The Blue View - Adding a Daytank

If you have decided to add a daytank to your engine, there are a few decisions to be made. The first decision is how fuel will be transferred from the main fuel tank to the daytank. One option is to use the existing engine lift pump. This is simple and inexpensive, but has two drawbacks. If the daytank is low on fuel, it takes a long time and a lot of hand pumping of the lift pump to transfer an adequate amount of fuel into the daytank. The second drawback is that once the engine is running, the rate of fuel transfer will be greater than the rate of fuel consumption by the engine, and eventually the daytank will overfill. To prevent the fuel from overflowing through the vent hose, a fuel return line must be added from the daytank back to the fuel tank. This return line could be tee'd into the existing return line, but the hose diameter may have to be increased to allow for the extra fuel flow. The other option for transferring fuel is to use an inexpensive electric fuel pump. An electric fuel pump will quickly fill the daytank. It can be controlled using a simple On/Off switch or a controller can be incorporated that will automatically keep the fuel at the desired level. Alternatively, the pump can be run continuously whenever the engine is running, as long as, as before, a large enough fuel return line is added. The sketch below is a diagram of a very simple implementation using an electric fuel pump.

The second decision is how large the tank should be. If your vessel is a typical sailboat, a daytank that holds 3-4 gallons (12-16 liters) will provide at least a couple of hours of motoring. If you have a power boat or large motorsailer, you will probably need a larger tank and/or an automatic controller.

The daytank must be mounted higher than the engine. Ideally, there is enough space in the engine room above the engine, but it can be located elsewhere. As I mentioned in the previous blog, I have seen other boats that had the daytank mounted in the lazarette. The fuel pump should be between the primary filter and the daytank, and the secondary filter(s) should be between the daytank and the engine. The daytank should be securely mounted for obvious reasons. A vent hose should be provided, and it should be vented outside the boat.

The next decision is what material the daytank should be made of. In my opinion, aluminum is the best choice. I made a temporary daytank out of plywood and epoxy which held up for more than five years, and had planned to replace it with a custom aluminum tank while we were in Australia. I found one online, however, at a race car supplier's website. It was quite inexpensive, came with all the necessary fittings already welded in place, and was the perfect size. I added a sight glass to the front using plastic barbed fittings and clear plastic tubing. I can tell at a glance how much fuel is in the daytank by the level in the sight glass.

Our original daytank used an electric fuel pump which was controlled by an On/Off switch. The tank held enough fuel to motor for about an hour and a half. We always checked the sight glass before starting the engine, and topped up the daytank as necessary. When we were motoring, we would set a timer to remind us when it was time to refill the daytank. When the timer went off, we would then flip the fuel pump switch on until the tank was filled. Once it was filled, the timer was started again. This system worked flawlessly 95% of the time. Sometimes, however, something would go wrong. I'd turn the pump on, then make a cup of coffee while it was filling and forget to turn it off. Sometimes one of us would forget to restart the timer, or the timer would stop running, and the daytank would run out of fuel.

We had problems often enough that I thought it worthwhile to add a controller. The next blog will provide the details of how to make an automatic controller for the daytank.

Daytank Series Pt. 1

Daytank Series Pt. 3

The Blue View - Why we like our daytank

 

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...

Daytank Series Pt. 2

Daytank Series Pt. 3