Wood Technology - Modern or Traditional?

1) Wooden Boatbuilding – Traditional and Modern

Timber can be seen as bundles of natural tube-like fibre (along the grain) which can be glued together in different ways to make up the different elements of a boat. We retain useful qualities of wood: the “feel”, beautiful appearance, resistance to fatigue, strength with low density, and insulation to heat and sound. The MODERN wooden boat combines plywood with raw timber, also often adding man-made fibre (mainly glass-fibre), with epoxy resins formulated for wood to hold it all together. Plywood is usually bought in as a factory-produced sheet material.

The fact is that problems with using a natural fibre still remain: wood swells and contracts with conditions, and needs to “breathe”. Glued structures tend to be rigid, moisture can be absorbed (and not get out), rot occur, and layers of wood work against each other to break down the glue joints between. Plywood edges are especially vulnerable to moisture – water penetrates deep and quick along the tube-like fibres inside but can't get out so easily!

The key to success is proper design with the use of the best surface coatings (epoxy resins & paint & glass fibre cloth) to prevent moisture reaching the wood which starts off dry. In TRADITIONAL boatbuilding by contrast, moisture content variation is accepted but but we make structures which can cope with, indeed make use of, dimensional change of the material.

“Modern” and “traditional” wooden boatbuilding are in fact distinct disciplines, though the former is a historical (even natural) development from the latter. Further, elements of both can appear in boats – the plywood deck on a traditional hull, for instance, where the properties of the sheet material can be a huge advantage over a traditional laid deck - while laminated stems and frames are often used with a traditionally-planked carvel (smooth-hulled) boat. We could say that most wooden boats being built nowadays are "composite" craft (analogous to the wood/grp composite). It is worth noting that the “modern” elements on a traditional boat often break down sooner than the rest of the boat – for various reasons, often poor maintenance.

The two disciplines can be muddled because wooden boats, both traditional and modern, can be made using the same workshops (heated up for modern!), tools, and skills. And both methods can produce wooden boats with all the good qualities of wood – but, scratch the surface, and there are different worlds within!

Here are examples of both - above right is a Norfolk Punt and to the left a traditionally built canoe.

2) Trailer Sailing - Traditional Wood Construction versus Plywood Construction

- some thoughts (leading to comments on how to care for wooden boats.........)

If I'm asked "Can a traditionally-built wooden boat be "dry-sailed" and used as a trailer-sailer?", I am bound to answer: "Of course! What did we do before plywood, GRP, and modern glues? What about those clinker Victorian sailing canoes which travelled by ship and train? What about gigs and life-boats carried aboard ships, dories aboard schooners, racing dinghies (eg National 12's) clinker rowing fours's and eights, Thames skiffs, etc etc?" - But I suppose now that we do have modern materials, we do have to ask about the pros and cons - is there a best choice? My conclusion is that if you choose a traditionally constructed boat, you need to know how timber behaves, a bit about how a boat is put together, and how to look after it; there is more to learn about here than for a modern boat on the whole, though the poor way many modern boats get treated will surely shorten their lives - no boat is "maintenance-free". In terms of durability, costs, usefulness and so on, the differences between modern and traditional wood construction are slight, all things being equal on the level of quality, so the choice has to be made on a more subjective basis over and above pure functionality - what materials do you like to live with? What looks and feels most beautiful? (I am here leaving out double-diagonal construction etc etc)

Nowadays we tend to think that the best choice for a dry-sailed wood boat will be glued or plywood construction - in fact a lot of plywood boats are sold on this basis - they will not leak. Another concern is weight: broadly speaking, it is "easier" to make a light plywood boat ("easier" means, usually, (but not always) less expensive -less labour) which makes launching and retrieval easier. A plywood boat may have built-in buoyancy compartments too. If a heavier boat design is chosen (for stability and capacity for instance) traditional construction becomes more realistic. Building a replica of a traditional boat (pre-1950) can free one from the restrictions of EU legislation which affects space and stowage inside for instance. The common experience of (older) traditionally constructed boats is that they leak, and being hauled in and out of the water only makes matters worse. Boats have suffered splits in the planking when taken on a long car-journeys in hot weather, as another example. Traditional boats often have to "take up" on being launched - be given time for the timber planking to swell and close off any leaky gaps - all inconvenient.

