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-   -   Plumbing options for 2 identical fuel tanks (http://www.sailnet.com/forums/gear-maintenance/62321-plumbing-options-2-identical-fuel-tanks.html)

Zanshin 02-25-2010 06:48 AM

Plumbing options for 2 identical fuel tanks
 
My boat currently has 250l fuel in one tank and 750l water in 3 tanks. Since I have a watermaker I will make do with just 500l of freshwater in 2 tanks and will convert one of those water tanks to diesel.

The manufacturer of the water tank has stated that the material is not suitable for fuel storage. I am ordering an identical tank in the correct plastic from the same manufacturer (through Jeanneau). Once it arrives I will have 2 tanks in the aft cabin side by side, the port one has 2 fuel pickups, returns & shutoffs (for main and genset), the fuel line going to a Racor filter.

Now I am in the process of trying to decide plumbing options for the fuel. The only decision I've made so far is to put in a dual Racor filter and a preference for KISS, i.e. keeping the fuel delivery system from just one tank and trying to come up with an optimal (and simple) method of getting fuel levels either automatically balanced or some simple manual transfer method to the port ('live') tank.

My goal in doing this is to increase my range and not necessarily to create redundant systems, but I could be convinced to have 2 separate systems.

I'd like to hear opinions and suggestions from the forum and will certainly hear some viewpoints or aspects that I hadn't thought about - thanks in advance.

jrd22 02-25-2010 11:25 AM

I prefer separate systems, although they require more direct management than just a simple crossover pipe between the two tanks (ultimate KISS). With a crossover pipe between the two tanks and all the supply/return lines from one tank only, the fuel can transfer to the leeward (low) side when the boat is heeled. This can cause fuel starvation while motor sailing if the fuel level is below half and all the fuel transfers to the "storage" tank. You can control this with valves on both sides of the crossover line (which should be installed at each tank so you can shut them off in the case of a leak in the crossover), but that requires management and access to areas that are generally difficult. If you simply run a redundant set of supply/return lines from the second tank to your dual racor, with a set of valves and "T's" you can select either tank and simply monitor levels in each. That is exactly the system that we have and have found it easy to manage, with the inherent safety of redundant systems.

klem 02-25-2010 11:50 AM

I agree with jrd22 on two completely separate systems. On some powerboats, the tanks are tied together (requires an extra pump) so that fuel can be moved to change trim but it shouldn't be necessary on a sailboat unless one tank is all the way in the bow or stern.

Cruisingdad 02-25-2010 02:22 PM

I suspect we will be doing the same thing soon. How I am going to do it:

Our second tank will be very close to the first tank, similar to yours.

I am going to put a Y in the fuel fill line. This Y can be selected for tank one or two. This is easy stuff.

I am going to put a T in the fuel vent to select between tanks. SO when you select Tank 2, you also change the vent to tank 2.

I am going to T the returns and supplies. This again is easy to do. With teh valves, I will be able to choose which tank to pull from and which to deliever to. If I were you, I would run the Filter Boss, if that is what yo uare going to use, between them so you can effectively filter between tanks as well as out. You could even put a manual pump inbetween them so that you could run the Filter Boss without having to run a engine. THis allows you to polish your diesel without running the engine. THis is popular amongst trawlers and larger motor sailors.

THe RIGHT way is to run new everything, including the deck fill and vent, but I am too lazy to do all that and do not get excited about another hole in my deck.

My opinions.

Brian

Cruisingdad 02-25-2010 02:28 PM

BTW, it may be better just to put a permanenet Y (not changeable) so that both tanks can use the Fuel Vent.

sailingdog 02-25-2010 04:20 PM

You really don't want to have the fuel vent line on a switch...better to leave both tanks properly vented...

Bene505 02-25-2010 04:34 PM

Quote:

Originally Posted by sailingdog (Post 574101)
You really don't want to have the fuel vent line on a switch...better to leave both tanks properly vented...

CD's just making sure we are awake.

If you fill your tank from a marina with a (cold) underground fuel tank on a hot day, the fuel will expand soon after. And with a full tank, there will be relatively little air above the fuel to absorb that fuel expansion (via an increase in air pressure). So the pressure in the tank will get pretty high until BANG you burst your very full fuel tank.

(BANG added for emphasis and color. Haven't really ever burst one of those, they may go BURP or CLANG for all I know.)

Regards,
Brad

Cruisingdad 02-26-2010 08:40 AM

You are right about the permanent venting. On the venting, follow the second idea I had and permanent T vent. No switching.

sailingdog 02-26-2010 09:15 AM

More importantly, what happens when you are running from a full tank and you forget to open said vent... and the engine dies just at the worst possible moment... and then you have to bleed the fuel lines to get the thing to work again... and if you've got a really good fuel pump, what happens if you drop the volume of the tank enough and the atmosphere decides to help you out—applying 14 lbs per square inch... If the top of the tank is 20" x 15" that's an awful lot of pressure.
Quote:

Originally Posted by Bene505 (Post 574108)
CD's just making sure we are awake.

If you fill your tank from a marina with a (cold) underground fuel tank on a hot day, the fuel will expand soon after. And with a full tank, there will be relatively little air above the fuel to absorb that fuel expansion (via an increase in air pressure). So the pressure in the tank will get pretty high until BANG you burst your very full fuel tank.

(BANG added for emphasis and color. Haven't really ever burst one of those, they may go BURP or CLANG for all I know.)

Regards,
Brad



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