MacNaughton Scantlings Rule
Thomas A. MacNaughton
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It is always a touchy thing when one reviews a book that you have written yourself. After all you are going to recommend it, aren’t you? Are you really going to have anything much to criticize? Nevertheless I’ll try to do my best:
We did up these rules over a period of years because we found that there was simply inadequate information on how to figure scantlings for virtually all forms of construction. This means that designers and builders must either just “guess” or must go through an enormous and uneconomical amount of labor and testing to determine whether the scantlings will reach some assumed level of strength. Very often this assumed level was a pretty arbitrary standard that again amounted to a “guess”.
In our case we did a large number of surveys of vessels which had survived a long time with special attention to any areas which didn’t hold up. By relating this to the cube root of the displacement in cubic feet, and other factors, we could relate the stresses to any size and type vessel. Using both vessels which had lasted a long time, and those which had not, we worked out the stress levels that it would take to damage the vessels. To simplify the description of a long and complex process which we continue examining today we found that a certain level of construction strength essentially never failed. Interestingly enough applied to conventional caulked seam carvel construction these came out as a very close approximation of Nevin’s rules. Since we had also been told repeatedly by long time boatbuilders that boats built to these standards were ones that they were confident would last a long time we have since then designed to these levels of stress adapted to all construction forms and materials. This has worked out very well and we have found that in practice designs done to our various rules always test out to reflect the best practice in scantlings accepted by custom boat builders who have worked a long time with any given materials.
It is interesting that only one class of builders ever challenges our scantlings. These are the builders of series built "mass marketed" stock fiberglass boats, who always claim that our scantlings are “overbuilt”. Interestingly enough custom fiberglass builders of long experience normally say we are “just right”. Of course since all our rules attempt to approximate the same standards of strength, stiffness, and fatigue resistance as nearly as we can with dissimilar materials it is obvious that our fiberglass rule is in fact on the money and that after decades of pressure to build fiberglass stock boats as cheaply as possible, the industry as a whole is now building soap bubble boats, whether consciously or not. Don’t get us wrong we’ve had boats built in fiberglass to our designs up to 90’ and have seen a number of custom and semi-custom fiberglass boat builders who are building really rugged boats, but they don’t complain about our scantlings.
Sheathed Strip Construction, in which a longitudinally planked frameless strip planked shell is sheathed transversely inside and out with either glass cloth and epoxy or carbon/graphite fiber and epoxy, was evolved from strip planked construction in the late 1950s by the naval architect Lindsay Lord. He had the backing of an a company making epoxy resins, which felt that epoxy would be very useful in custom and stock boat building when combined with wood. Mr. Lord was able to demonstrate that strong, fatigue resistant, stiff structures were very easy to build and would allow really excellent boats to be built. However he appears to have been more interested in developing the method per se, but apparently never really worked out the structural analysis needed to develop a versatile general scantlings rule suitable for use with all types of boats. While I did know Mr. Lord personally, at the time I was much too young to even know I was going to be a yacht designer, so I never did get to discuss this with him personally. However I did realize that he was a meticulous designer who had very high standards. So when my own structural studies demonstrated to me that the most cost effective, long lived, and easily repaired wood and epoxy boats would best be done using sheathed strip construction I went to work and very carefully developed the first and so far only Sheathed Strip Scantlings rule based upon real structural analysis work. It has proven itself well and has resulted in this method spreading all over the world since we first developed and started selling the rule in the 1970s. It has spread so widely and taken root so deeply that we have a number of times had builders coming to us urging us to design in sheathed strip for their clients, not realizing that they were working from methods developed or promoted originally by us. This is as it should be and I think we have already reached the point where the majority of builders using this method, even when they are working with formulas originally developed by us, have no idea that we originally developed these scantlings rules. So already in our lifetime this is becoming a “traditional” method of building. In our opinion this will eventually essentially completely take over from other wood and epoxy methods all over the world. Another interesting facet of this is that for different reasons it is very cost effective for both amateur builders and professional builders, whereas other methods are often cost effective only for the professional.
