How much fuel you carry on board will depend on how far and how often you cruise. Our boat has five tanks, two saddle "cubes" on each side of the engine room that hold 85 gallons each and a 60 gallon day tank in the bilge in the engine room. This give us a total of 400 gallons.
The engines and generator normally pull from the day tank which we refill periodically by gravity from the saddle tanks. The engine's fuel return also is normally valved to go to the day tank. Our fuel management process is to drain an opposing pair of saddle tanks into the day tank which normally takes a couple of months or more depending on how often we're able to take the boat out. When the front or rear pair of saddle tanks is empty we leave them that way while we start pulling from the other pair of opposing saddle tanks. When this pair is about 1/3 full we then fill the empty pair and repeat the process.
If 220 gallons is sufficient for your typical useage than there is no point in carting around the additional 110 gallons. As you say, weight at the back (or front) of a boat can affect its trim with regard to performance unless that weight was intended to be there and the boat was designed accordingly.
What is important is that your fuel stay as fresh as possible. Better that you run through your fuel reasonably quickly and then refuel than have fuel that sits on the boat for months and months and months.
How is the fuel pulled from your auxilliary tank? Is it a gravity flow to the other tanks or to the engine fuel feeds from the lowest point in the tank or does it use a typical pickup tube? The ideal situation is to be able to pull off fuel from the lowest point in the tank which will get water and contaminants out. But if the tank has a pickup tube that leaves a few inches of fuel in the bottom of the tank un-picked up, that could provide a chance for water to accumulate in the tank over time as well as contaminants.
So if you don't need that fuel on a regular basis, and if there is no risk of buidling up water in the aux tank from moisture from condensation or any other source, then leaving it empty may be a smart idea.* Ideally the aux tank is accessible and has at least one inspection port that can be used to examine and clean out (if necessary) the tank.* If the tank is empty now, or when it becomes empty, you could pull the inspection port and make sure the inside, particularly the bottom, is water and contaminant-free.* At that point it should be able to sit empty for years if you have no need of its capacity.
Additives can help extend the viability of fuel but that is even a matter of debates as a search of this forum's archives will show.
We use two additives in our fuel at the recommendation of our diesel shop and friends in the marine diesel industry. We have debated the effectiveness and necessity of additives on this forum, but at this point we've elected to follow the advice we've been given by the people we know. So we use Hammond's Biobor, which is a bug-killer for diesel fuel, and Hammond's Select 3 which is a stabilizer and lubricity additive.
There are people who believe biocide, stability, and lubricity addives are a good idea, people who believe they are a bad idea, and people who believe they are unnecessary but don't hurt anything. What I know about additives and petro-chemistry wouldn't fit on the head of a pin so all I can do is make a judgement based on my take on the people who have given us advice.
I have no experience with ValvTect as it was not one of the additives that was recommended to us. I'm not trying to imply that it's good, bad, or indifferent. I simply don't know.
-- Edited by Marin on Wednesday 28th of July 2010 08:20:49 PM