The connection of the keel to the hull

curtainsThe connection of the keel to the hull is very dependent on the type of yacht under consideration although some elements remain. With the traditional long keel under the wooden hull a clear distinction between hull and keel is not easy to make.

The hull construction consisted of a long wooden stem, connected to the keel beam which on his turn ends in the stem beam. Frames, beams, curtains transverse girders and stringers were used to supplement the construction of the hull planking. The ballast is separated from the hull by means of a piece of wood, called dead wood. The keel bolts run trough the deadwood and through the beams to take the transverse load and moment under sail.

Grounding loads are more difficult to handle with this construction. The rudder in these wooden long keel yachts is connected to the keel by means of hinges and the rudder stock has a through hull and trough deck bearings at the top and is supported at the bottom by an additional bearing on the keel balk. Figures 22 and 23 show examples of wooden and mixed metallic constructions.

Also with some elderly shaped yachts constructed in steel, aluminium alloy and GRP the integral keel is a often deployed hull shape. The keel is now integral part of the hull (S shaped hull) and the usual internal stiffeners are used to take the loading from the keel into the hull structure. The ballast is poured later into a void in the keel. In the separate keel design the keel in the metal hull construction is welded to the hull and forms also an integral part with the hull construction with the frames and the floors extending in the keel. In some designs a separate keel of lead and cast iron is bolted to the hull with a similar construction as used with the hulls constructed in GRP In GRP hulls, with a separate keel attached to the hull, the construction is generally similar to the construction used in wood, i.e. hull plating stiffened with floors, transverse beams and longitudinal girders. There is however one noticeable and important exception, i.e. the keel bolts run through the hull but not through any of the floors or the transverse beams. They are drilled through only the hull in between or just adjacent to the floors or transverse beams.

This less favourable construction technique is initiated by the fact that none of the floors or any of the transverse beams in the GRP construction are solid beams, as in the traditional wood construction, but they are made from GRP laid just over a core, usually high density foam or wood of much lower density. These stiffeners simply, by the very nature of their construction, can simply not take the high compression loads associated with the keel bolts passing through them. The keel bolts are put through the hull laminate and backed at the inside by steel plates of hopefully sufficient large size. The fairing of the edges of these backing plates is essential to prevent damage to the laminate of the hull with sometimes catastrophic results. With series production boats this construction technique poses another serious challenge for the builder because in those boats often internal moulds are being used, containing all the internal stiffeners used for engine support frame, the floors, the mast foot support, the shroud plates and the transverse and longitudinal stiffeners in the keel connection area.

This is placed as one piece in the hull. However solid this construction may seem the quality of the (secondary) bonding between the internal mould and the hull laminate is essential for the final strength of the construction. The absolute exact positioning of the internal mould with respect to the hull and the precise shape of both is crucial to prevent voids in between, which are marginally filled during bonding by resin only. The bonding material consists usually of thickened resin (putty) only and no fibres to yield any strength at all. The shortcomings of this kind of structure have become quite evident in real life after such structures have been exposed to grounding loads or extreme slamming loads. In some constructions this internal frame is made of metal, i.e. steel or aluminium, taking all the principal loads. The frame has sufficient strength and stiffness of its own and is laminated to the hull after hull laminate completion. This is generally considered to be a much more reliable and functional construction but also more expensive. Concerns include long-term corrosion resistance.

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