Originally Posted by sunchaser
Shall we take a step back and drift a bit? A few questions:
- What common problems does a day tank address?
- How large should a day tank be in terms of hours before hands on re-fill?
Nordhavns traditionally have had a day tank setup. I was going through the specs on a new Nordhavn build and noticed the model I'm interested in has only one very large fuel tank and no day tank. Hummm.
It's all about ensuring clean fuel.
The idea of a day tank is to run the machinery off an isolated supply of known-good fuel. To ensure it's know-good, fuel only get into the day tank by way of a filtered transfer system. Fuel is not allowed in via any other path since it might carry contaminants.
Purchased fuel always goes in one or more storage tanks where it is quarantined from the engines. It can be total shirt, but that can't get to the engines.
Even if you do have total shirt in the storage tanks, running through a water separating, 30 micron (give or take) filter will remove anything that could potentially stop the engines. Worst case is you need to change transfer filters while moving fuel, but the engines should remain safe and running the whole time.
As for size, there is really no hard rule. The trade off is how much time you spend filling the day tank. I don't see any down side to a large day tank, say one that can run the boat for a full 24 hrs or even multiple days. But I think more practically it will be less. I don't think I'd want to have to mess with it more than once every 6-8 hrs, and definitely not less than 4 hrs, or whatever your minimum shift time is. And don't forget that you want to fill before the tank get down below maybe 20%, and only fill to 80-90%, so allow some margin.
Nordhavn calls it a "Supply Tank" because on many models it's too small to be a useful day tank. On our 60, we took two of the small storage tanks and opened them to permanently gravity feed to the supply tank, and treated the three interconnected tanks as a single, large day tank. The deck fills for those tanks had never been opened when I sold the boat. It was 300 gal total and worked really well. We would fill when it got down to 60 gal or so, but there was plenty of slop if you weren't right on time. And we would fill to no more than 250gal, and often under 200 gal. When on a passage I would typically fill every 12 hrs, but we could go longer. It was nice to not have to be right on top of it.
I think the risk is that if you aren't minding the shop, you can perhaps more easily run the tank dry. We had the tank level alarmed at about 50 gal and the sound would wake the dead. My wife hated it when the alarm went off, so she ended up being the best alarm, "reminding" me to refill.
On our new boat I am making the supply tank a good bit larger than standard to make it an effective day tank. It will end up in the 175 to 200 gal range which should cover an 8 to 12 hr shift, depending on speed.