Fuel & Water Tanks

One of the key design elements in the FPB series is the approach to fuel and water tanks.

With a metal hull, the tanks are integral – the hull itself forming the outer skin of the tanks. In the photo above you are looking into the fuel tanks in the center of the hull, under the great room.

This has two advantages. First, it is possible to have enormous volumes of liquid carried low in the hull, where it assists with stability. Second, the integral tanks create an additional safety factor in collisions. The fuel tank tops, for example, are above the waterline in most load conditions.

Fuel Tanks

Fuel tanks are located below the raised salon, and contain approximately 3200 US gallons (12,000 L) of fuel. They are divided into four tanks. Forward there is a large central tank and two outboard “wing” tanks. The aft section is one large tank.

This arrangement allows for easy trimming of the boat. Wing tanks can be adjusted to trim out any heel, and the fore and aft tanks give good control of longitudinal trim.

Engine Room Day Tank

In addition there is a large “day tank” in the engine room for feeding the engine, genset, and diesel heater.

The day tank is arranged so that it is normally kept half full, and automatically refilled when it reaches the quarter level. Capacity is 410 liters/108 US gallons. When the heavy dinghy is launched the boat will often pick up a slight list to starboard. This can be eliminated by filling the day tank. The photo above shows the forward end of the day tank and its sight gauge.

This huge amount of fuel capacity gives enormous range, obviously more than is probably ever going to be used. So, why this huge capacity?

There are a number of reasons. First, the boat can handle the weight very efficiently. Second, carrying so much fuel offers a wide variety of options for when and where to fill up. If you are in Seattle at the end of the summer and heading south, you can probably save 30% on your fuel bill by waiting to buy until you clear into Mexico in Ensenada. The third reason is emotional. We don’t know about you, but we like knowing that the boat has the ability to go for years without needing fuel, should a disruption in supplies occur.

Fresh Water Tanks

Fresh water tanks are located under the soles of the sleeping cabins. The tops of these are 250mm (10″) below the datum waterline (75,000 pound/34,000kg displacement). The aft fresh water tank holds 750 US gallons (2840 L),

while the forward tank has the capacity to hold 1000 US gallons (3750 L).

That’s way more water than you would normally need for drinking and cleaning given the ability these boats have to make their own water and catch it off their decks. The capacity is there so that you can easily adjust trim and displacement.

Payload and Trim

Bow down trim is more comfortable heading into the waves, while stern down is preferred when surfing. You don’t have to change trim, but if a long passage is in the offing, it is a nice capability offering a smoother ride and the chance to save some fuel through increased efficiency. The variety of fuel and fresh water tanks give you wide trimming latitude.

The FPB series are optimized to cruise heavy. This means you will have maximum comfort with the boat at full load, while still being efficient under power at cruising speed. As fuel is burned the boat gets lighter. This is fine – and more efficient – in protected waters. If you are just powering for short hops, heading downhill in moderate ocean waves, or passaging in protected waters, having the boat light is great. But when you are heading into the waves for days on end, or crossing the ocean with a stiff breeze, those big water tanks, filled so that the boat remains at full load, will be much appreciated.

On the FPB 83, Wind Horse, we’ve found that even in conditions where we don’t need the weight, we tend to fill the water tanks on the last day of a passage (by running the watermaker). This allows us to take long showers and deep baths when we’re at anchor, without worrying about using the watermaker.

To find out more about the FPB 64 contact Sue Grant: Sue.Grant@Berthon.Co.UK..

Posted by Steve Dashew  (October 16, 2011)

7 Responses to “Fuel & Water Tanks”

  1. Yme Bosma Says:

    Do you use a coating inside the aluminum water tanks?

  2. Steve Dashew Says:

    We have never coated the inside of our aluminum tanks. Our feeling is that this can cause all sorts of problems if/when the coating begins to break down.

  3. Douwe Gorter Says:

    Hello Steve,

    We are building a new aluminum sailboat, Dykstra Design, 55 foot. The yard we’re building at here in the Netherlands informs us integral fresh water tanks need to be coated given that fresh water contains “bleach” that apparently leads to corrosion of fresh water tanks. Do you bunker fresh water or do you only produce your own water with the watermaker? The aluminum is 5083.

    Your website is highly informative to me, i got many good ideas that from your immense experience! Based on your recommendation we using Armaflex for insulation contrary to the normally used PU foam.

    Thank you for sharing this on your website.

    Thank you for your reply!

    Best regards,

    Douwe Gorter

  4. Steve Dashew Says:

    Many schools of thought on this and it depends on the water supply chlorine levels. But, we would NEVER coat a water tank as the risks if there is a coating failure, are very high for serious corrosion. If you are concerned fit a pre-fill filtration system to remove the chlorine.

  5. Yme Bosma Says:

    Douwe, Steve,

    We are having an aluminum boat build as well at the moment (in The Netherlands), and I got the exact same feedback from the yard. I was advised to use a thixotropic, two component, epoxy coating. For instance from De IJssel, they apparently use it for coating aluminum drink water tanks. I’ve decided not to do it, as it will be possible to have it done at a later stage if necessary.

    I too was inspired to use Armaflex for insulation by the way…

    Douwe, may be it would be a nice idea to be in touch since we are building a boat (in Holland) at the same time with some overlapping ideas? You can contact me at mail@yme.nl.

    Thanks again Steve for you feedback!

  6. Thomas Says:

    I don’t see the advantage to have the fuel tank tops above the waterline. In a collision integral tanks are really convenient, as the possibility to have the tanks damaged and the sea polluted by leaking fuel is quite low. But what is the benefit of above the water line tanks?

  7. Steve Dashew Says:

    The advantage of above the waterline tanks (depending of course on state of vessel loading) is if holed, there is nowhere for the water to collect, i.e. it eventually has to run back out of the hole. So that trim is not affected, or marginally affected by the breach in the hull’s integrity. Consider the opposite, say tanks are a foot/300mm below LWL. In which case a hole is going to allow that much volume of water into the breached area, with both major impact on trim and damage from the water rushing back and forth.