Salt water corrodes and rusts iron very rapidly and hence boats with plain iron fastenings and fittings should be avoided for salt water use. Copper or brass fastenings and brass or bronze fittings are far better, but these are expensive. Galvanized iron is therefore adopted very generally for salt water use on boats.
Even when a boat is well painted and the iron parts are thus protected, the salt water will corrode and destroy the iron work and just as soon as the paint becomes old, thin, worn or chipped off, the parts go to pieces very rapidly. For this reason boats should always be kept well painted and varnished at all times, and whenever a bit of paint is rubbed or knocked off, it should immediately be touched up with fresh paint.
In salt water, too, marine animals and seaweeds attach themselves to every submerged portion of a boat’s hull and grow very rapidly.
Not only do these growths hinder a boat from sailing well and rapidly, but they also destroy the paint and injure the wood beneath it. This paves the way for the water to soak into the planks and timbers and rot them and corrode the metal fastenings which hold the various parts of the boat together.
Still more injurious are the shipworms or teredos. These are marine animals which are not really worms at all, but are a species of mollusc related to the common clam. They do not eat the wood, as many people think, but merely bore into it to form their homes or burrows, and wherever they go they line their holes with a thin coating of lime or shell.
The shipworms are very small when they first enter the wood and as they increase in size they bore larger and larger holes until they riddle the wood with burrows and completely destroy it. No signs, however, save a few tiny holes, may be visible externally. So rapidly do they work if unchecked that large ships have been sunk by them in less than a year and there are several records of such catastrophes occurring.
Teredos seldom attack wood which is far below the surface but work mostly at or near the water line. For that reason small boats of shallow draft are often more seriously and rapidly injured by these pests than larger and deeper boats.
Moreover the shipworms seek spots which are out of sight for their depredations and unless the boat-owner is very careful he may overlook very serious injuries by the teredos without dreaming that they exist. The cracks between keels and sternposts, between keels and garboard planks and the interior or centerboard trunks and cases are favorite spots for teredos to bore and quite often the timbers in such situations are completely destroyed and the boat is rendered worthless before one realizes that teredos have attacked the boat at all.
But even without marine growths and teredos the planks and timbers of a boat may become rotten and useless through the action of the water. This is particularly the case where a boat rests upon a muddy bottom at low tide, for the mud contains gases and chemicals which destroy the paint and this allows the water to penetrate and rot the wood.
To guard against these three principal dangers every boat should be hauled out at frequent intervals, the bottom should be scrubbed, scraped and cleaned, and should then be allowed to dry thoroughly, after which it should be freshly painted with some reliable and good anti-fouling bottom paint such as the various copper paints. Large boats are usually sheathed or covered with copper plates below the water line in order to protect the wood, but small boats depend upon a coating of copper paint.