Offshore Canvas

The right fabrics, fastenings, closures and construction make a real difference

26th July 2001.
By Staff

img8397The well being of an offshore crew depends upon many things, but the ability to stand watch in some comfort, to remain dry and out of the wind in bad weather and to find shelter from rain and mid-day sun –even at sea– should never be under rated. A crew that can remain warm and dry and protected from the elements, will be a happier and safer crew than a crew that can not.

Offshore canvas should be considered essential equipment and given the same thought and consideration as the purchase of a sea anchor or GPS. Unfortunately, too often, the canvas work –despite the expense– languishes at the bottom of a “to do” list and does not get the attention it needs until the boat is well at sea and the problems of ill considered designs, shoddy workmanship and inappropriate fabric, fastenings and construction become apparent.

Moreover, canvas work around the cockpit should have a long useful life, should be easy to live with –ie to clean, adjust, repair– and should be attractive. It should be easy to erect and dismantle as weather dictates and, when stowed, should be protected from the elements and secured against damage from boarding waves.

A good dodger, weather cloths and sun protection need to be tailored to your boat and your needs. Below we’ll look at fabrics, fasteners and construction details that can help you get the best protection possible.

The Right Fabric For The Job

img8396Even though we speak of canvas, these days there are few if any practical applications for real canvas. Synthetics long ago replaced cotton as manufacturers discovered fibers that would not rot in the sun, were inert, stable and stain resistant. DuPont’s nylon and rayon started this move to synthetic fabrics but it wasn’t until the company developed Dacron — strong, low stretch and resistant to ultraviolet damage– that man-made fibers found their way onto sea-going vessels.

Dacron is commonly used today in sails and running rigging, often in combination with more exotic coatings and fibers. But because it has lower UV resistance than several other fabrics, Dacron is not used often in the canvas applications of dodgers and Bimini covers. Weather cloths, however, which need to be strongly made and often need to carry racing numbers or the vessel’s name, are a good application for Dacron cloth. Any good sail maker can whip up weather cloths out of 8 or 10 ounce material and will have the capability to apply permanent numbers and letters while many canvas shops may not be able to do so. Still, Dacron weather cloths will need to be removed when not specifically needed to preserve the cloth from becoming brittle in the sun.

Acrylic: The development of acrylic fiber and tightly woven cloth provided sailors with an excellent material for cockpit and on-deck canvas. One of the best known brands, Sunbrella, is used world wide. Acrylic fabric has proven to be easy to work with, resistant to ultraviolet damage and has the “feel” of traditional yacht canvas. Because the fabric repels water but retains the ability to “breathe”, it allows condensation and trapped water to evaporate, thereby cutting down on mildew growth. Acrylic fabrics are often used in dodgers, where the material’s slight stretch can be used to form neat, complex curves, and the ease of handling permits a fabricator to work quickly and efficiently.

But acrylic fabrics are not completely resistant to the sun’s rays; after six months of use, the crispness will be out of the fabric and in three or four years the fabric will no longer be water resistant. Over a period of year or more, acrylic fabric will lose its dimensional stability –ie, it will sag– and will become more susceptible to tearing, staining and fraying at the seams. Even though acrylic fabrics are colorfast, they will be discolored by rust, petroleum products and even the natural oils in the crew’s hands. Because the fabric has an open weave, dirt collects in the fibers, weakening the fabric and making it hard to clean. Acrylic fabric should have a useful life of three to five years of constant use –less in the tropics, more in high latitudes. While acrylics have long been used for dodgers and other applications, we suggest you save the fabric for uses in which breathability, stretch and look are the most important considerations, such as in sun awnings and sail covers.

Vinyl-backed acrylic: In an effort to make acrylic fabric more dimensionally stable, water resistant and durable while keeping the ultraviolet resistance and pleasant feel of the fabric, Glen Raven Mills (manufacturer of Sunbrella) created a hybrid cloth that has vinyl sprayed on one side. They call it Sunbrella Plus. The fabric is stronger than plain acrylic and has a longer life expectancy –five to seven years, we estimate. But the fabric retains most of the vices of plain acrylics -stretching, staining and sagging– with an added susceptibility to mildew because it won’t breath. That said, vinyl-backed acrylic fabric has a pleasant traditional feel, is easy to work with and would be a good choice for a number of applications, including dodgers and weather cloths.

