Fuel System and Tanks

On my previous boat, “SENTA”, I had two diesel tanks: one to starboard and one to port. Both tanks were installed with a slight slope so they could drain, with an outlet at the lowest point. Not through a thin fuel hose, but via a substantial 1½″ connection. Both outlets came together in a small manifold tank located centrally and low in the bilge. From this manifold, the engine drew its fuel. Between the manifold tank and the fine filter on the engine there was also a coarse (pre) filter. I occasionally replaced that filter, but more out of routine than necessity: it never contained water or contamination. The manifold tank had a drain valve at the lowest point, allowing any water or debris to be tapped off periodically. That way I always had clean diesel. The tanks themselves also stayed clean, because any contamination could sink via the large 1½″ hoses to the manifold tank, where it could be removed.

This approach differs from what is common on many pleasure yachts. There, the tank is often placed low in the boat and fuel is drawn via a fitting on top of the tank, using a pickup tube that ends just above the tank bottom. The major disadvantage is that over the years contamination accumulates on the tank bottom. At some point it will start clogging filters, potentially causing engine problems. With today’s “green diesel” and the associated risk of microbial growth, this conventional setup can be quite vulnerable.

There are roughly three common approaches in our type of boats to reduce or eliminate this vulnerability:

1. Equip the tank with a water separator and drain valve (often difficult to fit if the tank is low in the bilge).
2. Install a higher “day tank”, a solution more often seen on larger vessels (space is not always available).
3. Fram’s solution: two draining tanks feeding a lower manifold tank.

Fram’s primary fuel is diesel. It is used not only for the engine, but also for two hot-air heaters and for cooking. The pump for the diesel stove is located above the level of the main tanks, but for the hot-air heaters the pumps sit lower. I prefer not to have static fuel pressure on these small pumps. Therefore, I added two small auxiliary tanks dedicated to the heaters. These tanks have their own shut-off valves and are fed from the manifold based on the principle of communicating vessels.