Carvel (boat building)
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In contrast with clinker built hulls, where planked edges overlap, carvel construction gives a stronger hull, capable of taking a variety of full-rigged sail plans, albeit one of greater weight. In addition, it enables greater length and breadth of hull and superior sail rigs because of its strong framing, and is one of the critical developments that led to the preeminence of Western European seapower during the Age of Sail and beyond.
Carvel construction developed through transition from the age-old Mediterranean mortise and tenon joint method to the skeleton-first hull building technique, which gradually emerged in the medieval period.
Carvel boat construction is well-attested, in Western Europe, in the late-Roman / barbarian period. The first carvel-built ships were the carracks and caravels of the fourteenth and fifteenth centuries. Both were developed in Iberia and were sailed by the kingdoms of Portugal and Spain[clarification needed] in the early trans-oceanic voyages of the Age of Discovery. At that time the battles of the centuries-long effort to expel the Muslims from Iberia were gradually swinging to the Christian side, represented by the Kingdoms of Aragon and Castile (later united as Spain, around the time of Christopher Columbus). The invention is generally credited to the Portuguese, who first explored the Atlantic islands and south along the coast of Africa, searching for a trade route to the Far East in order to avoid the costly middlemen of the Eastern Mediterranean civilizations who sat upon the land routes of the spice trade. Spices, in the era, were expensive luxuries and were used medicinally.
The smoother surface of a carvel boat gives the impression at first sight that it is hydrodynamically more efficient. By contrast, the exposed edges of the clinker planking appear likely to disturb the streamline. This distribution of relative efficiency between the two forms of construction is an illusion: For given hull strength, the clinker boat is lighter, because it has far less heavy timber framing, and so displaces less water. This means it has less to push aside while moving.
Where, for example, draught is limited by the available depth of water, reduced displacement may also make it possible to reshape the hull, making the lines finer so that passage through the water is easier still.
For cargo vessels, the hydrodynamic advantage loses much of its importance as the vessel is laden and the hull weight becomes small in comparison with total displacement. But an economic advantage remains: the clinker vessel is more efficient, for cargos which are bulky rather than dense. The internal structure of the vessel occupies less of the space. For a given external volume, there is greater internal hull space available. That means that a greater quantity of a bulky cargo can be carried within the hull (rather than as deck cargo, which affects stability and general sea-keeping qualities).
A third, structural benefit of clinker construction is that it produces a vessel that can twist and flex around its long axis (running from bow to stern). This gave advantage in North Atlantic rollers so long as the vessel was small in overall displacement. But due to the light nature of the construction method, increasing the beam did not commensurately increase the vessel's survivability under the twisting forces arising if, for example, when sailing downwind, the wave-train impinges on the quarter rather than dead astern. In these conditions greater beam widths may in fact have made the resultant vessels more vulnerable.
Thus, as these torsional forces grew in proportion to displaced (or cargo) weight, the physics imposed an upper limit on the size of clinker-built vessels. The greater rigidity of carvel construction became necessary for larger non-coastal cargo vessels. Later carvel-built sailing vessels exceeded the maximum size of clinker-built ships several times over.
A further limitation on clinker construction is that it does not readily support the point loads associated with, for example, lateen or sloop sailing rigs. At least some fore-and-aft sails are needed for manoeuvrability, if only at start and end of a voyage: especially so for larger vessels. The same problem in providing for concentrated loads makes for difficulties supporting a centerboard or deep keel, much needed when sailing across or close to the wind. Timbers can be added as necessary compromise, but always with some loss of the fundamental benefits of the construction method.
However, clinker construction remains to this day a valuable method of construction for small wooden vessels, especially small dinghies and tenders which need to be readily moved and stored when out of the water.
A number of boat building texts are available that describe the carvel planking process in detail.
Modern carvel methods
Traditional carvel methods leave a small gap between each plank that in the past was filled with any suitable soft, flexible, fibrous material, sometimes combined with a thick binding substance. This caulking would gradually wear out and the hull would leak. Likewise, when the boat was beached for a length of time, the planks would dry and shrink, so when first refloated, the hull would leak badly unless recaulked—a very time-consuming and physically demanding job. The modern variation is to use much narrower planks that are edge-glued instead of being caulked. With modern power sanders a much smoother hull is produced, as all the small ridges between the planks can be removed. This method started to become more common in the 1960s with the more widespread availability of waterproof glues, such as resorcinol (red glue) and then epoxy resin. Modern waterproof glues, especially epoxy resin, have caused revolutionary changes in both carvel and clinker style construction. Whereas in traditional construction it was the nails that provided the fastening strength, now it is the glue. It has become quite common since the 1980s for both carvel and clinker construction to rely almost completely on glue for fastening. Many small boats, especially light plywood skiffs, are built without any mechanical fasteners such as nails and lag screws at all, as the glue is far stronger.
A further modern development of carvel is cold moulding or strip-planking. The latter is used for small boats and kayaks, where the hull is made up of narrow strips of wood built up on a wooden jig and glued together. Larger vessels may have two or three layers of strips which, (being a pre-shaped marine ply), forms a light, strong and torsionally stiff monococque which need little or no framing. The cold moulded hull may have an epoxy sheathing (both on the outside and inside) for durability and watertightness. The epoxy may be clear (to show off the beauty of the wood) or coloured, and it may be reinforced with grp.
- Clinker—by way of contrast, the predominant method of ship construction used in Northern Europe before the carvel.
- Haywood, John (2006). Dark Age Naval Power: Frankish & Anglo-Saxon Seafaring Activity. Routledge. p. 18. ISBN 978-1898281436.
- Carvel Planking Texts for Sailboats—Richard Joyce Montana Tech
- West System International http://www.westsysteminternational.com/en/welcome/an-illustrated-history