To A New Paradigm With FPB

Wicked FPB 97 Tank Volumes

FPB 97 Tank layout

The FPB 97 has a wicked amount of fuel and water capacity, so much so that only in special circumstances would the full amounts of either ever be carried. The weight and position of these liquids obviously have a substantial impact on fore and aft trim and stability. We are dealing with close to 37,000 liters/9,800US gallons of capacity (after deducting for structure, separating coffer dams, and stabilizer coffers).

The drawing above shows the tanks at half height. There are many ways to calculate volume, and we normally employ two methods as added insurance, it being easy to get these figures wrong. The data above is from our 3D modeling software. It tells us the volume and center of each tank section. We double check this using sectional area at each station, as indicated in our hydrostatic analysis. A deduction is then made for the structure. Being a metal boat, this is usually in the range of three percent (fiberglass tanks can cost as much as 10% of the volume for structure).

FPB 97 Tank Layout 2

Between diesel and fresh water tanks there is an open area, called a coffer dam. This separation exists to isolate the tanks in case a leak develops.

Deciding how to divide the total volume is relatively simple, at least in theory. There needs to be enough fresh water capacity so its weight can replace diesel burned on a long passage if there are risks of severe weather. Unless the FPB 97 were caught by a vicious storm and very light on diesel, you would not use more than half the fresh water capacity. But in rare situations it might all be warranted.

That leaves the diesel, and how much fuel do you really need or want to carry? While there are some absolute stability issues involved with the fresh water, beyond a certain level extra fuel is not usually going to be warranted.

How does one decide the correct ratios? This is where we encounter the baseline issue: how is the boat going to be used? For everyday, local cruising, where fuel supply disruption or availability is not a concern, a thousand gallons/4000 liters, is going to last a long time. Let’s say you are spending the summer on the East Coast of the US. Lets assume 800 US gallons/3000 liters is aboard. That is enough to take you something like 1100+ NM at 11 knots. With the solar array supplying at-anchor power, daily generator consumption will be minimal.

On the other hand, if you think prices are going up, or you have found a deal, it might pay to load up. We’d probably be content with 4000 US gallons/15,000 liters. This stretches the range to well past trans-Atlantic. Concerned about the geopolitical risks? Then fill her up. We’re now talking 6800 US gallons/25,700 liters. If you want to really stretch the range, change one of the two props to a folding wheel, reduce pitch on the fixed prop a touch, and back off to 200 NM per day. Watch your weather carefully, and minimize the air conditioning loads, and you just might do a circumnavigation without refueling.

The last part of this decision-making process is where the fuel and water is placed in the hull, and how the tanks are segmented. Here the issue is minimizing trim impact as fuel is consumed, and maximizing the leverage of the fresh water to change the trim, should it be desired.

The centralized fuel tanks have little impact on longitudinal trim as fuel load is changed. As the fuel load lightens, there will be a tendency to trim by the stern, generally a good thing for the performance and control. But if you prefer even trim, this can be easily compensated by adding a little water forward. Three water and four fuel tanks allow a high degree of flexibility,

We’ll discuss fuel consumption at various speeds, and trim considerations, in separate posts.


Posted by Steve Dashew  (April 22, 2012)




13 Responses to “Wicked FPB 97 Tank Volumes”

  1. Matt Marsh Says:
    Hi Steve, Although the thought of a $31,000 fuel bill is terrifying, I do think you’ve hit on a very important design feature with the tankage in these FBPs. It is far, far better to have the capacity and not use it, than to need the capacity and not have it. As supporting evidence, I submit the thousands of cruising boats with jerry cans tied to the deck because the builder thought that 300 litres would be more than enough for any conceivable purpose.

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    Steve Dashew Reply:

    When you are buying in tanker load quantities, Matt, the price comes down a lot, probably a quarter or more. Also, think of the money you would be saving compared to what a 100 foot sailboat would run. The per mile cost of the FPB 97 is probably half of the sailing cost.

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  2. David Says:
    6800 gal x $5.00/gal = $34,000 USD. That makes my home heating oil bill seem pretty tame. Of course eating through high tech sails and running rigging for a 97′ boat isn’t cheap either.

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    Steve Dashew Reply:

    The cost of running the FPB 97 will be a lot less than the sailing equivalent (which we know from experience), and the fuel cost per gallon would be significantly less than the pump price since you’d be ordering a tanker for the fill up.

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  3. SARAH SARAH Says:
    Steve Will the 97 have wing tanks for athwartship trim adjustment or, once the dink is deployed, is there no appreciable trim alteration? Bill.

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    Steve Dashew Reply:

    Hi Bill: There will be less change in trim than with the 64 since the upright stability is twice the FPB 64, but the dinghy moments off center are less. We will use the day tank auto adjustment, as with the 64, to level the boat out. There will also be wing tanks for longer term leveling.

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  4. Martin Says:
    On the British perpective: 25700 l @ £1.50 = £38,550. Or at $1.60 to the £, $61,680. Given how much of our fuel price is tax, I’m not sure you save much buying by the tanker-load either. It’s not so much trans-Atlantic range you need to visit us, as twice that to get you back home again! There must be a way of sticking a giant wind turbine on the foredeck so you can collect the subsidy…

    [Reply]

    Steve Dashew Reply:

    When we were in your part of the world in 2008/2010 fuel in Gibraltar was half the cost of the EU. Or, you could fuel up in the US, spend a season or two in Europe, and then return west without the Eurocentric fuel issues.

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    Bob Reply:

    Gas Oil is currently circa £0.71 per litre (plus VAT) for a full tanker load in the UK.

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    Martin Reply:

    Ok, yes. I hadn’t looked at the price of a tanker-load of the red stuff. Adding 20% VAT makes it about 86½ pence a litre and leads to a fill-up price of £22,200/$35,500. My bad. Ok everyone, come visit us after all!

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  5. Patrick S Lasswell Says:
    The most extreme example of fuel inflation was during UN sanctions on Haiti where the efficacy of the US and Canadian Navies was able to drive gas up to $40/gallon circa spring 1994. It was then worth the price of buying a ship, loading her with fuel, delivering a small cargo of humanitarian aid, getting the ship banned from returning, selling the ship, buying a ship, painting on a new name, rinse and repeat ad infinitum. Much of the price of fuel is political excess. While this craft might not take you beyond the reach of politics, it stands a fair chance to take you out of its grasp.

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  6. Brian Says:
    Folks – unrelated to the fuel tank issue – people have spoken here about using the ipad on their boat – and just wanted to share this new product that makes it safer to take your ipad on the water: http://www.gizmag.com/lifedge-waterproof-ipad2-case/22286/ Brian

    [Reply]

    Ben Hines MD Reply:

    Check out http://www.drycase.com. Way cool for ALL I Pads,phones etc. Ben

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