Pitch was used as a preservative both above and below the water line.
There are three basic problems:
Stuff that fouls (grows on) your vessel, such as barnacles, slime, seaweed, etc.
Stuff that rots your vessel, usually fungus, causing dry rot.
Stuff that eats your vessel, specifically toredos (sp?) (shipworms).
In the southern latitudes, and especially in the tropics, all three work at a furious rate. However, with Viking ships, you are blessedly free of almost all of these. The Baltic is very brackish (high fresh water to salt ratio) and does not support shipworms. That's why the Vasa was in such good shape when they found her. The North Sea and North Atlantic are saltier, but still cold, which also slows down the buggers a bit. A method used down on the Chesapeake was to anchor up in a fresh water creek for a week and kill off the shipworms.
Dry rot also thrives in warm, moist conditions. Longships are, essentially, big open boats. There are no cabins or holds to keep in the dampness. Water comes aboard, but things dry out. It's fresh water that's the problem. Dry rot doesn't live in salt water, so below the waterline it's not a problem. Decked wooden vessels used to have small compartments along the hull/deck joint to put rock salt in to stop the rot.
Barnacles and fuzzy stuff just slow you down. You have to scrape them off the hull, but for the most part it's no big deal. They, also, do not thrive in colder waters. Beyond that, it's no big deal to careen these vessels for a quickie scraping, caulking, or other repairs. (Trust me, I've done it!) Add to that a short season and pulling her out for most of the winter and the need for bottom coatings is minimal.
Smaller vessels required no caulking. Larger vessels were traditionally caulked with pitch and cow hair, but to add to what Bascot said, one find had been caulked with most of a woman's dress, which was a wonderful find for the folks studying clothing, and the sort of thing that leaves me wondering: "THIS most be a good story!"
Anyway, the real fun and games were further south, in the Mediterranean. There they had all of the problems and coated their bottoms, at least by the Age of Discovery, with a mixture of white lead, tallow and ground glass. I would suppose frequent scraping and replacement was the order of the day. Once you were away from you harbor facilities and in the Caribbean, you were on your own, and ships literally fell apart. Your great 17th through 19th century wooden warships were, literally, rot-factories, and had to undergo almost a complete re-build on a regular basis.
In the 18th century, the British discovered that copper sheathing stopped shipworms and just about anything else. In the early 20th century folks on the Chesapeake used to mix Paris Green (arsenic) into their bottom paint. For a while in the 1970s tributyl (sp?) tin was great stuff, until it was noticed that it killed everything near the boats. It's now restricted to the USN, and when last I heard, even they won't use it. So now we're back to copper compounds. Most of the vessels from Scandinavia that we saw up at L'Anse aux Meadows in 2000 used no bottom paint whatever, but they also seem to have been mostly dry berthed.
Just a note: A couple of decades back the Maryland Dove's shore staff, against the advise of the Captain, skipped the bottom paint for a year to save money. The shipworms ate their way through most of the oak bottom planking. My father was called in as part of the inspection team and said that parts of it looked like a kitchen sponge. The repairs exceeded the cost of hauling, scraping and painting by many times. Ironically, the original Dove was lost at sea and never heard from again. There is some indication that the loss may have been due to shipworm damage!
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Full time civil servant, part time blacksmith, and seasonal Viking ship captain.
Visit your National Parks:
www.nps.gov Go viking:
www.wam.umd.edu/~eowyn/Longship/ Hit hot iron:
www.anvilfire.com