Cost-Effective Boatbuilding With Sheet Material for Hulls and Decks

CONIC DEVELOPMENT OF HULL AND DECK SURFACES IS KEY TO USING LOWER-COST SHEET CONSTRUCTION, YET IN THIS AGE OF MOLDED FIBERGLASS, REQUISITE KNOWLEDGE AND EXPERIENCE IS WANTING

Preface: This article is nominally about boatbuilding materials and processes, but in reality, it's also about a cost-effective approach to small craft manufacturing, recreational marine marketing, and perhaps even the future of the small-craft industry itself. The materials and methods discussed here represent a pathway for returning to an era when owning a small (under 30 feet LOA) "starter" sail or power cruiser was within the financial reach of the average middle-class family. As well, it's about the marketing of small craft being, in part, the nurture of future markets, and not just the pumping of immediate sales. All of which are topics of concern to those who love yachting and the recreational marine industry, including marine professionals, DIY-ers, and various and sundry boating enthusiasts in general. There is a wide variety of issues at stake here, and you are welcome to join the conversation with your views, opinions, and relevant experiences.

A short while ago, I was involved in a discussion of plywood boatbuilding, in both the DIY and professional small-shop sectors. In that discussion, I mused about how frequently builders of plywood boats talk about measures they've had to take to "torture" the plywood skin on the boat hull into conforming to the designed curves, especially below the chine forward. And also how often I’ve seen comments by small-shop metal boat builders about the high costs of equipment for plate rolling and cold-forming. All of which moved me to wonder how many, in this age of molded fiberglass boatbuilding, understand — or even know about — the employment of “developed” or “multi-conic” surfaces in boat boat design and construction.

A developed surface is one without any compound curves, namely, a surface which is a segment of a cylinder or a cone or a combination of such segments laid tangent to one another, designed to create a surface that can be flattened without stretching or compressing the material from which it is made. Think, for example, of how you can form a cone from a single sheet of paper, but how you cannot smoothly cover a globe (whose surface has compound curvature) with the same flat sheet.

As a result of that brief discussion, I received a passel of push back from both several DIY-ers and a few professionals, detailing the cons of doing so. I even received a few edgy messages telling me that conic development is a BS myth, that it’s impossible to find anyone who has actually done it. The more substantive remarks included the following:

1. A developed hull surface in plywood construction generally forces the builder to forego straight timber frames in favor of sawn or built-up curved frames forward, which is too complicated and time consuming.
2. In metal construction, it’s easier and faster to stretch the forward plates into the right compound shapes over the framework with come-alongs than to develop the surface on the drawing board, then loft and cut the correct plate shape out of a larger flat panel.
3. Developed surfaces yield only clumsy-looking boats -- crude, boxy, single-chine scows that nobody loves.

To which I answer that numbers 1 and 2 above are just plain false because, in this time of CADCAM, you can radically reduce your own shop labor hours by having a pre-cut frame and plating kit cut, crated and shipped to you. And with respect to objection 3 above, a double or triple chine hull form can be developed using a "multi-conic" approach that enables the above-water chine to be feathered out so that it is virtually invisible in the forward 1/4 to 1/3 of the hull — with the result the hull is relatively easy to build with sheet material, while capturing most of the look of a round bottom hull built using brake-formed metal plating or molded fiberglass construction.

When Theory Turns Into Reality

A typical example of the potential for multi-conic development in the real world is the all-welded aluminum 41-footer you see here, which has two chines even though, to all intents and purposes, her chines forward meld into a visually single curved, but fully developed surface in her forward topsides. I designed her in the early 1980s, and she was built entirely without brake forming any of her plating. Which means that she could have been built in plywood without "torturing" (forcing fiber damage into) any of her skin panels.

I personally learned the technique from a now obscure paper written by small craft designer, Samuel S. Rabl, reprinted in a publication of the Society of Small Craft Designers (SSCD) some fifty years ago. I have a copy of that original publication in my personal library, but for those interested, although long out of print, an inventory of used copies can be reached via the link supplied at the end of this article.

But What About Fiberglass Construction?

I’d give you ten to one odds that a majority of yacht designers and boatbuilders (even if familiar with multi-conic development of hull surfaces) believes that it applies only to metal and plywood construction. However, it doesn’t — or at least it doesn't have to be so restricted. Over the years, I developed, but never had the opportunity to implement a system for building multi-conically developed hull and deck shells using pre-molded fiberglass sheets.

The system provided for bulk molding FRP panels on a 4’x8’ or 4’x10’ glass or melamine surfaced table, then producing a framing/plating kit using either an NC 2-1/2 or 3-axis mill, or by manually patterning and cutting the pieces. For manual cutting, full size Mylar patterns could be printed for the transverse webs and frames, and spiled patterns produced for the plating segments. Match up tabs would be incorporated for dry assembly and a specially-designed jointing system bonded with a high-strength adhesive like methyl methacrylate (e.g., Plexus ?) or polyurethane (e.g., 3M 5200 ?).

Because an NC cut parts kit would be flat, it could be readily crated and shipped to a DIY-er or small boatbuilding shop, where the transverse frames would be erected on a strongback and the plating pieces applied and bonded to the assembled framework. The molding table surface would produce a high quality exterior surface for the hull and deck plating. Some touch up work might be needed at the plating seams and butt joints — but likely significantly less than the amount of post-molding corrective work performed on the average female molded FRP hull/deck shell. And without facing the prohibitively high cost of producing even limited-production female tooling.

Moreover, this FRP system would avoid the almost universal drawback of plywood construction, namely, incipient rot, which often takes hold and spreads faster than the progress toward completion in a prolonged DIY project. That, however, is a discussion for a different time. Let me know in the comments if you like to have that conversation sometime. Fair winds and safe harbors!

— Phil Friedman

Copyright ? 2024 by Phil Friedman — All Rights Reserved

Post Script: If you are interested in multi-conic development of hull and deck surfaces, you can purchase an economical used copy of this out-of-print book containing Sam Rabl’s paper on the topic. The paper includes step-by-step instructions for performing a graphical development using ordinary drafting tools on a drawing board. See the link below.

Note: If you purchase through this convenience link, we earn a small commission at no added cost to you. This revenue goes directly to helping support the publication of this newsletter.

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