Vinyl laminates: Vinyl fabric is manufactured by fusing vinyl to a woven substrate (cloth) of either nylon or polyester fibers. Weblon is one popular brand. Being inert and highly resistant to UV damage, tearing and staining, vinyl fabrics are useful in a number of applications including dodgers, weather cloths and Bimini tops. It is easy to clean and will have a long life, even in the topics, where it is common for the fabric to last eight to 10 years. Choose a fabric with a polyester substrate for the longest life.

Vinyl fabric is not as easy to work with as acrylic because it will shrink slightly causing bunches along seams and around zippers and fastenings. Also, because it does not breathe, mildew can be a problem. Vinyl laminates behave more like a membrane than a fabric, which means they are liable to crack in cold weather and after repeatedly being folding. Lastly, vinyl has the aesthetic drawback of feeling and looking like plastic instead of canvas. Many canvas shops and skippers prefer to use vinyl fabrics on overhead protection instead of in dodgers and other on-deck applications.

Vinyl-coated polyester: Although a close relative to vinyl laminates, vinyl-coated polyester fabrics have the texture and high UV resistance of acrylics with the stain resistance, durability and dimensional stability of vinyl. One of the more popular brands, Stamoid, is manufactured in Sweden. An attractive quality of vinyl-coated fabric is its low stretch, which makes cutting pieces of a dodger or Bimini more exacting, but ensures that the finished product won’t sag or bunch. With a very high strength to weight ratio, a 9-ounce vinyl-coated fabric can be used where you would have to use 12-ounce vinyl laminate or 10-ounce acrylic. Of the four types of fabric, we like vinyl-coated polyester, Stamoid, for all offshore canvas applications and would select 9-ounce fabric for a dodger and 12 ounce for weather cloths and Bimini tops.

Fasteners Hold It all Together

img8395No matter how good the cloth your select for your cockpit and deck canvas, the fasteners attaching the canvas to the boat determine how strong and secure the gear will be.

The dodger and sun protection — a Bimini– should be designed to withstand storm force winds and water coming aboard. But, a prudent skipper will also want to fold down and store fabric and tube constructions when the going gets really rough. A Bimini in particular should be designed so it can be folded down on deck and secured quickly and easily.

Zippers do the heavy work of assembling dodgers and Biminis, so they need to be strongly made, well installed and protected from the elements. Most canvas shops and sail makers use molded Delrin, toothed zippers to attach canvas and frame constructions. The world’s largest manufacturer of zippers and other fastenings, YKK Inc., distributes Delrin zippers world wide so chances are you can find a replacement in any sailing port.

While Delrin zippers work well, they tend to grow brittle after a few years due to UV damage, causing teeth to break. Once the process has begun, the zipper must be thrown out. To protect the zippers, cloth flaps with Velcro closures should be sewn in over the length of the zippers.

Continuous-coil zippers could be a better solution to the zipper problem for they are stronger, more flexible than toothed zippers and don’t cease to work when one of more teeth are broken. But to date, YKK, which manufactures the zipper, has not seen fit to equip it with a plastic or stainless-steel slider. The existing slider is plastic-coated white metal that rots quickly in the marine environment. While we like the continuous-coil design, we have to recommend Delrin-toothed zippers with fabric covers for offshore and long distance use.

There are several ways to attach a dodger to the cabin house and some are better than others. The simplest and strongest system for the base of the dodger is a bolt rope fed through a snug track. A bolt-rope system keeps water out and spreads strain along the entire lower seam instead of at the snaps or fasteners only.

A bolt-rope slide needs to be installed on a flat surface, something hard to find on most boats. A wood or composite track can be fabricated to support the slide, but in most cases, skippers choose the simpler option of using one of the other fasteners.

Simple snap fasteners are the most common attachments in use and work moderately well as long as they are only side loaded. Any vertical pull with quickly cause a snap to fail. When using snap fasteners, use stainless steel if possible or make certain, at least, that chrome plated fasteners are bronze and not steel underneath the plating. If necessary, use a magnet to cull steel based snaps and screws from the bronze.

Common Sense or turning fasteners work well in many places around a dodger and are especially useful where the fastener will have a lot of side load, such as on the aft flaps which pull the dodger back and into shape. But, Common Sense fasteners rely on a flimsy moving part that sticks out from the edge of the cloth. A snapping genoa or the side of a shoe can rip the top off the fastener, leaving a jagged pin exposed and destroying the fastener. Also, chrome plating on the fastener is rubbed off every time it is done up or undone, leaving a bare bronze surface that will corrode and eventually seize up.

