Keel. A sailboat requires a form of keel under the water, to prevent the boat being blown sideways by the wind. When the keel is part of a monohull, the keel’s weight forms ballast underwater which holds the boat upright, allowing it to carry more sail than possible without the weight. The keel also converts the wind’s force on the sails into a vector which moves the boat forwards. A compromise must be achieved between the depth of the keel and the friction generated by the keel. Deep keels work better sailing close-hauled, while shallower keels allow the boat to sail in less water.

Various forms of keels exist, just four of which are fixed, swing, centerboards and daggerboards. Each one has a typical application: for example, many cats have daggerboards, dinghies are often equipped with centerboards and cruising sailboats usually have fixed keels while long-distance racing boats typically have swing-keels.

These photos show Bit-by-Bit’s underwater shape. The first shows the rudder and feathering propeller (folds when sailing to reduce friction), while the second shows the bottom of the daggerboard protruding from the hull. Note that the rest of the hull underwater is simply a smooth, curved surface, ideally kept free of barnacles and algae to reduce the friction of the hull against the sea.

BeeBee: Stern View Stbd in the sun

On old ships the keel was the main longitudinal structural form and was the first component to be laid out in the yard. The Stem, the main component of the bow, and the Sternpost, the main piece of the aft end of the ship, were attached to the keel on either end. Between these two upright forms ran the rib bulkheads.

The next image was scanned from the official HMS Victory Tour Guide, purchased at the site. While you can’t see the keel here, you can see the bulkheads and if you can read the fine print, you can learn a fair amount about the ship. The next two images were copied, with permission, from drawings found at the Greenwich Maritime Museum, of HMS Jason, a fifth rate frigate from 1794.

HMS Victory, from her official tour guide HMS Jason, from drawings at the Greenwich Maritime Museum HMS Jason, from drawings at the Greenwich Maritime Museum

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