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Old 21-02-2008, 14:58   #1
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Water-displaced fuel system?

i've been do a little reading on design of cruising yachts and ran across this Water-displaced fuel system. My only thinking on this is I would like to have as much ballast as possible come from useable weight rather than lead on the keel and wdf is a way to help with this. you have to maintain pressure in the tanks and add water as you take fuel out. It this something to look at? is it something practial? i could see where it would help. but how much would it help?

any thought on this?
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Old 21-02-2008, 16:07   #2
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Originally Posted by nylarlathotep View Post
i've been do a little reading on design of cruising yachts and ran across this Water-displaced fuel system. My only thinking on this is I would like to have as much ballast as possible come from useable weight rather than lead on the keel and wdf is a way to help with this. you have to maintain pressure in the tanks and add water as you take fuel out. It this something to look at? is it something practial? i could see where it would help. but how much would it help?

any thought on this?
to me, it sounds like a bad idea
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Old 21-02-2008, 17:05   #3
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Along with the expense of such a system there is the possibility for major things to go wrong and unless you plan to compete in the Americas cup I really don't see the point.
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Old 21-02-2008, 17:21   #4
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It sounds complicated! Simplicity is more reliable.

Once a vessel reaches a certain displacement the adding and subtracting of liquid supplies usually makes little difference in it's sailing ability. Not including racing vessels, of course. Then lighter is better.

The transfer of products takes power and then one has to replenish the power with another product that requires space and weight. It's a vicious circle that takes experience to balance out.

My thoughts up to this point.........................._/)
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Old 21-02-2008, 17:50   #5
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over all for the long cruise its not worth anything. i was just wondering. something i was reading made a refrance to it, and i jut wanted to know others thoughts about it


thanks
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Old 21-02-2008, 18:41   #6
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If I remember correctly, US Navy Spruance class destroyers, Oliver Hazard Perry class frigates and the Aegis cruisers all use a water-displacement fuel system. Much easier to maintain trim because you only have to compensate for the difference in weight between the fuel and water.


If you have very large fuel tanks this could help. For anything smaller than the megayachts I doubt it would be worth the complexity.

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Old 21-02-2008, 20:39   #7
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All diesel submarines use this system. But in order to maintain fuel quality, you need a very good water separation and filtration system. I don't think this system is something that makes a lot of sense on a cruising vessel.
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Old 21-02-2008, 20:43   #8
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The German Diesel U-Boats during WW2 used this system. Possibly other countries too, but I know for sure the Germans did it since I just finished a large section of research on them.

Their system was very simple. their external "saddle" tanks just were open at the bottom. The diesel would float on top of the seawater. as the fuel was drawn out, more water would come up to fill the extra space.

granted, this system would never float(pardon my pun) today given environmental regulations, but it worked well enough for them back then.
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Old 21-02-2008, 21:16   #9
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The only reason I can think of for doing this is to increase the stability of the vessel by eliminating "slack tanks". Any liquid sloshing around in a very large tank has a huge impact on the vessels stability. Anybody who has taken the USGC captain's exam surely remembers at least a little bit about this because the resulting stability calculations figure promenently on part of the test.

Not very many cruising boats have tankage large enough to even begin to think about this.
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Old 22-02-2008, 01:29   #10
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There is another way to do it. I was invloved with a vessel here that needed ballast, but could not have a keel and did not have room for lead or anything extra. All the lower part of the hull was taken up with fuel. But the Fuel was kept inside flexible fuel bag tanks, which was inside the main fuel tank. So as Fuel was used, the bag squashed down and water filled the outer area. It was some years ago now, but if I remember rightly is was something like 5000ltrs or maybe more in fuel. The vessel had no engine by the way, the fuel was for a 50ft support vessel. It was a Maori canoe of 100ft or more and it was people powered. They were going to paddle to Raratonga or some similar Island. But on a practice around NZ, they got into difficulty and it sank I think. Not sure, it was kinda kept a little hush hush so as not to embarrase some. I ftiied 100G's worth of Electronic gear to the support vessel. It went back to commercial fishing. All funded by us Kiwi taxpayers, but I won't go there.
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