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Metal Boat Framing...? You Bet...!

Copyright 1990 - 2011 Michael Kasten

We have here an entire issue dedicated to strategies for plating and framing metal boats.

It takes over 60,000 psi to tear apart a chunk of mild steel (the usual stuff of steel boats). It takes 30,000 psi to deform the same piece. Thus the extreme benefit of steel as a hull material: The "plastic range" of steel is quite large, and the material is therefore able to take a terrific beating without failure.

When I saw Moitessier's Joshua sail into Port Townsend after the infamous Cabo San Lucas storm had washed her ashore, destroying three other boats in the process, the dents were quite impressive. One large dent below the waterline, if plated over, could have hidden several illegal immigrants. I'm not stretching the truth, either. Moitessier was so despondent that he sold the hull to some opportunist beach bums for a dollar!

The boat was bulldozed back into the water, the sand shoveled out, and parts salvaged from the wreckage of several boats on the beach. Soon they had a boat again, though severely battered. Joshua was still completely watertight, and soon made the voyage here to Port Townsend. She is now in Europe, having sailed there dents and all.

The Joshua definitely had framing. Quite a bit of it. All arranged transversely.

There is potentially misleading and incorrect information in the implied promise of "frameless" steel boats, pandered by some. In truth, in order to achieve the required strength in a steel vessel without using framing would require an enormous weight penalty due to the required increase in the thickness of the plating. Metal boats, whether steel or aluminum, which are properly and responsibly built do definitely use framing. The framing may be in the form of devious strategies, such as the use of bulkheads and other interior or exterior features to achieve the required reinforcement.

The concept of frameless metal boats is attractive, but flawed. It is true that many metal boats are successfully plated and the plating then welded together without the aid of metal internal framing. This provides a very big plus when trying to maintain fairness. In the final product however, in order to provide adequate strength, frames must be added before the hull can be considered finished. Responsible builders, such as Gary Noble Curtis, though they erect and weld up the plating on their metal boats without using frames as a support structure, very definitely make proper use of framing in the finished hull.

Many so-called "frameless" boats make extensive use of longitudinals. True, they may not have transverse frames per-se, but they do have "framing," and lots of it. Bulkheads or other internal structure will usually be used to reduce the span of the long's.

There are those who will disagree with what I've said here. The fact is, though, that if you apply well proven engineering principles to the problem, you quickly discover that frames are simply a requirement. ABS rules, Lloyds, and Det Norske Veritas may all be somewhat conservative in their approach, but working through their formulae will show the benefit of framing - primarily to bring the weight of the vessel within a reasonable range, while maintaining the required hull rigidity. In general, the ABS rules are easy to use, and are fairly uniform from one to the next.

The ABS rules are changing. ABS will no longer be providing design information for vessels smaller than 79 feet. Presumably, another ruling body will pick up the torch for those of us unable to afford 80 foot yachts. Still, the ABS rule for Steel Vessels under 200 feet, while aimed at larger boats, is useful. Additionally, the ABS rule for Aluminum Vessels remains available. There exists an ABS rule for Motor Pleasure Yachts, published in 1990, and updated twice since then. This rule covers all materials except ferro-cement, and is quite valuable. Recent studies of failures in smaller crew and offshore supply vessels built of aluminum show the need for parts of the ABS rule to be made even more restrictive in terms of the allowable areas of unsupported plating.

Anyone interested in reviewing the plating failure studies will find them in Marine Technology, April '96, from the Society of Naval Architects and Marine Engineers.

Michael Kasten

Metal Boat Quarterly #8 - Fall 1996 Editorial - Updated 2009


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Other Articles on Boat Structure

Metal Boats for Blue Water | Aluminum vs Steel | Steel Boats | Aluminum for Boats
Metal Boat Framing | Metal Boat Building Methods | Metal Boat Welding Sequence | Designing Metal Boat Structure
Composites for Boats | The Evolution of a Wooden Sailing Type