Steel has remained my favorite material for boat and shipbuilding. It is readily available, of consistent quality, and it can be welded, forged and cast. The steel used in boat and shipbuilding is readily welded, and even though the several alloys used may differ in chemical composition are readily welded to each other without fear of one attacking the other. Having said this, there are preferred grades of steel (alloys) which give greater corrosion resistance and strength without an increase in weight. The higher the carbon content, the greater the corrosion rate. Thus one seeks low carbon steel A569 that has a maximum of .10; whereas, merchant quality A36 has a maximum of .25. With the former, there is a loss of yield strength, making it easier to form but must often be increased in thickness to obtain the required strength. A36 is, however, suitable for framing and heavier plates such as those used for stems and keels and also can be used for the shell plating. It would be safe to say that the majority of steel hulls are built from this merchant quality steel or, to use the generic term, mild steel. It is the least expensive, and a hull properly built, painted, and maintained will have a long service life. The alloy that I use and recommend for plating 1/4” and under is known as “Corten” which is the proprietary name used by US Steel Corp. for its High Strength Low Alloy (HSLA) steel. It is designated in the steel handbooks as A242. This steel has a maximum carbon content of .09 and, because of its alloying, has a yield strength of 55,000 psi compared to the A36 yield strength of 36,000 psi, and the price is about twice that of mild steel. This suggests that the plating thickness can be reduced to save weight and cost. Sometimes it can in the heavier structures, but there is a practical lower limit of plate thickness that is readily welded. This minimum is generally conceded to be 11 gauge sheet, which is .1196” thick and weighs 5# per square foot. Because of the ease of welding, most of us would rather weld 10 gauge sheet .1345” thick and weighing 5.63# per square foot, and this is the minimum with which some builders will work. I have built several hulls using 16 gauge .0598” thick weighing 2.5# per square foot. With stick electrodes they were very difficult to build and their life quite short as there was no margin for corrosion. This is another advantage of “Corten” since the resistance to atmospheric corrosion is 4 to 6 times that of carbon steel (mild/merchant). This has led some builders to not paint or sandblast the “Corten” which will then deteriorate just as quickly as unblasted and unpainted mild carbon steel. “Corten,” for use in salt water, must be prepared for painting and painted just as carefully as mild carbon steel. If the protective coating is then damaged, the rate of corrosion will be less than carbon steel. Some of the early carbon steel hulls that I built are approaching 50 years of age with no major repairs required. Others using “Corten” for plating are over 35 years old and are still active, and when they are sold it is not uncommon for them to bring two to three times their original building cost. Gauge plate pickled and oiled is also low carbon, and has the further advantage of no mill scale due to processing.
Preparing to construct in steel need not be a expensive undertaking as the equipment costs for the necessary tools are very modest, i.e., $500 to $800 in the year 2000. Required is a 225 amp AC/DC welding machine for stick electrodes, helmet, gloves, oxygen/acetylene cutting torch, heavy duty grinder, electric drill, 6 C clamps, come-along, wrecking bar, chipping hammer, cold chisel, punch, vise, assorted malls, sledge hammers 4# to 15#, and a 2” to 3”slab of steel 16” x 30” for use as an anvil. If only one hull is to be built, this is all most builders will need. The time needed for construction will be higher than that for a builder that makes a further expenditure, which will ease the physical labor as well lower the cost. The equipment I find beneficial is a plasma arc cutting torch, which is not only faster but causes the least distortion and, when correctly done, eliminates 95% of plate edge grinding. The saving here is in not working a backbreaking arm-tiring 15# grinder and the consumable grinding wheels. An advantage is the ability to cut openings, say, for portholes in the middle of the plate without distortion. The next is a MIG welding machine. This method uses a continuous solid or flux cored wire of small diameter (.035) versus 1/8” stick electrode. Like the stick electrode, the flux cored wire can be used in the open on a windy day and have consistently sound welds. The use of a solid wire requires a shielding gas, which leaves a clean weld that does not need to have slag removed. This is practical when construction is done under cover, since anything more than a light breeze will blow away most if not all of the shielding gas, thus prohibiting sound welds. In some instances, a portable screen can alleviate some of this problem. The lower heat input and speed of metal disposition makes it easier to control distortion, the gun is very small and lightweight, and also there is no stub loss. An auto-darkening shade glass for the welding helmet eliminates the need to lift the helmet to see where to start and where to move.
