WHITE PAPER: Steel Hulls for Small Vessels


by Mal Low, Naval Architect, SmallTugs LLC, Gloucester, MA USA
www.smalltugs.com

Copyright 1997, 2006, SmallTugs LLC, Gloucester, MA USA
Excerpted from a longer technical paper.


"Steel is NOT a suitable building material for small vessels under about 32 ft LOA"

This is a true statement with a few exceptions. The reasoning behind this is that the weight of the material so far exceeds the weight carrying ability (normally devoted to the hull structure alone) of a small hull as to be impractical. Would you build a 12 ft. skiff from steel and expect it to be seaworthy, handy, durable, and movable with a normal amount of Hp (or oar-power)? No.

The exception that I will discuss here is in small flat bottom or small high displacement v-bottom hulls. These, by necessity, must be considered workboats and that classification, luckily, also recommends itself to the use of steel (or aluminum) for its strength, durability, and repairability.

Any hull with a high waterplane area at increasing displacement and an appropriate amount of freeboard is inherently extremely buoyant. However this description does not lend itself well to the typical smallcraft shape. If you picture a small barge form, nothing more than an elongated shallow "cube", and calculate the displacement at varying weights, you will see that the barge has very little depth and draws very little water for very high loads. These loads can be be in the structure of the barge itself and in the deck load combined.

Lets take this shape and play with it a little; if we stretch it out a bit and put a pointy end on it, maybe a little rocker in its form longitudinally, (and hey: how about a fantail stern as well?) we end up with a shape that looks a lot like a tug hull. Its still a barge hull, but a better looking one, and one that can be self-powered in a reasonable range.

And what if this hull is only 21 ft. LOA? It doesn't matter; the barge form has an excess of load carrying ability, such that we can even build this hull form in a heavy material like steel. What does matter is whether we can build a hull this small for any useful purpose. Here's where the naval architect comes in. Properly designed and laid out the 21 ft. small tug hull is a useful, workable, buildable, powerable, repairable, long-lived workboat.

The same attention to load-carrying ability of the hull form can be applied to a heavy v-bottom hull. The double chine v-bottom hulls in the SmallTug TM series are designed with an abundance of displacement. This allows them to be built in small LOA versions (25 ft. and up) without sacrificing metal construction. Unfortunately it does make them more unsuitable for wood or fiberglass construction without extra attention to inside ballast or full accommodation packages. However, the types of vessels in the SmallTug double chine series are those that most usually are built in steel or aluminum.

So why isn't the 12 ft. skiff practical in steel? Its a barge shape isn't it? Because steel corrodes. This fact means that steel members and plating in a hull must be of a certain minimum size to provide long term strength with the losses expected due to corrosion over time in the marine environment. That 12 ft. skiff would have to be built from a minimum of 7 ga. mild steel plate and would sink at launching unless the freeboard was grotesquely large.

You will notice that my designs call for the use of A-242 CorTen steel for plating. This is not just a whim. The use of CorTen allows a thinner and therefore lighter guage of steel to be used with the same long term life/strength expectancy as thicker mild steel. This is because CorTen is: 1. a higher tensile strength steel than conventional boatbuilding steel, and 2. it forms its own rust barrier when exposed, much like aluminum does, although it is not as visible in aluminum. Even CorTen must be properly surface prepped and painted, just as with other steels. It is not a shortcut and can be a tougher material to work. But it will result in a lighter hull, giving more usable weight carrying ability for other purposes than in the hull structure alone.

Also be aware that my SmallTug TM designs only allow for the use of steel or aluminum as construction materials if you use the Offsets Table, not if you scale from the paper drawing. The paper drawing dimensions are suitable for wood or fiberglass construction only. I highly recommend that you use the Offsets Table for all hulls, regardless of material, as this will give a drier vessel, with drier decks, and not incidentally, more interior headroom as well. This is much better especially for a tug-yacht style vessel.

Steel can be used as the construction material in a vessel as small as 21 ft. LOA with proper design. My SmallTug hulls are designed to this requirement as described in the Offsets Table.