We Build Wooden Ships

New wooden ship construction

Tri-Coastal Marine builds new wooden vessels. We've specialized in large sailing vessels, built near they will eventually homeport. The construction is part of the exciting process and usually open to the public.  Here's a magnificent vessel we designed and built in Newport, Virginia:

Schooner Virginia

Counter framing

Planking her up

More Virginia construction photos

 


Sail Ferry

We designed and built this odd little vessel that operates on Lake Champlain.  Check out the sail ferry:

Weatherwax


Wood

Wood was the only ship building material available until the middle of the 19th century when the first iron ships were painfully rivetted together from small iron plates. Even the largest warships such as Nelson's Victory, the USS Constellation and the USS Constitution ("Old Ironsides") were built entirely from wood. The largest wooden ships ever were built in the early part of this century. They were the sailing bulk carriers like the gigantic six masted schooner, Wyoming. These large ships all suffered from structural problems, particularly hog.


Plank on frame or carvel construction

Carvel construction probably derives from the Portuguese word "caravela" or caravel, a ship type that was current iin the 15th century. It a boat and ship building system where the planks are flush: the edges meeting and giving the shell a smooth surface instead of overlapping as in the clinker system. The planks are fastened to transverse frames, ribs, with nails or wooden pegs called trunnels (treenails). The seam between two adjacent planks is caulked by driving home fiber strands made of cotton or hemp and then covering or paying the seams with putty or hot pitch. It is practical to build larger vessels with carvel construction then it is with the lapstrake method, but the upward practical limit is really about 40 m LOD and 500 tons displacement. Larger vessels were indeed built with this method but they inevitably had huge structural problems.


Cold molded construction

Cold molded construction implies building boats and ships from several or many layers of relatively thin planking. These layers are oriented in different directions relative to the each other, that is layer one may be fore and aft, layer two may be at 45 degrees to the fore and aft planking, layer three may be at 90 degrees, etc. The different layers are glued to each other forming a stiff shell.

Hull laminate detail, USS Constellation

The largest cold molded ships built to date are the US Navy's MCM class minesweepers


Hogging

Until the 1920's a large percentage of the world's shipping consisted of large wooden ships and their plague, after plain old rot, was "hog". A ship floating quietly in still water is subjected to external forces. These are the weight of the vessel on its cargo (downwards) and the buoyancy force (upwards). Archimedes showed us that for a floating vessel, these two forces must be equal in magnitude. For a floating rectangular piece of wood, they are also equal in distribution. For most normally shaped ships, the distribution is not equal. For example, when an empty ship has more weight (relatively heavy structure, engines and equipment) in the ends, and more buoyancy in the middle. This "excess" of buoyancy in the middle cause the middle to rise up and the ends to bend down -- a hog in profile. The opposite condition is sagging. For old wooden ships, this resulted in a long term, plastic deformation. The total curvature could be a meter or more in larger vessels. Some vessels like the Wapama hogged so much that they nearly broke in two. Hogging is no longer the problem it was in the 1920's when it threatened the nation's merchant fleet -- because those ships have sunk!

Wooden ships, even wooden warships like USS Constitution, are actually quite weak even when new. Although solid shot may have ricocheted from their sides, they are generally unable, over time, to resist the fairly small forces they are subjected to moored in still water. There is a false idea that amazingly still has some following, that wooden ships were strong because they would flex. In fact, relative movement between structural members allows fresh water to enter the hull structure, carrying rot fungus spores deep inside.

Engineers have often attempted to analyze the structures of wooden ships as if they were homogeneous box girders. This is a common misapplication of beam theory. Actually, a wooden ship, especially as it ages, more closely resembles a rather weakly bound bundle of reeds. These reeds are free to slide past each other. If traditionally built wooden ships were box girders, then one would expect to see many tensile failures amidships in the upper deck of a severely hogged vessel; however, this is not the case. Failures in longitudinal structure are infrequent and tend to be scattered almost uniformly throughout the vessel. The idea of "strength decks" or "extreme fiber" is largely irrelevant to the meaningful analysis of old wooden ships. Microscopic investigation reveal a generally low level of stress in "hogged" structural members. There often is evidence of plastic behavior, creep, around fastenings. Large overall deflections in the hull can be achieved with a very small amount of creep around the fastenings.

The bundle of reeds metaphor implies that the ship is comparatively poor at resisting longitudinal loads due to a weakness in shear. Wooden ships are generally stiffer in lateral loading since the transverse frames are like individual beams. As a vessel ages and softens, even these relatively stiff beams can suffer large creep deflections. USS Constellation is an extreme example of an old, soft wooden ship and probably has large lateral deflections as well as hog -- behaving more like a wet wicker basket than a bundle of reeds. Pushing up on the bottom of the basket causes the sides to bulge out and the bilges to drop. This is evidently the case since the keel has deflected over two feet and there is much less curvature in the upper decks. The vessel is also soft transversely. That is apparent from the curvature of the gun deck which is hogged in several distinct undulations. The upward force on the bottom comes from an unequal distribution of the weight and buoyancy forces on the vessel. In a newer, stiffer vessel it is possible to minimize this net force by the judicious placement of ballast both longitudinally and transversely in the bottom of the vessel.   Here's a link to a TCM paper on hogging


The Gaff Rig

The gaff rig is a fore and aft rig, like the modern marconi rig, but gaff sails are quadrilateral instead of triangular. The upper boundary of a gaff main, the head, is supported by a gaff much like the boom supports the foot of the sail. The modern triangular main sail wasn't invented until the early part of this century and didn't truly supplant the gaff main sail until after WWII when waterproof glues and cheap aluminum allowed for the mass production of reliable hollow masts. Now they are thought to be old fashioned, hard to set and inefficient - nothing could be further from the truth. Here're some of the advantages of gaff sails:

  • It's low tech -- you can fix everything yourself. Let's face it, if the twisted titanium shackle at the foot of the roller furling drum snaps, you're not going sailing!
  • It is actually more efficient for sailing to weather than marconi sails when the aspect ratio is less than 2.
  • For large heavy sailing yachts or passenger carrying vessels more effective sail can be set.
  • It looks great!

 

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