Several years ago we fitted Nine of Cups with a daytank, which is a small fuel tank that sits higher than the engine. Fuel is pumped from one of the main fuel tanks into the daytank and then gravity fed to the engine.
There are a number of reasons why we like our daytank. The primary reason is that it greatly reduces the possibility of an air bubble making its way to the injector pump, causing the engine to sputter and die. For all the years we owned Cups prior to our daytank, it seemed we were always having problems with air in the fuel lines. If the fuel tanks were less than half full and we were sailing on a heel, or the seas were rough and the fuel sloshed around in the tanks as the boat rolled, air would often make its way into the pickup tube inside the tank. The air would stay in the pickup tube until the next time we started the engine, then make its way to the injector pump where it would create an airlock. The injector pump would then be starved of fuel and the engine would die. Sometimes, tiny air bubbles would get sucked into the fuel line due to a small air leak, and over some period of time, ranging from a couple of hours to maybe 12 hours of motoring, the tiny bubbles would combine into larger bubbles until they became big enough to create an airlock. I changed hoses, valves, filter assemblies, gaskets and hose clamps over the years trying to get rid of air leaks. Sometimes the problem would go away for a few months, but it always eventually returned. It was very frustrating. If the engine died when we had plenty of sea room, it was an easy matter to bleed the fuel system, ridding it of any air. If the engine died when we really needed it, things could get a little tense.
The daytank all but eliminates this problem. It is the highest point in the fuel system, so any air bubbles that do get introduced into the fuel line make their way upwards and into the daytank where they dissipate rather than create an airlock.
The next reason we like our daytank is that it gives us some warning if the main fuel tank is running out, the fuel pump has failed or the filters are clogged. Our daytank has a low fuel alarm on it. If the fuel level drops below a certain point, a warning light comes on in the cockpit and the alarm sounds below. When the alarm goes off, there is enough fuel left in the daytank to continue motoring for about ten minutes, so we have a little time to either resolve the problem or find a place to safely stop the engine. Without the daytank, often the first indication of a problem was when the engine died.
A third reason is that it provides a small backup tank in the event the main tanks become contaminated. A friend of ours once filled both his tanks with fuel that was so dirty, it was clogging his filters after every hour of motoring. With a daytank, we could siphon fuel from the main tank, through our Baja filter and into a fuel can. Then we could use our electric fuel pump to transfer the fuel through the primary filter and into the daytank. Not perfect, but it would buy some time until we could polish the fuel and clean the tanks.
There are a couple of other reasons I can think of as well: Adding a fuel supply for a diesel fired heater or stove is trivial - simply add another outlet on the daytank; bleeding the fuel system is easier – the gravity forced flow of fuel eliminates the necessity of having to manually operate the engine lift pump.
There are a couple of reasons why you might not want a daytank. If your fuel system has always worked flawlessly, a daytank adds complexity to the system. And some engine rooms simply don't have room to add a daytank, although there is usually some place it could be fitted. Our friend, Noel on Sadko, put his daytank in the lazarette.
I made our first temporary daytank to get us into Puerto Montt, Chile. It was constructed out of an old 10-liter water container and some fittings we had aboard (the Importance of Junk!) We were heading into some treacherous waters and needed to be able to rely on our engine long enough to get us to port, which it did. As you can see from the photo, it was a real work of art. Nonetheless, while there, I made a more permanent, temporary daytank out of epoxy and plywood. I hoped it would last until we got to New Zealand. It is now more than five years old and I have just now replaced it with its permanent upgrade – a shiny new aluminum (or aluminium if you're a Brit or from downunder) daytank that will last a good long time.
Next, I'll talk about how to go about adding a daytank...