construction methods, and how salt water affects wood

[Charles Neuman]

I have read that a carvel planked boat must be kept wet (at least through a season) if kept in salt water.

First of all, why is this so? What happens if you don't follow that advice? And how does the construction method affect what happens to the boat if kept in salt water?

Second, I have also read that epoxy seals the wood and therefore doesn't allow moisture to enter or leave the wood.
If that's so, how can the wood be affected by what kind of water it's in?

Thanks,

Charles

[ 12-17-2002, 11:43 AM: Message edited by: Charles Neuman ]

[Ian McColgin]

Standard carvel planking presents a smooth outer hull. The planks are nailed or screwed or occasionally rivited to frames. To accomodate the natural swelling of wood as it gets wet, the planks typically have caulking between them.

The caulking, by the way, is structural as well as a sealer. The planks must make fairly firm contact each to the next so that the skin joins with the frames in providing diagonal and gunnel to gunnel strength.

On some small boats, the caulking is dispensed with in favor of a tight seam approach. Usually the edge of the plank is distressed a bit to leave crushed fibers that act in lieu of caulking. Sometimes you'll see a kind of strip planking where narrow planks are riveted closely spaced frames, like the fameous yawl Cimmeron.

No conventional carvel is comnpletly tight when launched.

By the way, most lapstrake or clinker hulls need to take up also.

Epoxy is not exactly waterproof and is not at all vapor proof. Epoxy construction from jump has usually utilized smallish individual pieces of wood - lke in cold molding or epoxy strip planking - so that the shrinking and swelling will not overly stress any individual joint. The few boats that were attempted epoxy sealing fairly large bits of wood without engineering for the stresses failed rather spectacularly. Many moons back now, John Garden did a devestating review of a St Pierre dory book in the National Fisherman on just this point.

Epoxy can be used on carvel hulls in a couple of ways, sometimes together. Grana had her seams reefed and was epoxy splined and then sealed inside and out. Works fine, but I'd hate to take her out of the water for very long. The moisture balance in the hull seems nice and stabile as things are now.

You could also wood sheath a wooden hull, typically a couple of layers ashcroft style but perhaps one layer fore and aft.

In either case, you are massivly changing the mechanical dynamics of the hull structure to something that's more like a monocoque structure.

Finally, there's CPES as a sealer. Done right this will help the wood settle in to a more or less constant moisture content and it's a great base for paint. But there's still moisture, still moisture content changes, still vapor migration, and therefore still dimensional changes in the wood that must be allowed for one way or another.

G'luck

[Ed Nye]

I am not the most knowledgeable member on this subject, but here is what I think. The reason a carvel hull should be kept wet is that the planks swell and shrink with each wet to dry cycle. You build the boat, cork her tight and launch. The planks swell, if you haul her and let her set the planks will shrink. The trouble is that when the planks swell the first time some of the fibers are crushed. With the next wet dry cycle the planks don't swell as big as they were the first time. So the hull is not as tight (strong, from the planks and corking pushing against each other) as it was. It is thought that repeated wet dry cycles would ruin the strength of the hull and also cause leaking.

Salt water - fresh water. Some think that a boat in salt water will last longer because the salt pickles the wood. I don't know whether to believe that or not. I know that fresh water inside a closed boat will allow rot. But, I also have heard of a lot of boats kept in salt that have soft frame heels. One would assume that the water lying around in the bilges is salt water. Is salt water being good or bad for the boat? The fastener material matters more than the construction method. Salt water is much more corrosive than fresh water.

Epoxy - What Ian said.

Ed

[Ian G Wright]

Originally posted by Ian McColgin:
No conventional carvel is comnpletly tight when launched.

Er,,,,,, mine was, and after 13 years still is, just. Iroko on steamed oak with cotton caulking and putty. Hauled out and painted every year. Dry as a bone 'til this year, though the stern gland will soon need attention.
The hull, once planked, had several coats of cuprinol slathered on inside until it soaked through the caulking then a few coats on the outside. A year or so to dry prior to painting and Bob's your Uncle.
Maybe I'm just lucky?

IanW.

[Ian McColgin]

Perhaps I could parse my statement to show that a carvel boat that doen't need to take up isn't 'conventional.'

There are woods that shrink and swell less than others and sometimes the nature of the seam work will keep drips out even while the boat swells.

It would have been more correct to say that most leak a little after launching.

I do think it safe to note that prudent launching allows the boat to take up for a few days to a week before setting the rig and using her. In sheltered harbors you might get away with having the rig up but not fully tuned. If a boat is used before she's taken up, the stresses in the hull will be inordinatly placed on the fastenings rather than shared across the skin. It might not do much damage in one year, but is a bad practice in the long run. Most of the time.

[Ian G Wright]

Originally posted by Ian McColgin:

I do think it safe to note that prudent launching allows the boat to take up for a few days to a week before setting the rig and using her.Good advice. In my case the sails were not ready on launch day, I had to wait two months before bending them on and when I did sail the rigging was not even nearly tuned. Still isn't, hit one of Patience's shrouds with a spanner and you get a dull thud rather than a twang, but then she is a gaffer,,,,,, low stress+ well built= long life with few leaks.

IanW

[Ken Hall]

Sorry for being too lazy to go look it up myself, but is "Ashcroft style" double-diagonal?

Ever since I read Riddle of the Sands, I've been interested in Dulcibella's construction...not least, the (estimated) scantlings, and what the heck you'd have to do to get teak to bend into place.

[Scott Rosen]

Carvel boats don't have to stay in the water. As others have noted, some (most) carvel boats need a week or so when they are first launched for the season to take up.

It's not rocket science, although it may not be obvious to someone who's never owned or cared for a carvel boat. Dry wood changes dimension when it gets wet. The greatest change is across the grain. The smallest change is with the grain. In other words, a dry plank will expand its width when wet, but will not expand its length. Some woods change more than others. Iroko, teak and some other tropical woods don't change much at all. White cedar is at the other extreme; it changes so much that after drying out you can sometimes see daylight through the seams.

This all means that when building a boat, the builder has to allow for the natural movement of the wood. Carvel boats are planked to have a tight fit when wet. That means that the builder has to allow room in the seams for expansion. The dry hull will have a very narrow space in the seam which is filled with cotton caulking. When the boat is launched, the planks expand and compress the cotton between the seams, forming a watertight seal. When the boat is hauled and dries out, the wood returns to its previous dimensions and there will be a narrow space in the seams again.

Some woods, especially soft woods like cedar, are susceptible to a phenomenon called edge set, or something like that. When the planks expand against each other, the wood fibers on the edge of the seams will compress and crush the cells. This changes the dimension of the wood permanently, so that when it dries out, the seam will be wider than it was before it was launched. Other woods like mahogany, teak and iroko are very resistant to this and will keep their shape and size even after many wetting/drying cycles.