Duckworks - Projects
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The Online Magazine For Amateur Boat Builders


Canvas Coverings

by Paul Butler
http://www.butlerprojects.com/
illustrations by Marya Butler

Fabric decks, spray shields, and cockpit tents are comfortable, space saving, and lightweight. They can significantly increase your living space, make a rough passage a bit safer and dryer, or turn sleeping aboard on a cold, wet night into a pleasant experience.

Fabric structures should be as low as possible to keep windage to a minimum. Well designed fabric foredecks and small shelters may sometimes be left in place when under way. If you want to, you can sew plastic windows, vent flaps, and mosquito netting into the fabric.

A wide variety of fabric types, weights, and colors is available. Quality is related to price — you get what you pay for. Some fabrics stretch, fade, or wear more than others, and even shock cords may not be able to take up the slack of cheap fabric or wet, untreated canvas. We've found heavy-weight fabric advantageous in building these structures. And when in doubt as to the best fabric weight for a specific application, we call our supplier and ask his opinion.

No matter how it's designed or which fabric is used, a structure that flaps, sags, or leaks is as bad as none at all. A good support system is the key to success. Every boat offers different options for securing and supporting fabric structures. Battens, ridgepoles, upright poles, and shock cords can provide tension and support. Fabric also needs slope to effectively shed moisture. Even a heavy dew will fill pockets in a flat and loose-fitting piece of fabric. And a bit of flex is necessary because a stiff and unyielding fabric structure will strain lines, bend supports, and stretch fabric.

Narrow boats with small spray shields don't usually pose difficult design problems, but wide hulls often require a more complex structure to provide proper support.

Simple Supports

Options for framing and support of a fabric structure are limited only by the imagination. A ridgepole is the simplest support and does a creditable job (Fig. 1). Ridgepoles usually run fore and aft down the centerline, although they can also be positioned on one side of a hull for a "lean-to" shelter. A boom is the classic ridgepole, but we've also used a 10-foot sculling oar supported by a forward tripod and a notch in the transom.

Halyards attached to sewn-in D-rings provide prime, adjustable overhead support (Fig. 2). However, they offer such strong leverage that you must be careful not to tear the whole affair off the hull and hoist it to the masthead. When using halyards, you may have to snub the line to the mast to quiet flapping in a breeze.

Telescoping supports may also be situated inside the structure, braced up from thwarts or the sole (Fig. 3). I made three good aluminum supports from an old tripod. I adjust them by tightening two knurled knobs on each leg. These supports can be placed in a grommet or reinforced patch in the fabric.

Battens & Beams

Support battens may be rigged under or above the fabric or sewn in between layers (Fig. 4). One method for supporting fabric is an external batten much like a mountain tent (Fig. 5). All you need is a pocket at either end for the batten and small loops or ties along the rest of the batten length. If these ties are small diameter shock cord, they can be adjusted to keep the fabric at exactly the correct tension to shed water and wind. This structure only requires tie-downs at corners and along the edges to be self supporting.

You may find fiberglass, plastic, or aluminum battens that work well and do not require any finishing, but wood battens are the best looking. They finish nicely and bend to a uniform curve. Hardwoods like ash or mahogany or straight-grained softwoods like spruce and fir make fine battens. All wood battens should be well sealed to keep their strength, since soaked wood will lose much of its stiffness and may snap in response to a gust of wind or a sudden load. Three coats of epoxy, followed by varnish if exposed to sunlight, are best for sealing.

Wood battens made for supporting a fabric structure usually need to be wider than thick to prevent twisting and make rigging easier. A successful size for us has been 1/4 inch by 1-1/2-inches, but each application seems to require some experimentation as to best size and shape. Start big and keep shaving the batten with a block plane until it bends easily to the shape you want. Keep in mind that laminated and epoxy-sealed battens will be stiffer than plain, unfinished battens, but they will also retain uniform strength under all weather conditions.

You may wish to drill small holes in the ends of the battens to provide a tie off point. Straight battens maybe slipped out of the batten pockets and rolled in the fabric, allowing a large assembly to be stored easily in a small place, in many fabric structures, the battens are different lengths, and having each batten labeled makes it easier to rig. If fhe batten pockels start to wear, consider wrapping the batten ends in duct tape.

Fabric decks can be supported with beams that fit into sockets or notches in the gunwale or deck structure (Fig. 6); Sometimes these beams can be permanent and the fabric covering removed or left in place according to the weather. By laminating, you can make the beams to almost any shape for a small boat.

click to enlarge
(click image above for larger view)

These beams are best lofted and laminated right on the lofting board (Fig. 7). Make the lofting board of 3/4 inch ply and do all the layout right on the board, then screw down short sections of aluminum angle and use the lofting board for laminating the beams. We build ours out of 1/8 inch veneer.

Tie-Downs

The outboard ends of decks, tents, and spray shields can be held in place with snaps or loops and hooks (Figs. 8-10). They can be attached to the outwale, under the outwale, or in the case of hooks even inside the boat if you drill a small hole under the gunwale to provide access for a small hook.

We favor shock cord loops and hooks over snaps as a means of attachment. Shock cord stretches in response to pressure or slack and will keep a fabric panel at about the same tautness whether wet, dry, hot, or cold. Using cords of various size and length also allows a degree of fine tuning. Be sure to get the quality cord that comes in rolls in various diameters from 1/8 inch, good for small adjusting and
lace lines, up to a whopping 1/2 inch, which defies efforts to stretch it more than a few inches.

If hooks are permanently attached to the boat, the fabric structure, once adjusted, can be set up easily, even late at night or in wind or rain. With any fabric structure, you may need to rig additional lines fore and aft to provide proper support tension. We always throw a few small C-clamps in the boat for overnight trips, since you can easily clamp a structure together in an emergency. Small ply pads protect the hull from marring.

A couple of words of caution. Anchor your boat so it can weathercock into the wind. If the wind blows into the tent from the backside, your ears will pop every time the thing flaps.

Also, test everything before your first night aboard. For some reason, nothing ever seems to happen to fabric decks or tents till late at night or miserable weather, when you least want to get up and deal with it.