FPB Structural Grid

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Since we have been showing you photos of how the FPB 64 is being built, we thought it might be helpful to have a look at how the pieces fit together on a macro basis. The 3D images above and following are based on the cut files for the FPB 64, from which the boat is actually built.

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A fish eye view from the bottom. Framing has to work with plating. The more closely spaced the grid, the thinner the plating can be, as it has less distance to span. This is a very tightly spaced grid system, so plate thickness could be reduced to 6mm (1/4″) and still be considered conservative with the Lloyds Special Service Rule. Yet we are using 12mm (15/32″) for the bottom and 8mm (5/16″) for the topsides. Why? Because we can afford the weight, and the combination of plating thickness and closely spaced grid increases the ability of the boat to absorb punishment.

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We are looking at the fuel tank area between the great room bulkheads. The closely spaced frames afford another advantage – they act as efficient baffles within the fuel tanks (here) and water tanks (ahead and behind fuel). Notice the partial frame immediately forward (right side) and aft (left side of the image) of the great room bulkheads. These mark the ends of the fresh water tanks. The space between these and the bulkheads are called coffer dams. Coffer dams are used to isolate fuel and water supplies.

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Baffles reduce liquid movement which is important to motion, affects safety in heavy weather, and reduces sloshing noise of tank contents. We are looking up at the bottom here (fish eye view again).

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The forepeak can become highly loaded when working through big head seas. So it sports the most closely spaced framing of the entire structure. You can also see the massive framing which supports the stem bar (bow). This is to improve the odds in a collision. Further increase in safety comes from the collision bulkhead just behind the stem bar supports. The aft end of the forepeak is defined by another watertight bulkhead. The area between the forward collision bulkhead and the next partial bulkhead is for storage of anchor chain.

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This view is of the engine room, looking down from just aft of the middle of the starboard side. One of the advantages of aluminum is that all of the engine room structure – beds for engine, genset, and other systems – are welded into place. This is strong and allows the structure to be tuned so it minimizes machinery vibration transmission to nearby structure.

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The structure has to deal with local loads – stabilizers, rudder, prop, log impact – as well as global loading, as when dried out on a mud flat, or supported between a pair of steep seas. What you want to avoid are hard spots/stress risers in the system, which could create weak spots. Study the topside and bottom framing above and you will see how continuity is maintained from one end of the hull the the other.

The FPB 64 has the strongest structural system of any vessel in which we have been involved.

For more information contact Sue Grant: Sue.Grant@Berthon.Co.UK.

Posted by Steve Dashew  (November 7, 2008)

One Response to “FPB Structural Grid”

  1. Dev Site » Blog Archive » Aesthetics – Form Does Follow Function Says:

    […] both materials, cost being the major aspect in favor of FRP. But when it comes to powerboats, where tankage and range are paramount, aluminum wins hands down. The difference in toughness, and the ability to […]