Top off tanks?

[/U]Is it preferable to keep my tanks topped off to prevent condensation or can I keep them about half full to enjoy the lighter weight?
Thanks
Tim tiernan

I keep my tanks near empty for 8 months before a 2500 mile round trip to Fl and back to NJ.


Most experienced boaters have found condensation is USUALLY not an issue at all.


A few things do alter the equation...but if you burn off your fuel every year and have clean tanks, clean fuel and a decent fitration system....keep your tanks wherever you want.


At least with diesel...you will just wind up with water in the bottom of the tank and not phase separation like in gas boats...diesel is more easily remedied.

If your diesel top them off prior to winter layup in the north.

It depends on environmental conditions around your tanks and the climate. The bigger the temperature changes in the tank, the more air is pushed out by heated fuel and more pulled back in as the tank cools. Air coming into the tank brings moisture. In the winter or summer with little change in tank temps brings little new air whatever the amount of fuel. But if you're using your engines, the fuel is being heated because of the fuel returned to the tanks. My Detroit mains pump about 70 gallons an hour but return all but 8. If my day tank is kept full, there is little room for changing air even if the fuel heats. One reason tugs and ships use a day tank and keep their larger tanks cool and stable if they're not located where the engine room can heat them.

Nothing wrong with less than full. Assume you have a separator filter.

Ted

Here are some tidbits about condensation in fuel tanks....




Does An Empty Tank Condensate? Photo Gallery by Compass Marine How To at pbase.com

My tanks are normally around 25% full unless on a long trip, then I fill them accordingly. Never a drop of water in the racor. But my tank fills are not on deck, but on the tanks themselves, and vents not on hullside.

Ski in NC said:

My tanks are normally around 25% full unless on a long trip, then I fill them accordingly. Never a drop of water in the racor. But my tank fills are not on deck, but on the tanks themselves, and vents not on hullside.

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I have similar experience. My tanks vent into the lazaret, but fillers are on deck. I never have them more than half full, and never get more than a teaspoonful of water in the separator under the primary filter. However, my tanks drain from the bottom, (Nordhavns also have this set-up - they would have a reason), so stuff does not tend to accumulate, in effect being polished continually, yet the filters last for ages, so sludge must be minimal. What I don't like is the way so many marine tanks have a fuel pick-up which enters at the top and does not reach to the bottom, which seems to be a recipe for sludge accumulation, and it must get stirred up in rough weather. I suspect that is why so many comment on experiencing clogged filters, something I have never had happen, and I only change filters about every 7 years, and even then, the engine performance appears fine. I don't do huge mileage I must admit.

My vents and fuel fills are mounted on the back wall under the veranda roof. Fuel fill Orings are replaced annually. Fuel tanks are polished from the lowest point of the tank to keep anything from accumulating in the tanks. It's a simple formula for keeping fuel clean.





Ted

Ted, are you saying you engine draw off is from the bottom, or just the fuel polishing circuit..? If the former, that's fine. Good filters will deal with any crud continually, so it will never build up in a way it could clog the filters, even in rough water. If the draw off is via pick-up tubes, then having a polishing system drawing from the bottom is better than not having it, but it makes you wonder why it was done that way, as an engine draw-off from the bottom, going through decent filters tends to make a special polishing circulation a bit superfluous in my view.

Unless I am going on a longe trip I keep low fuel level. That way my fuel is filtered more rapidly. despite all that I have never seen any sign of water in the fuel.


IMO leaking inlet orings and buried tanks at the fuel dock are the source of most water.

With our DD 6-71 we keep the tanks (small 100G each) full when coastal cruising.

The DD return a huge amount of fuel for cooling purposes ,20GPH pumped 3GPH burned , 17 gallons returned , warm..

I do not know if harm would come from running on the last 10gal of fuel, but I do know the fuel tank is really warm!

Our fuel tank has a metal plate with hundreds of holes about 2 inches up from the bottom.this is supposed to trap water and bug residue under the plate.

A bottom drain valve allows easy servicing , once a year.

My 2 centavos worth:

So, a coin flip on this question it would appear - or not. A short discussion with those who operate tank farms (I have) will tell you large amounts of water can seasonally occur as tank levels rise and fall. We analyzed incoming for water and noted % water picked up due to seasonal changes in the farm itself. Very good (if maintained) water traps and filters kept outgoing in spec.

But there is always water in fuel. How much is in new fuel and/or self induced and what to do about it is the routine question. Bottom off takes (often not known with in tank tubes) seems a good remedy for our little vessels. Larger boats use a wide variety of filters, centrifuges and protocols.

Peter B said:

........ as an engine draw-off from the bottom, going through decent filters tends to make a special polishing circulation a bit superfluous in my view.

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Peter B said:

Ted, are you saying you engine draw off is from the bottom, or just the fuel polishing circuit..? If the former, that's fine. Good filters will deal with any crud continually, so it will never build up in a way it could clog the filters, even in rough water. If the draw off is via pick-up tubes, then having a polishing system drawing from the bottom is better than not having it, but it makes you wonder why it was done that way, as an engine draw-off from the bottom, going through decent filters tends to make a special polishing circulation a bit superfluous in my view.

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The fuel polishing circuit draws off the bottom. Tanks are contoured to the hull and slopes forward so that all but the last cup full will come out. The draws for the engine and generator are several inches off the bottom on the starboard tank. While there are fuel fills for both tanks, I only fill the port tank. Fuel is transferred through the fuel polisher (4 GPM) as need to the starboard tank. Simply, all new fuel runs though the polisher before going into the tank for use. In an emergency, the tanks can be equalized without the pump. My normal routine is to trim the boat (by transferring fuel) every 2 cruising days. Before transferring fuel, I polish the starboard tank for a couple of minutes to clean the lowest point. Normal 8 hour cruising day is under 20 gallons. So, polishing the starboard tank and transferring fuel is less than 10 minutes while doing my normal engine room checks. I know this is excessive, but with the exception of the Racor 1000 filter ($325) and a few fittings, the boat came with all the other parts necessary.

Ted