Well, let's look at this. At the start of a traditional boat's life, carvel or clinker, it should NOT leak when first launched - then, typically, the leaks following winter storage ashore get worse over the years until there comes a point when they don't stop and some sort of repair needs to be done. Putting a time-scale on this, I maintain a Salcombe Yawl (mahogany planking) which has a regular in-and-out regime and which is still fine after 35 years. Similarly, "Dainty Lady" an Osea Brig I built in 1984, after spending several years on a mooring, is now dry-sailed - i.e. lives out of the water most of the time - this doesn't mean she won't leak at all, but the amount is obviously bearable.

I might digress here to add that our attitude to leaks has changed - considering that being afloat, especially in small craft, is often a wet business , one wonders why anyone is so bothered about the odd leak anyway. Well, we ARE bothered because a leak in a MODERN boat is serious in that it implies either a serious malfunction of a fitting like a stern-gland, or a fracture in the (laminate) hull which usually means, in the case of plywood, a serious threat of rapid deterioration and/or rot. Or, again, take decks - a weep in a teak-laid deck will be inconvenient and we need to check that the supporting structure of beams etc is not threatened, but it does not portend "doom" - but in a light plywood deck, (often with a somewhat different structural function) any leak certainly does demand immediate attention to preserve the structural integrity of the boat. But water shipped over the side is a different matter........... we are used to having to bail out the bilge now and then. Of course a lot of modern boats, partly because they don't leak (we hope), don't have any sort of built-in bilge or sole, anyway, to keep our feet dry, which makes our sensitivity to such things even worse. Whereas those of us brought up sailing around in a leaky (OLD) boat, are more used to coping with the odd bit of water sloshing around. Comfort, when day-sailing, depends upon design around such details.

For trailing, dry-sailing, and being kept ashore (on or off a trailer), it simplifies matters if a hull is intrinsically stiff, and incorporates "support points". Plywood boats tend to be stiff, but if they are light some care needs to be taken to avoid ruptures caused by point loading. Many small traditional boats are quite flexible, and lightly, perhaps inadequately, framed - if that is the case, they need careful handling and support out of the water, especially for trailing. On the positive side, the traditional hull may flex to absorb point loading, and so avoid rupture, even if slight damage could occur in the long run if subjected to this repetitively. In short, a traditional craft should be constructed with dry-sailing in mind from the outset if it is to be used in that way (eg the Osea Brig has a substantial centreline and floor structure with this in mind). Or, put another way, we can always build traditionally to meet the requirements of dry-sailing though this might entail some structural modification to some designs.

Given the above, the next consideration is the swelling and shrinking of timber with moisture content. The main reason that older boats leak is that over time the fastenings become loose. Copper nails/rivets stretch under the pressure of swelling wood (keel bolts on yachts can break under the pressures of a swelling keel if too tight initially; design should keep them as short as practicable), fastening heads draw down into the wood, the wood softens around fastenings so they don't grip as well, and so on; finally, if the boat has discontinuities in the structure (like the end of a centreboard case on the keel) the concentration of loading in these places will cause the fastenings/timber to fail as the timber deteriorates with age and especially if it becomes saturated over time. Timber is plastic, it deforms under stress over time. High moisture content is bad all round, so how do we combat all this in a waterborne object? (leaving aside choices of timber......)

With glued/ply craft, which are more threatened by water penetration, the answer lies in the coatings - epoxies, glass sheathing, 2-pot paints, and the fact that there are no unfilled cracks or pores for moisture to lodge in or enter the structure. If these sort of coatings are used with traditional structures, they do not usually last, because no coating is TOTALLY secure to moisture vapour penetration; also, it is almost impossible to prevent some moisture being in the timber which will cause dimensional instability with temperature, and so on, and once you get movement, you get cracks in the coatings and....... However, it has to be said that there have been many restorations and re-builds where every part of the hull is epoxy-encapsulated (every rib prior to fastening eg) where the life has been good - however I suspect such boats need to be treated and used with a constant awareness that problems can arise, as in a plywood boat, if the surface coatings are broken.