Cold Molded Construction is a very interesting method, often requested because of the loose use of the term “cold molded” to cover a huge variety of methods which usually amount to a planked and framed hull covered with two or three veneers of cold-molding. This is generally done because the designer or builder has found that cutting and gluing up a large number of layers of veneer in a true cold molded hull is extremely labor intensive. This is quite true and even though a considerable amount of weight is saved by this method if done entirely as layers of veneer without any frames generally pure cold molded must be reserved for vessels where money is not a constraint at all. If this is not the case you are better off going straight to sheathed strip construction. About the only case in which cold molded construction might make any sense would be in very small dinghies intended to be very high finish, varnished inside and out and presented as much as small works of art as anything else. If someone wishes to use cold molded simply because it can be varnished, unlike the surfaces of sheathed strip boats, which are glass cloth and epoxy or carbon fiber and epoxy, there is no reason not to build a sheathed strip vessel and add a 1/8” layer of a really nice grained wood on the outside which can be varnished. This looks very nice.
Having said all that the fact remains that when a customer a number of years ago wanted a cold molded hull for his 30’ sloop, we could not find a single rule for cold molded construction that was based upon real structural analysis. The only rule we could fine was an obvious attempt to “guess” at how thick a layer of cold molding it would take to replace the caulked seam carvel planking on an otherwise perfectly conventional traditional wood boat. This did not take advantage of the virtues of cold molding and resulted in an expensive boat about the same weight as a conventional carvel planked boat but at much greater cost. One way to tell a cold molded rule which doesn’t make any sense is if it has transverse frames. Transverse frames serve two purposes in caulked seam carvel construction, neither of which apply to cold molded. First they help to keep the planks in line which is obviously not needed in a hull which has a solid shell. Second they provide transverse strength and stiffness. However since in the cold molded hull you are running the veneers crisscrossed in multiple directions you have both strength longitudinally and strength transversely. Once you have a thick enough section to provide the longitudinal strength you have far more transverse strength then you actually need and framing becomes just excess weight and cost.
Despite our reservations on cost effectiveness, true cold molded construction as codified in this scantlings rule can produce a strong, long lived vessel in the hands of a good builder.
Fiberglass Construction scantlings rule is intended for conventional hand layup or vacuum bagged lay up using conventional “E” type glass cloth and polyester resin as is used in the majority of fiberglass boat construction. It can also be used with vinylester and epoxy resins and, especially with epoxy will be the better for it, but it would be better to have a yacht designer trained in fiber and resin laminate design do a special set of scantlings as it would be likely to be somewhat over strength in some areas otherwise. If you use epoxy and “S” glass or more exotic fibers, such as carbon or graphite fibers you definitely in an area where the laminate must be optimized by a designer well versed in structural analysis of fiber and resin laminates. Also the more exotic the laminates and resins the more critical the influence of the actual layup practices used and it then becomes necessary for optimum performance to require the builder to have samples of the laminate made up by the layup person in whom they have the least confidence for testing in the designer’s own facilities to be sure that the laminate need not be adjusted for less than expert layup practice.
Having said all this the scantlings rule we give will work for solid conventional fiberglass layups, longitudinally framed layup using fiberglass hat section frames, layup over a structural core of wood, or a solid fiberglass shell reinforced by longitudinal wood and epoxy framing. These scantlings, as with all our scantlings, are suitable for prolonged service not only in coastal work but in offshore and commercial work.
Various Forms of Carvel Planked Wood Construction. Though commonly thought of as, and definitely encompassing conventional caulked seam carvel construction. Carvel planked construction may also use various types of planking, some with glued seams, and anything from steam bent, through laminated, or sawn transverse framing. Also it can encompass a variety of fastening methods including wooden trunnels, drifts, various types of nails and rivets, screws, etc. As such this method covers a lot of ground and a lot of different types. In the past there have been two excellent Carvel Planked Construction Rules. One of the most famous is the Herreshoff Rules, which are excellent if used with the very particular design and construction favored by the Herreshoff Manufacturing Company. The other is Henry Nevins Rule developed for use at Nevins Yacht Yard and no doubt optimized for his particular methods. We have structured our rules to cover a much broader range of methods and regardless of the particular selection of methods you should be able to work out from this rule how to structure any conventionally constructed wooden boat.
One thing that this book is lacking which we wish we could easily supply is a set of rules for building steel and aluminum boats. We do have these rules for the use of our students and our own design work, but I must admit that we have not, as yet, been able to put them into a form which can be used by the average builder, especially the amateur builder, with confidence.
If you wish to have a set of scantlings prepared for a metal boat we can help you for a quite moderate cost and we do apologize for not yet having worked out how to produce a truly versatile metal rule that is easy enough understand as a simple straight forward set of calculations. (47 pages) (tm)