Even with the unusual flexibility offered by the Nordhavn fuel manifold, we found it didn’t do some of the things we wanted it to be able to do so we made fairly substantial fuel manifold modifications on Dirona. Some of these modification were driven by us extending some of the applications of the manifold and some were driven by us operating the fuel systems a bit differently from some. Let’s start first with how the standard manifold works, look at the most common operating modes, and then at the manifold changes we made and why.

Most Nordhavns come delivered with a separate day tank to feed just the get-home (also called wing) engine. One of the most common causes of diesel engine fault is dirty fuel, so having a separate fuel tank where no fuel is even placed there unless first proven to be good via use for days in the main engine adds considerable security. The wing engine has a separate, known clean fuel in addition to its own mechanical control system, transmission, prop shaft, and prop. It shares almost nothing with the main engine, reducing the likelihood of a correlated main and wing failure.

In addition to the day tank feeding the wing engine, there is a supply tank which feeds all other engines on the boat. The supply tank is always the fuel source for the engine(s) and generator(s). There are also multiple bulk storage tanks. On Dirona, we have two side tanks of 835 gallons each, a supply tank of 65 gallons, and a day tank of 15 gallons. The day tank feeds only the wing engine, and the supply tank feeds all others engines and generators. The bulk tank contents are moved into the wing or supply tanks prior to using the fuel.

The picture above shows the fuel transfer manifold on Dirona when it was delivered in early 2010. It’s similar in functionality and design to the manifold delivered on most Nordhavns, although many have more tanks, engines, and generators. The lower manifold is the transfer manifold and the upper is the return manifold. All engines, except the wing, draw fuel from the supply tank and return it to the return manifold. The fuel transfer pump sources from the transfer manifold (the lower one). This transfer manifold selects which tank the transfer pumps draws from. The return manifold gets the output of the transfer pump and the return from all engines except the wing. It’s this manifold that sets which tank the return goes into. Understanding how the systems are laid out, let’s look at how they are typically used and why some of our usage models are different and the design extensions we implemented to support these other operating modes.

The most common operating mode for Nordhavns is to choose one of the bulk fuel tanks to draw fuel from and to open the valve at the bottom of that tank to gravity feed into the supply tank. The return manifold is set to send return fuel back to the supply tank. Since the supply tank bottom is below the bulk tank bottoms, the supply tank won’t run out in this mode. As the fuel draws down, the selected bulk tank gets lighter and the boat will eventually start to list away from it. At that point, the gravity feed from the first selected bulk tank is closed and another is opened on the other side. This keeps the supply tank full and keeps the boat relatively well trimmed.

To further improve the trim, some owners chose to have all the bulk tank gravity lines open. This has the advantage of pulling them all down equally but there are two downsides: 1) you might want to more more fuel on one side to correct a list (perhaps the dinghy is down) and 2) having tanks on both sides of the boat connected allows fuel to move side-to-side which isn’t ideal from a stability perspective. Consequently, I don’t recommend running with more than one of the gravity feed lines open at a time.

Another variant of the single-gravity-feed-at-a-time model is to return fuel to the bulk tank that is currently gravity feeding into the supply tank. The tanks will all run at the same levels in this mode of operation, and it can allow cooler operation. Here’s why. The bulk of the fuel the engine draws from the supply tank is not consumed, but is used to cool the injectors and other fuel parts and the warmer fuel is returned. If just the supply tank fuel load is in circulation, that fuel will heat up. Whereas, if the entire bulk tank and supply tank fuel load is in circulation, there is much more fuel and much more fuel tank surface area to cool and the fuel will run cooler. Modern engines measure fuel temperature and take into account changes in temperature when computing the amount to inject, and cooler fuel does allow just slightly more power. This mostly is irrelevant but just barely useful enough that, if you do chose to gravity feed as most do, I recommend transferring back to the bulk tank that is currently gravity feeding rather than directly back to the supply.

We chose to not gravity feed to the supply tank even though, as described above, this is an easy to manage and reliable way to operate the fuel system, and it would keep the fuel cooler. Instead we chose to explicitly pump fuel from the appropriate bulk tank to the supply tank every four hours rather than gravity feed. This is a slightly more manual operating mode but has some advantages that we really like. The first advantage is if there is a leak on the engine, at the filters, or in any of the fuel lines, you can’t possible loose more than the volume of the supply tank. If you are gravity feeding, you could lose the entire bulk fuel load and could end up out of fuel and risking environmental damage via a large fuel spill. Avoiding this is important any time but even more important when doing long ocean crossings sometimes more than 1,000nm from the closest shore. Having no fuel when days from shore could really be a disappointment.

The second advantage of the explicit fuel transfer system is all fuel has to pass through the transfer filter before it gets to the supply tank. Given the uncertainty of fuel quality world-wide, we really like a layer of filtering prior to the fuel even getting to the supply tank. The combination of keeping the bulk fuel locked up and safe from leaks and the additional layer of filtering makes this operating mode important to us. It is a bit more manual work but it feels worth it. This is the source of the first fuel system modification we made. The standard fuel pump, a Walbro 6802, is incredibly slow at 43 gallons per hour. In fact, so slow that this way of operating the boat can be frustrating. So we replaced it with a Jabsco VR050-1122 pump capable of 660 gallons per hour.

Like many modifications, when you make one change, it can drive others. To accommodate the transfer rate of this pump, we needed to go with a much larger transfer filter. We went with Racor FBO 10, pictured below, which is commonly used in bulk transfer commercial fuel management applications. This filter has the advantage of supporting large transfer rates but it also has large filtration area so few filter changes are needed.