Ironically, these hard moisture-resistant coatings will work most successfully with the best substrates, like teak, because of the timber's intrinsic stability - but a teak boat hardly needs modern coatings! It can be added that it is an illusion to think we can compensate for poor quality (or condition) timber or a shoddy substrate structure simply by coating with epoxy - there will be movement and the epoxy will crack. One way to get round these problems has been tried - build the boat "dry", drive out moisture ("completely") in an oven, put the boat in a vacuum for a while, then infuse the whole structure with epoxy hoping that it will penetrate every crack, joint, pore. As far as ply building goes, inferior ply faces will let the coating down unless the epoxy is combined with glass cloth, and any surface discontinuites (like butt-jointed ply) must be glass-epoxy taped or the coatings will crack at this point.

Paint coatings have pores left by the evaporation of the solvents. (Epoxy (glue) binds the solvent in - hence no pores (almost) - epoxy paints use solvents which flash off - so........ .) If we have a well-painted surface (ie many coats) liquid water will not cross the barrier on the whole, except by a sort of osmosis which causes the paint to bubble and break up (which is why we can't paint yachts with ordinary paint below the waterline). But the paint layer CAN be penetrated by water vapour at the molecular level from the atmosphere - so the moisture content of wood will depend, in the end, upon the surrounding relative humidity. The main thing we can do is SLOW the RATE OF moisture content CHANGE in the wood, by adequate painting (- up to a point, the more coats the merrier!), This needs to be done to meet the challenge of seasonal temperature and humidity conditions too.

Wood is plastic. If moisture content change is slow, dimensional change is slow, and the stresses on the timber and fastenings is less because time allows stress to be partially absorbed by dimensional deformation of the wood.

From the point of view of dry-sailing, a well-painted boat will probably not change much when afloat for several days - so if it is used like that from new, the chances are that it won't develop "leakiness" for many years.

For trailing, we need to watch the weather - taking a boat out of a damp garage and then trailing for a day in hot sun could cause a high rate of drying out and hence damage - a precaution could be simply to wrap the boat or put it in a "bag" made from a cheap tarpaulin.

A boat can be "swelled" by covering the interior planking with damp towels (for example) before taking to the water, but this shouldn't usually be necessary.

When a traditional boat HAS dried out after normally being kept afloat and the seams have opened up, great care must be taken to avoid dust and dirt getting into open gaps and shakes. A lot of boats have been damaged by being painted carelessly when dried out, with paint allowed to run into the lands and harden, probably binding in the dust and lumps left from scraping and sanding the old coatings. It is a moot point when building the boat how to treat the lands. In the short run, it might be OK to use a masitc in the seams, but in the long run it may be better that the surfaces are almost bare - then when the boat is launched WATER can get into the wood quickly and close the seam. (I lute my lands with a coat of varnish as a compromise - a thin coating, not thick enough to prevent water wetting the wood, yet enough to give the surface a bit of extra hardness.)

Narrower planking makes dimensional changes less so seams and fastenings are less stressed over time. It can also help to paint with lighter colours which reflect more of the heat from the sun.

Depending upon the design of course, it is often good to use heavier fastening than the minimum to allow for the ravages of time, and to protect the fastenings too by adequate countersinking and filling over the heads.

And finally an exhortation.....
If a boat is kept afloat, it is important that it is kept dry and ventilated inside - to exemplify: in a traditional yacht, if there is any water in the bilge below the sole the relative humidity in the bilge will be 100% - this is bound to lead to gradual saturation of the surrounding timber; big yachts have a system of internal lining so that convection right round the inside of the planking (caused by the sun warming the topsides during the day) ventillates the inside of the planking, which would be unpainted or very lightly treated - thus a big boat can be kept afloat yet the planking not become saturated - moisture will migrate to the inside of the planking and get taken away - this is a very slow, almost glacial process, but one can easily see when it has been interrupted - after a period..... Covers are essential equipment to protect a boat especially from sun and frost, but one must make sure a cover does not prevent air flowing round - the more one can do to ventillate a boat the better it will last - one reason I believe boats that are often used last well - every time people are aboard or go sailing the boat is ventillated - at rest, stagnation occurs somewhere. Similarly, clear out the clutter! And clean off dirt and grime! Strip the boat in winter and open all the hatches under covers! - I have talked lastly of bigger boats, but the principles are the same for all ......