Day Tank

The HR 41 does not have a day tank fitted as part of its fuel system.

We wanted to fit a day tank since we purchased our 41. On sister vessel Ondine (hull #14) the aft holding tank had been converted to a day tank and so we decided to do the same.

Day tanks give a number of benefits to marine diesels:

  • if the engine’s fuel pump (also known as the “lift pump”) fails then the day tank will gravity feed fuel to the engine
  • the fuel that has been passed through the engine without being burned is returned to the day tank. This fuel has been heated by passing through the engine block and so any condensation issues between it and the cooler fuel in the main tanks are avoided as the temperature of the day tank is raised with all the fuel being at the same temperature – diesel engines pump much more fuel than they burn (often 4 times as much) and the excess fuel serves as both lubrication and cooling
  • the main fuel filters are between the day tank and the engine so the fuel is filtered multiple times as the unburned portion is returned to the day tank

Following the approach from Ondine we were able to remove the holding tank, have it power washed and sterilized, then have stainless fittings welded to it. The general layout is:

(A) air vent (part of holding tank)
(B) drain (part of holding tank)
(C) heater outlet
(D) engine outlet
(E) lift pump inlet
(F) engine return inlet

The mounting position under the cockpit sole is higher than the engine meaning that should the main fuel pump fail then the fuel in the day tank will supply the engine due to gravity. Also the tank is higher than the heater location in the saloon and so the day tank will also supply the heater (click here for more details on the heater installation).

The outlet “B” is the previous outlet from the holding tank, we had it welded shut and then a hose tail fitted. There are approximately 5 litres below outlets”C” and “D” which is where water and sediment will accumulate. By opening the outlet “B” these impurities can be removed.

The inlet “F” which is the engine return has a pipe on the inside so that the fuel returned under pressure is released against the aft wall to minimize foaming. The inlet “E” which is from the manual lift pump simply ends on the front wall as the fuel is not being moved at any appreciable rate.

There is a clear plastic fuel line on the left hand side which serves as a fuel gauge. There are 2 vertical baffles inside the day tank to stop fuel sloshing around when the boat moves.

Assuming the Westerbeke uses 4 litres per hour of diesel fuel, we should have had a day tank of 50 or 60 litres so that we could adequately cover a 12 period so we could refill it twice per day. As we used the previously sized holding tank we made due with the tank volume which is just over 40 litres and therefore slightly less than 12 hours running.

<more to come>

  •  even metal fittings with tapered threads will leak,  we sealed the fittings with Hylomar Universal Blue sealant and stopped any leakage

  •  the heater runs perfectly off the day tank

© The contents of this site are the copyright property of the authors.  Visitors may read, copy, or print any material for their own use, free of charge.  No material printed or copied from this site, electronically or in any other form, may be sold or included in any work to be sold without explicit permission from the authors.