One of our goals is to be able draw fuel from the supply tank and return it to the supply tank while polishing one of the bulk tanks. The standard manifold design doesn’t support this. The engine return goes to whatever tank the transfer pump is returning into. Unless you are gravity-feeding, polishing one of the bulk tanks while underway has the downside of the supply tank being completely pumped out every 30 to 60 minutes and runs the risk of running the main engine out of fuel. So we made manifold changes to support what we wanted.

Dirona‘s manifold pictured at the top of this post supports many extension from standard. The first to address the issue we just brought up. If you look closely you’ll see that we can polish fuel from a bulk tank back to the same bulk tank but still direct the main engine fuel return to the supply tank. There is a bypass that runs between the engine return and the supply tank fill that allow the main engine to return fuel to the supply tank while still being able to polish fuel in any other tank. This bypass hose can be seen running through a valve on the right side of the manifold.

Another addition we made to the manifold is provision to drain pump out of the supply tank. We have added a hose from the bottom of the supply tank into the transfer manifold allow the supply tank to be polished if a fuel problem is encountered. It also allows the supply tank to be pumped out if there is a need to service it or some of the fuel lines in that area. Because we can pump out the supply tank, and the supply tank is below the wing tank, we can actually pump out the wing tank as well by first pumping the supply tank level to below the bottom of the wing tank and then opening the wing and supply return manifold valves and allowing the wing tank to drain down into the supply tank. We think it is super important to be able to pump out, service, or re-filter the fuel in any tank and especially the wing and supply tanks. These changes allow the supply tank to be directly polished underway and supports draining the wing and supply tanks if needed.

The next extension is to allow Dirona to carry more fuel in those rare times when greater range or higher speed over long distances are needed. Dirona as delivered is capable of around a 2,400nm range and this is more than enough for 99% of all she will ever do. However, there are times when very long crossings are planned or when we want to run faster on a passage that is within range. The nicest solution is to put more tankage on Dirona but it’s impractical to install more and it’s probably not worth the space compromise that has to be paid every day for the entire life of the boat just to get more range or speed on a long crossing. You may only need this greater range every few years and yet more tankage take up more space all the time. Our solution is to run on-deck fuel tanks when we do want to run more or ran faster. This is more of a hassle but, since the extra fuel is rarely needed, it feels like a better answer on a small boat than giving up more space inside the boat. Our longest run has been 2,600nm , and having more fuel made this much more practical. But, in five years, we have only needed this additional capacity once and only used it twice. On-deck fuel bladders are a good compromise when you don’t want to give up more space and very rarely need more fuel.

To make the bladders easier to manage, we have a bulkhead fuel fitting in the cockpit plumbed into the fuel manifold at bottom left (and pictured above). When we install the bladders, we install a short length of fuel hose between the bladders and the bulkhead fitting using cam lock snap fittings. This allows us to drain the bladders without going on deck and without having the fuel intakes open to potential water ingress. When we are ready to draw them down, we just turn on the fuel transfer pump, select the tank we want to pump into, and the transfer pump quickly does the work. This has the added advantage of putting all bladder fuel through a filtration phase before bringing it into the fuel tanks.

We’ve mostly gotten good fuel, but there have been a couple of times over the last fifteen years when we’ve bought some expensive water, or picked up some fuel with lots of foreign matter. We buy fuel all over the world and the good news is that bad few is fairly rare. But it does happen. Our defense against it is mass filtration with lots of spare filters. The way we use the boat, fuel will be filtered at least four times before reaching the engine injection pump: 1) through the transfer filter to the supply tanks, 2) through the primary filters to the main engine, 3) through the first on-engine filter, and 4) through the final on-engine filter. We have a lot of filter spares on board, with more than 40 of our primary filters stored away. If we get bad fuel, we probably have the filtration to be able to manage the problem.

The final issue is complexity and human error. Nordhavns have very flexible fuel transfer systems but with flexibility comes some complexity. On Dirona, we have extended the design but, with those extensions comes some additional complexity. It’s hard to avoid. And where there is complexity and potential tired boat operators, mistakes can happen. The most common mistake is to close an engine return valve or close the return manifold tank connection. This causes the running engines to not be able to return, which will very quickly lead to leak or pump failures. You can disable an engine quickly this way. Another mistake is to accidentally pump fuel overboard.

We battle complexity and potential error every way we can think of, including posting the fuel transfer diagram at the manifold and having all valves brightly and clearly labeled. We have also calibrated the sight gauges in our all our tanks and installed redundant digital tank level monitors. We have installed a digital fuel transfer timer and both calibrated it and labeled it for the number of gallons transferred per minute. So, if you are moving 17 gallons, you can see exactly how many minutes of transfer time is needed, substantially driving down the risk of mistake. But it is still possible. To catch mistakes in either direction, we also have digital level indicators on all tanks, a high-level alarm on the supply tank, and low level alarms on the wing and supply tanks.

Finally we label all fuel transfer valves as normally off or normally on to make it clear where they should be in normal operating mode. But, even this isn’t enough. In a storm with only two people on the boat, there is a risk of getting tired. And, if there is a fault at the same time, mistakes get harder to avoid. So, we tie-tag all valves open that need to be open to avoid the blocked return problem described above. The only way to close a valve that could hurt an engine is to go and get wire cutters and cut the tie tag off.

All these design changes give Dirona a flexible system that can polish fuel while operating at sea, can’t lose all the fuel in a fault, supports easy service, and helps manage human error while still offering a fairly flexible system.