FPB 64 – Structure


The hull, deck, and house structure for the FPB 64 are engineered to our normal high standards, with exceptional factors of safety.

This starts with the Lloyds Special Service Classification (SSC – for lots more detail on these standards and how we go about this see “Hull and Deck Structure” and “Glazing” under “Design objectives” on the sidebar).

The stem (bow) of the boat, for example, is over two inches thick (56mm) where the topsides are welded to the stem bar. We don’t need anything nearly this strong for ocean loadings, but if you are going to be bumping into logs, docks, or other hard items, that thick, deep stem bar is really nice to have.

Bottom Plating

The conservative SSC rule calls for 5/16″ (8mm) plate for the bottom, based on the framing system we are using. We have upped this to 7/16″ (12mm), which is over three times as stiff (stiffness is a cube function of thickness). This is not because we need the thickness for pounding upwind, or for anything else to do with the ocean loads. That 12mm plate is being used because it gives us a warm, pleasant feeling when we’re offshore knowing we have huge factors of safety in the bottom plating for things which go bump in the night. This extra thickness extends six to seven inches (150 to 175 mm) above the datum waterline.

This bottom plate is supported transversely by a series of deep web frames. These occur at the middle of the forepeak, the watertight bulkhead, center of the forward cabin, each end and center of the salon, aft water tight bulkhead and center of the engine room.

Framing System

Running the length of the hull are a series of deep longitudinal girders. These start with at the centerline and then at approximately 22″ (550mm) centers.

There are a total of seven of these deep lengthwise girders down in the middle sections of the hull (they taper off as the hull narrows towards the ends).

Tanks are integral with the hull, as we have discussed in detail elsewhere. Tank tops form the upper flange of the girder system.

And the girders act as baffles for the contents of the tanks. This is an immensely strong and efficient system.

Window Structure

Probably the hardest part of engineering a structure like the FPB are the house windows. The various rule making societies, like Lloyds and ABS, do not contemplate the loads we might see in a severe knock down. We’ve been through this exercise in detail on the FPB 83 and come up with 19mm (3/4″) toughened safety glass for house windows. This is hugely over engineered by ABS and Lloyds rules, but, like that thick bottom plate, we prefer the emotional security which comes with those massively strong windows.

If you are being knocked down, or dropped off a wave, window loading is related to the boat’s displacement. Although the FPB 64 is lighter than the FPB 83 we are using the same window thickness.

Avoiding Deck Leaks

Welded aluminum construction has many benefits amongst which are elimination of leaks, if the hardware is detailed correctly. As is our standard, all deck hardware (winches for example) is blind fastened to threaded plates which are welded to the deck. This way there are no penetrations of the deck over the interior, except for antennae (which are mounted on pipe stubs welded to the house roof).

This approach even extends to bollards (cleats) welded to the rub rail extrusion.

Bottom line, this hull is the toughest with which we have been associated to date.
For more information contact Sue Grant: Sue.Grant@Berthon.Co.UK.

Posted by Steve Dashew  (October 17, 2011)

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