability etc mentioned above.

 

Have you even seen a boat where a hose has burst? I have, not a pretty sight.

 

There are significant power losses through the hoses, more than the manufacturers claim. So instead of say twin 20hps you need one 50hp to do the same job. That means instead of using a 20hp for battery charging, or on tick over when motorsailing you are using a 50hp. Not good news for the engine lifespan. And fluid plus hoses tend to be heavier than you expect

 

The hydraulic setup tends to be noisier, not just because of the bigger engine but also pump and fluid noise.

 

And where do you fit the engine? It has to be central, and you need to lead the hoses round the boat and have access to them.

 

Lots of cons, few pros. Probably why its not a popular setup

 

Richard Woods of Woods Designs

 

www.sailingcatamarans.com

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Most cruising cats seem to have two engines, one in each ama. This is absurd as it is twice as complex as it shoud be. So, why not a single engine driving a hydraulic pump. Then there could either be a simple hydraulic motor in each ama or a single hydraulic motor in the middle. You would have simpe hydraulic lines running from the pump to each motor. The motors could easily be driven separately.

For that matter, a single centered hydraulic motor could be on a system that can be raised from the water for less drag.

The best thing, about two engines and drive-lines, is, You have a complete, and full set of, spare parts. I always loved that.

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Besides its pretty simple task to have a symmetrical assembly weightwise with twin engines. One bigger engine should be placed on the middle of the boat, so it makes the layout more complicated. Also having the motor on the end of a hull is a lot more quieter for the prime travelling space, the bridge, than a central mounted engine.

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tired of motoring at 7 knots with dual small inboard diesels, a local cat owner dropped in a small toyota engine and dual hydraulic motors. Lot's of unexpected engineering and installation problems. He told me hydraulics always leak, from his experience. When it was all finally dialed in I think he hit 7.5 knots. A very dissappointing project.

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Don't add hydraulics unless you really need them, especially high pressure, high flow hydraulics. They need real engineering, and all of the parts are expensive and heavy. They also fail in spectacular and dangerous ways, do a quick google on high pressure fluid injection injury and you will see how a small pinhole injection can lead to loss of a limb.

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Don't add hydraulics unless you really need them, especially high pressure, high flow hydraulics. They need real engineering, and all of the parts are expensive and heavy. They also fail in spectacular and dangerous ways, do a quick google on high pressure fluid injection injury and you will see how a small pinhole injection can lead to loss of a limb.

OUCH!! I knew they can be messy but had no idea of the injury potential!

 

1230552-1241999-2340.jpg180527d1285852784-took-hydraulic-fluid-simage8.jpg

 

P.S. On reading further, some of those cuts may be surgical, not directly from the injury:

 

Hydraulic Injection Injury Finger that has been lanced in attempt to save the finger and the hand. The injury was caused by a high - pressure injection of hydraulic fluid. If there is a pinhole leak in the hydraulic line and someone runs there hand along it, at 2000 psi, they can easily incur and injection of hydraulic fluid and may not even be aware that it happened until gangrene begins to set in. Use cardboard held above the line to check for leaks.

finger-hydraulic.jpg

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Most cruising cats seem to have two engines, one in each ama. This is absurd as it is twice as complex as it shoud be. So, why not a single engine driving a hydraulic pump. Then there could either be a simple hydraulic motor in each ama or a single hydraulic motor in the middle. You would have simpe hydraulic lines running from the pump to each motor. The motors could easily be driven separately.

For that matter, a single centered hydraulic motor could be on a system that can be raised from the water for less drag.

 

Single engine twin screw hydraulics:

I toyed with hydraulics in my 51' cat in the 1980's, and determined it was not worthwhile. I used little Vickers landing gear pumps mounted to twin retractable legs that held I/O bottom end/prop, and used standard twin Volvo MD2's with stock hydraulic pumps mounted. This removed the props from the water tucking them into the bridgedeck. Problem with hydraulics is that they are extremely noisy..... noisier than the diesel engine itself. Next is the heat, and will require an additional heat exchanger. Then there is hose chafe, and was always breaking a hose every 6 months, despite my best attempts to chafe proof it and use of ss braided protection. Then there is the loss of horsepower... like 20% at EACH pump. One motor to one prop still = a 40% hp loss. Now if you want to use one motor to power two props, there are tecnical problems, like prop RPM control..... you cannot use standard twin screw manuvers, because both the forward and reverse props will be spinning at the same speed, whick can cause handling complications. There is a way to spin your props and different RPMS in foreward and reverse, but this would require the use of a very expensive and very heavy rotary valve, with all of it's attenuant controls, heat, and yet another 20% power loss. Also, single engine hydraulics to twin props will never give you the same redundance safety as two separate engnes and props, which is far more reliable, especially with their own transmissions and direct fdrives. Hydraulics in the end will not save any weight either, when taking all into consideration. Note: the retractable drives were not a good idea either..... they slowed the boat down radically when down, eliminating the possiblity of effective motorsailing.

 

If you are hell bent of trying someting new, worth pursuing may well be diesel-electric, using DC generators insteasd of propulsion engines, in which one could be shut down when powering at slower speeds, and this would also cover all power requirements. The mounting of the genrators would be not as critical as propulsion engines, and could be set on larger rubber dampeners.

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aw, man, I just had breakfast... I installed a faryman diesel with a single hydraulic drive

in my trimaran in 1975 after i sold boat buyer removed it too much weight and went outboard,

it didn't have an american shaft, german motor: straight shaft, metric? with hole through it

was able to find a prop to fit. I thought maybe it would be an easier install..?

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