Of course other things that are useful are quick-acting bar clamps in lieu of C clamps. I have 40 and am only building for my own use. Also helpful are lightweight grinders (4”) in addition to the 15# grinder, one with a grinding wheel, one with a wire brush, and one with a sanding disc, and several 3/8” heavy duty drills with quick acting chucks. The above probably saves me 1/3 of the total hours to construct a hull than IF I had stayed with just the minimum equipment.
Flat, V, and multi chine hulls are suitable for owner-built construction.
37' steel cargo schooner, ANTELOPE, built with no engine installed.
For the most part, straight sections are used, and cutting the correct length and mitering the joints of the frame is all that is required to making the frame. Plating is usually straightforward with no compound curves except in the bottom forward portion of the bottom plates. Here the solution is to use narrow strips, usually about 12” wide and 8’ to 10’ in length, and trim them to fit each other. In other words, in metal construction, one need not use a conical section because of material limitations. The framing systems are basically all transverse frames closely spaced, transverse frames approximately double-spaced with longitudinal stringers, and widely spaced transverse webs with closely spaced longitudinal stringers. Transverse frames are either Flat Bars or angles. Angles are the strongest, lightest, and stiffest, and occupy less transverse space. The objection to them so often heard is that they are harder to paint. They can be, but how often in the life of a vessel will they have to be painted again? My personal preference is for only transverse framing on vessels plated with 7 gauge (7.5#) and over. Below this, the use of longitudinals often produces a fairer hull, especially those that are not meticulously lofted. Transverse Web frames are usually T sections and occupy more space than regular frames. It is well to remember when using longitudinals that they are one of the prime sources for the deterioration of the shell plating from the inside of the hull, so limbers must be carefully located to prevent any water buildup.
Round bottom construction in metal requires a knowledge of stretching metal, which is not difficult to learn. With learning comes experience and, at times, when one is just beginning round bottom construction, it can be frustrating. One edge of a frame can be hammered with closely spaced blows to form the required curve. This is hard work. Heat can also be used via a rosebud tip on the acetylene torch; however, a helper is almost a necessity since, before the torch can be laid aside, the metal has cooled enough to inhibit a true bend. A small forge with open ends allows one to remove the metal quickly and work alone without the need of a helper. Some of the larger boatyards have a furnace, and the whole frame comes out white hot and is quickly bent to a pattern. The small boatyards usually make a hydraulic frame-bending machine. These are of numerous individual designs based on the yard’s need and experience; and they are either horizontal or vertical from 5 to about 150 tons capacity and range from hand operated to machine driven. A 10-ton bender seems to be adequate for vessels up to about 30’; a 20-ton bender for vessels up to about 45'. The frames are usually bent in pairs so that one complete bending operation of a frame will result in a complete port and starboard section. The same holds true for deck beams by bending in pairs which saves time, even though this results in some scrap. A method of obtaining a bent frame that is often used by the inexperienced is to cut the frame out of plate and weld on the flange or just leave it as the equivalent of a flat bar. The edges must be ground smooth and the corners rounded off to hold paint. There is a lot of labor and scrap using this method.
Plating of a round bottom hull requires a plating model on which to lay out the plates so that the builder is assured that they will not only wrap around the hull, but will also fay against each frame. It is of course possible to do this on a trial and error basis directly on the hull, but it will be found that the errors, if significant, result in either a large scrap pile and/or a rough hull. The lesson learned is once is enough--make a model.
This picture shows paper patterns (scaled to plate size) pinned in place to establish the sight edges of the plate on the hull. A special drawing, known as the shell expansion drawing, locates the sight edges for the builder plus locating tank to shell interaction, through-hulls, hawse pipes, etc.
48' steel owner/built pinky, PAPILLON, which, after numerous Atlantic crossings and a voyage to Australia under sail only, completed her circumnavigation after installing an auxiliary engine.
42' steel Australian owner/built GAZELLE, rigged with Chinese lug sail.
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