The Lift-a-Dot fastener relies on a short stud and a simple, flat clip to secure the edge of a dodger. The stud is attached to the cabin top and the clip is pressed onto the dodger’s lower edge. When the two pieces are snapped together, a small spring and a stainless-steel washer hold the stud in place. Although water can collect inside the fastener, the fasteners hold up well and resist corrosion. Lift-a-Dots hold side loading well and has a low profile so it can not be damaged in normal sailing conditions. We always select these fasteners when we are fitting a dodger or other on deck canvas and recommend them in place of Common Sense and snap fasteners.

Lastly, Velcro is a useful tool in the construction of on-deck canvas, but it is not a fastener. It should be used only as a closure for flaps and not to take loads. Use only Velcro with polyester backing; nylon backed Velcro will deteriorate quickly when exposed to sunlight and moisture.

Construction Details Several small details in the manufacture and construction of cockpit canvas will make a large difference how the gear performs. As a rule of thumb, a dodger or Bimini –both the fabric and the frame–should be build strongly enough to support the weight of a large person falling hard against it. Weather cloths and the dodger should be strong enough to withstand the force of a wave breaking on deck.

Thread holds all of these fabric creations together, so beware of inexpensive and inferior products. Dacron or nylon thread often used by canvas shops is not up to years of exposure to sunlight. Instead, specify bonded polyester thread, which will last as long as the canvas work. If you only go with the best, then Gore-tex thread is what you want. When the weather cloths or dodger are long gone, the Gore-tex thread will still be like new.

Reinforcing patches, similar to the corner patches sewn into sails, should be considered essential to the design of weather cloths and dodges, particularly if acrylic fabric is used. The reinforcing should back up any grommets, cringles, zippers and snaps and should be wide enough to distribute loads evenly. Vinyl coated polyester makes a good reinforcing material due to its low stretch and strength.

In dodgers and Biminis, tube size and frame construction determine the overall strength of the gear. Even on small pramhoods or hatch covers, we suggest using 1-inch stainless tube as a minimum –unless weight is a serious consideration. On larger boats, or in situations where large spans are covered, 1 1/4 -inch tubes should be used.

Dodgers for offshore vessels should have a low profile and curved bows. Although a square shape will provide more shelter and room for stowing cockpit gear and cushions, a bowed dodger will have more strength and presents a lower profile to an unexpected boarding wave. On medium size boats, a two-bow frame will work well up to spans of about 40″ in a fore and aft dimension. Spans larger then that require a triple-bow design, which is both stronger and more expensive.

Windows in dodgers are generally made of polished vinyl which can be sewn to acrylic fabric or bonded with glue to vinyl fabrics. We don’t recommend gluing windows in place; they are usually the first part of the dodger to require repair or replacement so it is useful to be able to remove a window with a sharp knife and replace it with a simple sewing machine.

In Bimini construction, use 1-inch tubes for short spans and 1 1/4 for longer spans. In aft-cockpit vessels, try to design the frame structure to incorporate the stern pullpit for added strength. Most Bimini tops are designed with flat sections on top. However, a slightly bowed top adds strength to the frame and improves drainage in rain storms. Biminis for use at sea should be kept to a minimum size, preferable directly over the helmsman’s station, however, each boat requires slightly different treatment.

In the design of weather cloths, keep in mind that cloth and thread, stanchions and life lines are not engineered to carry the weight of a large waved rushing down the leeward deck at 7 knots –a ton or more of water. Stanchions can be bent or ripped out of the deck, weather cloths torn, and life lines broken by such forces. The bottom of the cloth should not be laced to the toe rail , but should be attached firmly at the corners and in the middle with heavy shock cord. Water on deck should be able to run freely under the weather cloths and the force of large waves should be absorbed by the stretch of the shock chord.

By choosing the best fabrics, fasteners and design details for your cockpit canvas, you can improve the quality of life in your cockpit and get the most for your investment in cockpit protection. More importantly, a secure and comfortable cockpit is a safe place from which to run the vessel, a place where the crew will remain warm, dry and alert to the needs of the boat instead of their own discomfort.

This review was prepared with the able assistance of Paul DiMartino, president of
S & S Fabric Products
1 Maritime Dr., Portsmouth, RI 02871
Phone: (800) 441-2252
Fax (401) 683-5858
info@ssfabricproducts.com
www.ssfabricproducts.com


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