Scantlings Rule for Various Forms of Fiberglass Construction

At present the only other scantlings rules for fiberglass vessels available to small boat builders are Lloyd's Rule and American Bureau of Ships Scantlings. Unfortunately both rules have some restrictions on their versatility.

Since in the U.S. there has been no practical standard to go by, people have become accustomed to having and hearing about major structural problems. The general level of construction is so bad that any boat that is even marginal overly impresses people. Designers have no dependable way to judge what to draw. Builders are reduced to figuring scantlings by looking at the competition. Owners have no one to tell them what is solid enough or how much and in what ways to beef up an obviously inadequate boat. Those advocating or building light scantlings boats should reflect on what value they would place on human life.

This rule gives several alternative methods of producing similar strength, stiffness, and fatigue resistance. The weights vary considerably between different methods. The strength and fatigue resistance is equivalent to time proven methods that have survived well at sea.

This one rule is equally applicable to yachts, both power and sail, and commercial craft. It is suitable for designers, builders, and owners to use quickly and without hesitation. We hope this rule and others that we have and will formulate for other materials may someday form the basis of a system of classification for small craft. We believe this would help quality builders stabilize their market, and make bank loans and insurance available at lower rates. We need something like this in the U.S. Otherwise the government will tell us what to do and presumably make a complete mess of things.

It is important to realize, that no direct conversion of another material to fiberglass is possible. Scantlings in one material must be reduced to strength and fatigue resistance figures and the new material examined in the light of these figures before any idea can be obtained of what the new construction will look like. A fiberglass boat bears little resemblance structurally to other methods.

The above is condensed from the Introduction to the Rule. We will further comment here that the state of fiberglass boat building in the United States has gotten so bad that we are told repeatedly by builders that our rule is "too strong" or "over building" despite the fact that it is done to the same standards as our other rules which no one has ever claimed were excessively conservative. We know of several people or groups that have made extensive studies of today's fiberglass production boats. All of them have found that most of the "production" boats in the United States are so under strength as to be dangerous and extremely poor long term investments yet I know of no practicioner of either practical or theoretical marine engineering who has been able to get such studies published in the general marine press for fear of advertiser pressures. One study that I received information on privately focused on fatigue resistance and concluded that the average US built fiberglass boat was good for about 35 days of hard sailing before it suffered massive failure. The authors of this sold products to fiberglass boat builders and concluded they dared not publish their study if they expected to continue to sell product. Fortunately most people rarely take their boats out except on light wind days in protected waters.

Another problem we have seen recently is that people have found that standard test methods do not produce the results they want so they change the test methods. This has been used to justify light cores with insufficient shear strength in fiberglass and other composite methods. We use standard simple engineering theory and test methods. If the results aren't what we expected we accept what the real world tells us. We do not devise different tests for different materials to get the results we want.

None of this is to say fiberglass is not an excellent material. In our shop we often do extensive rebuilding projects on fiberglass boats and will build in fiberglass when an owner will allow us to specify epoxy resins and scantlings like those in this book.

The contents of this rule are:

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