When ordering the plate, make sure you specify plate-mill and not strip-mill plate. Plate-mill stock is plate that has never been coiled. Strip-mill is plate that has been rolled into large coils after manufacture; later, the steel is unrolled and sold as flat plate. But it has a “memory,” so it won’t be absolutely flat and unstressed before you start to bend it. If you’re forced to use strip-mill material, try to ascertain the natural curve of the plate, and use it to your advantage.
Our choice for steel boat building is plate with a low to medium carbon content. You’ll find there are many different grades of steel, but we recommend low-carbon steel with a carbon content of between 0.15 and 0.28 percent. The highest carbon content acceptable to most classification authorities is 0.28 percent, so we recommend you stay within the range quoted above. Low-carbon steel is available in various shapes, strips, and plate, and has good welding characteristics. As code numbers vary from country to country, you should seek advice from your steel supplier to ensure that you receive the correct materials as suggested above. Lloyd’s A-grade shipbuilding steel will be one of your preferred choices if you live in Europe, or build from a pre-cut kit that is cut from Lloyd's-approved steel.
The plate thickness will be specified in your plans. Remember that it’s harder to avoid distortion when welding materials that are thinner than 1/8 inch (3 mm). Even this thickness should be restricted to decks and cabins, as well as to hulls on boats under 35 feet (10.66 m) in length. Your designer will specify the plate thickness recommended for your boat. When you’re building small steel boats, it’s better to reduce the amount of framing than to reduce the plate thickness. Some builders increase the plate thickness without consulting the designer, which, in a steel boat, can have disastrous results. If you are unable to obtain plating as specified in your plans, always contact the designer for advice. Changing the plating thickness may require rescheduling the spacing and sizing of the framing.
As you may be responsible for the quality of the steel being used in your boat, you should be aware of the common defects. Check for “wavy” areas in the sheet. This defect can appear as small, uneven areas with a wavy appearance. Another defect is rolled-in mill scale, which is caused when impurities on the surface of the plate are rolled into the surface. Buckles or kinks in the plate can be caused by improper handling after manufacture. You may also find thin areas in the center and ends of pipe.
