Hull Construction  

Although the use of iron for hull construction was tried successfully on small craft very early in the 19th century, the old traditional methods of wooden construction, in which almost all the shipyards and their personnel were well versed, were still considered the best by the merchant ship owners at the time that the first tea clippers were ordered. In North America, wooden construction persisted until quite late in the century, chiefly because the raw material was readily available, and also because of a high import duty on manufactured iron from Europe. America in those days did not have a self-sufficient iron industry.

  There were noticeable differences in the construction methods of merchant ships in British and American yards, due mainly to the much greater length of the American clippers and the necessity of counteracting the hogging strains which tended to bend a ship, the upperworks being in tension and the bottom in compression.

  Royal Naval designers had already introduced a series of innovations, such as internal diagonal lattice-work framing over the normal vertical framing, and a solid bottom to resist compression. The latter was achieved by filling in the spacing between the frames either with wooden chocks or planking, up to a point above the round of the bilge. Since the bottom was solid and flush, the bilge water lay above it, and drainage holes were not required in the lower surface of the frames. Although this mode of construction was a great advance, it was not taken up by the merchant ship builders, who kept to the older system of vertical framing spaced out and covered by longitudinal planking, internally (ceiling) as well as externally. The consequent sealing up of heavy timbering was a great inducement to the start of rot. Any wooden structure which has great bulk made up of separate timbers facing onto each other is a source of rot, as clean fresh air cannot circulate. The ancient method of minimizing this was to char the adjoining surfaces beforehand with hot irons, or indulge in the practice of'worm chasing'. This involved gouging out the surfaces in haphazard grooves, which it was hoped would allow air circulation.

  The final solution, however, lay in salting. It was found that wood well soaked beforehand in salt water, or packed with plain salt in any interstices, resisted rot. Lloyd's allowed an extra year of classification if salting was carried out. This was done during construction by completely filling up the gaps between frames, progressively, as the inner limber strakes and ceiling were fitted. The upper surface of deck beams also had a long groove about an inch deep filled with salt before the deck planking was laid, and the same system was applied elsewhere in the timbering. It will be noticed on the drawing of typical wooden hull construction in British-built ships that each frame unit is made up of two sets of timbers or futtocks which although abutted together across the keel gradually open up towards their extremities as their sizes diminish, although the outside surfaces remain parallel to each other (20). The purpose of this rather awkward form of construction was originally to allow air circulation, but it also became useful for salting.