click to enlargeOdd Looking Dinghies
The purpose of this section is to explain why I plan to make sailing dinghies unlike anything that have been built. Remember that the dinghies we build are intended to be prototypes for somewhat larger well-built and seaworthy cruising barges. To that end the first dinghies will have two unusual features that will certainly slow the boat down:
1. The central section will be box shaped, with parallel sides and a flat bottom
This facilitates construction and permits the smallest possible draft. The parallel sides permits us to locate the leeboard anywhere we want.
2. All surfaces will be flat with transverse "hard chines" under the surface of the water
This permits construction out of unbent planks. Only the large barges need be built out of planks; the small dinghies are most cheaply made of exterior grade plywood. A consequence of these flat surfaces is that transverse sharp corners (chines) will characterize the underwater surface.
Go back to the three boats that inspired me as I thought about a sailing barge for Grand Lake
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The first (Martha Jane) was scientifically designed for the home boatbuilder. The second (Weekender) is quaint and easy to build, but not seaworthy. Nor do I believe it can it be kept in the water for long with its submerged wooden keel. Only the third (Folkboat) is a true cruising sailboat. None resemble a sailing barge in any way, yet all have something I want in a cruising barge.
The "over-built" sailing barge
The FB (Folkboat) stands out as the only ocean crosser. Don't take my opinion too seriously when it comes to such matters, but I think a truly seaworthy sailboat needs only two features:
1. So well built that it will never fail
2. A plan for capsizing
The FB achieves the first requirement with a thick and heavy hull. The 25 x 7.5 ft Folkboat is "clinker" built from overlapping planks, each with a thickness that exceeds a half inch. To almost eliminate the possibility of capsizing, a typical Folkboat has a draft of 4 ft and a displacement of 4400 lbs, with over 50% of that displacement in the keel. In contrast, a typical Catalina-22 has a displacement of 2300 lbs, with only about 23% of that displacement in the retractable keel.
Allowing water to evaporate from a wooden boat
I'm pretty sure the bannana-shaped planks of a FB must be bent by steaming them first. This is not within the grasp of most amateur boatbuilders. For one thing, boards intended to be severely bent must be carefully selected for a grain pattern that is parallel to the board. Boats like the Martha Jane are usually constructed from plywood that is thinner on the sides (perhaps 3/8 inch) and usually doubled on the bottom. But a double layer of wood has problems of its own. It might cost $100 in epoxy to do it, and you need to drill and then fill lots of holes. Boatbuilders have told me that this is no big deal, but it sounds like a lot of trouble to me.
I also worry about the problem of rotting. My understanding is that a well designed wooden boat has a method by which moisture can evaporate through the wood and into the air. Shown below is a schematic of a wooden boat with a bottom, two sides and a keel. An evaporation "plan" through a single layer of planking is shown as arrows through the wood for every member, except the keel. The Weekender is fitted with such a keel.
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A rugged and seaworthy cruising barge might be accomplished with straight or nearly straight planks. Reasonably good 3/4 inch planks cost about as much as exterior-grade plywood of the same thickness. Marine grade ply costs at least twice as much, I think. One problem is that I can't find wooden boards less than 3/4 inch at a good price. Hence the small dinghies are best made of exterior ply or luan underlay.
Freeboard and energy to capsize or knock down a sailboat
The heaviness of a planked barge suggests that we give it a very low profile by copying the extremely low freeboard of the Folkboat (the height above the waterline is about 10% of the length). This yields a "wet" boat, but if you want to stay dry -- don't go sailing! The reduced freeboard will also allow us to reduce sail area. Polytarp is too thin for large sails, anyway.
Safety can be added by giving the barge a "false" cargo of flotation material whenever one plans to sail dangerous waters. Such floatation above the waterline cancels sail area by letting wind push the boat in unwanted directions. But it also helps keep the boat upright. An upside down boat with large freeboard has more gravitational potential energy, provided some ballast is placed on the bottom. Shown below are boats that have been either knocked down or capsized to illustrate the advantage of large freeboard on a boat like the Martha Jane. The ballast is shown in BLACK, and the arrows show the height of the center of mass above the waterline. Recall that the formula for gravitational potential energy is, PE = mgh, so that the length of these arrows indicate how much energy was required to put the boat into each state.
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Easy to build shapes for a wooden-planked barge
Shown below are two profiles of sailing dinghies that resemble my concept of a cruising barge. Two versions are shown (red and blue). In both versions, all underwater flat surfaces are quadrilaterals with at least two parallel sides. This removes the need to taper the planks when we switch from small plywood dinghies to the larger planked cruising barge.
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A control group of dinghies
A simple (and fun) way to investigate the consequences of the sharp corners associated with planked barges is to construct plywood dinghies with rounded hulls. One good season of racing should settle the question.
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Links
I. A problem with 3 Solutions II. Constructing a Sailing Barge III. Odd Looking Dinghies