Diesel Fuel Tanks

[Dale Genther]

I'm working on a boat with two diesel fuel tanks. They are about 12 gal each and located down low in the cockpit seat lockers, one on each side. From what I can see there are two appreaches to plumbing the fuel from these tanks to the 30 hp engine. The first approach would be to tee the fuel line leading from the tanks together to the engines primary fuel filter. In this way the fuel would feed roughly the same from each tank and the tanks would be at the same level with the boat being level. The other option would be to valve the tanks so that only one tank is being fed from at a time. The tanks would have to be manually switched via valves. Which of these is the preferred approach? Also what about the fuel return lines in either of the above two approaches. With the first option, are two return lines needed (one to each tank) or is only a single return needed as the tanks are self equalizing in level by themselves? In the second option I assume two return lines with appropriate isolation valves are needed to make certain the tank feeding the fuel is the one getting the fuel returned to it. The problem I see with this is somebody forgetting to switch the return line valves when switching tanks. This will result in overfilling the standby tank.

[Gary E]

Selector valves so that fuel from one tank is drawn to the engine and returned to the same tank. Yeah, it's 2 valves per tank, and if the user dont know what he's doing or is to lazy and forgets, he should only have one tank. Label each valve and if your dealing with some one who needs instructions make a big Instruction Sheet in BIG PRINT and attach it to the boat near the valves. Try imagining the valving so that twin engines can run from either tank.

Draw your piping diagrams, and folow it.

[Ken Hutchins]

I have 3 tanks a filter for each tank, 4 way valves for both supply and return. You would need 3 way valves for 2 tanks.
Teeing together would be a problem because the fuel would flow from one tank to the other when heeled over, you have to be concerned with fuel flow through the vent lines also.
You could use a 6 or 8 way valve to insure the fuel going back to the tank it came from, but this may not be desireable because you might want to move fuel to the other tank. So the seperate valves would be preferred, along with lables and instructions posted about how the valves should be used.

[ron ll]

I have two tanks (that need to be replaced soon) that were installed in 1943. Not saying this is the best way, but they are plumbed with separate supply valves teed to the main filters. The return is piped to the starboard tank only and in this way can be used to correct the trim.

One disadvantage to this is that if one tank is contaminated, they both are. Also, I think I would rather have two separate filters, but still have the two tanks teed before the filters so that if one tank goes bad, you switch to the other tank. Or if one filter gets plugged, switch to the other filter regardless of which tank is supplying fuel.

If you return fuel to only its own tank, I suppose you could correct trim by switching tanks often as fuel is used ?? But I do like the idea of never mixing the fuel between the tanks.

[Gary E]

Correct trim????????

How many gal tank are you talking about??

[ron ll]

Oh. Sorry. My tanks are 200 gallons each :) .

[Gary E]

If your gona replace them, I'd change it so that either could be empty or full and not affect trim.

[ron ll]

That would involve some pretty significant remodeling of the boat. :D These tanks are each 6' long, 4' tall, and about 24" athwart (tapered top to bottom). I still think the most important is to have the two tanks isolated from each other so that one contaminated tank doesn't spoil your whole day.

[Gary E]

So your going to REPEAT a louzy design? when you know there's a better way?

[ron ll]

Ha ha. As soon as I posted that I realized my mistake. You're right, I could separate each tank in half fore and aft, and then plumb the two halves across the boat. The tanks have to be baffled anyway so no big deal. I'll have to think about that some more. Thanks for blowing some cobwebs out of my brain.

[ron ll]

Dale:
Didn't mean to hijack your thread. Hopefully some of this is of interest in your situation as well.

[py]

My previous boat had 2 tanks, about 500 litres each, each had an inline cock before going into a T, which then went to the filters. Return went to one tank only. I could leave both cocks on and let the levels look after themselves, or I could turn one off and draw from port or stbd only. I'd usually run on the tank with the return line for quite a while, then switch to the other tank, knowing that the return would slowly add fuel to the first tank.

[HR]

I would be very careful about not switching the return line along with the feed line. Some diesels, particularly newer ones, return a LOT of fuel and it's pretty easy to pump a tank dry that way. Don't ask how I know!

[suedebriar]

Does anyone have any experience with using a day tank? I am converting my boat from gas to diesel and always thought about buying a new tank. When I got the old tank out and looked at it closely I discovered it is made of monel. My choices are (as I understand it) is to have a return installed in the tank or use a day tank of about 10 gallons. The day tank would be replenished when it gets low through a diesel pump. Big generators use this arrangement and I would like to know if anyone has any experience with it?

[Stiletto]

I crewed on a yacht that had what might be called a day tank. It was a header tank that held at most a couple of gallons. The boat was a 30footer used mostly for racing during the season so the motor was used mainly for getting on and off the mooring and out to the starting area.

[RandyT]

I have 6 tanks in my 1967 Garden Wanderer 44; 3 midship, 2 aft and one topside. The aft tanks are 250 gals. each, the midship tanks are 170, 170 and 190 gals each and the topside tank is 12 gals. The midship 190 gal. tank is the day tank and with the 2 Perkins 130's running at 1800 RPM and 4-7 gals/hour total, it takes a full day to drain it. Trim is a big deal with the fuel capacity (not to mention the 500 gals of water aft) and the system works fine. The 12 gal tank is a day tank for the heater. The main day tank filling is via valves and the heater tank is filled with an electric pump. Each engine and the heater have have their own primary filter off of the 190 gal day tank.