PS Advisor: 05/15/05


C-Flex Construction
I ran across a an owner who said his boat was built with C-flex construction. What do you think of this method, and how does it compare with other fiberglass building techniques?

Miles Avery
Via e-mail

C-Flex is a fiberglass material used principally for building custom boats and one-off projects. C-Flex essentially allows builders to create a single-skin fiberglass boat without a full mold, and its proponents say that it gives builders the opportunity to save weight relative to a conventional single-skin fiberglass laminate, without sacrificing strength.

Since we’ve never had the occasion to work with it ourselves, we sought more details on its use from naval architect and boatbuilder Bruce Roberts, who often works on custom projects. Hal Whitacre, a naval architect who manages Bruce Roberts’ office in the U.S., told us that “C-Flex, on a micro level, is a unique combination of rods held together loosely, with pliable areas in between.” Both Roberts and Whitacre regard this product as the only rigid fiberglass material available that will conform to compound curves without having to be stretched or deformed in some way.

“C-Flex will bend sideways, a property that virtually eliminates the problem of having to fin or spile the ‘planks’ while building,” said Whitacre.

He told us that C-Flex’s chief advantage is that its a unidirectionally reinforced material, so it’s considerably stronger in the direction of its fiber orientation than ordinary fiberglass mat or roving. This property allows C-Flex to be placed in wide spaced mold frames as opposed to a full contact type of mold needed for standard, single-skin fiberglass construction. C-Flex is run fore and aft, set in casting resin, and then covered in fiberglass roving or bi-directional cloth, resulting in a single shell with good strength properties in all directions.

C-Flex is manufactured by Seemans Fiberglass in Harahan, LA (504/738-6035). It costs $3per square foot. Readers can find an article depicting the use of C-Flex online at this website: Just click the tab labeled “Boatbuilding Methods,” and then click on “Boatbuilding with One-Off Fiberglass.”


Sharpening Knives
What kind of material do you recommend for knife blades if someone’s principal interest is in keeping a sharp edge? Also, what should I use to keep a serrated blade sharp?

Ed Hooper
Jacksonville, FL

For most people, the best sharpener is one which will consistently maintain the knife’s original cutting-edge bevel. The knife industry doesn’t appear to observe standards for edge bevels, however, and neither does the sharpener industry. Yet we consider the bevel-angle issue more important than the kind or aggressiveness of the sharpener’s abrasive material.

Most of the nautical blades we’ve tested are of high-carbon stainless steel, a material that effectively resists the abrasive action of a sharpener, so matching the original edge bevel is critical, unless you’re willing to perform the one-time labor necessary to change that angle-dull work indeed. We’ve found that blades of cobalt and titanium are the easiest to sharpen; these materials are less hard than most steels, making it easier for the sharpener to abrade the edge.

If the knives you own are really dull, return them to the manufacturer to get back the original edge. If youre buying a new knife, consider the ones we’ve rated highest. In both cases you start out with a fresh factory edge. Measuring blade bevel-angle isn’t easy, and we don’t recommend spending time on it.

Sharpeners can be classified into three general types: freehand (bench stones and pocket sharpeners wherein it’s your skill that establishes and maintains the sharpening angle); angle-assist (crock sticks and other devices that help you maintain an angle), and variable-angle-lockup (high-end devices that let you set the angle, and lock the blade in position).

For plain-edge blades, consider a freehand or angle-assist device. Depending on your skill level, this could mean a fairly inexpensive flat diamond-imbedded sharpener (Eze-Lap, DMT, Ultimate Edge), or a moderately priced Spyderco Tri-Angle Sharpmaker.

To sharpen serrations, you need something that will get into the serrations (don’t just sharpen the back of the blade). The Tri-Angle will handle this, as will several of the thin tapered diamond sharpeners on the market, including the Mission Knives NMS. Ideally, your sharpener should be exactly as round and wide as the serration.

Practical Sailor has been independently testing and reporting on marine products for serious sailors for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising or any form of compensation from manufacturers whose products we test. Testing is carried out by a team of experts from a wide range of fields including marine electronics, marine safety, marine surveying, sailboat rigging, sailmaking, engineering, ocean sailing, sailboat racing, and sailboat construction and design. This diversity of expertise allows us to carry out in-depth, objective evaluation of virtually every product available to serious sailors. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser with more than three decades of experience as a marine writer, photographer, boat captain, and product tester. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


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