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Brian, Yes, that's what it is. There are two tests you can perform for out of roundness. The first is to simply measure the diameter (ID or OD) of the cylinder at one (or both) of the ends, at various compass locations. So let's say you identify the top and bottom of they cylinder. You might take a diameter measurement spanning the top and bottom (0-180 degrees), then a diameter measurement left and right (90-270 degrees), and compare them. You might then take measurements spanning 45-225 degrees, 135-315 degrees, and then compare them all to get an average out of roundness. This test will give you a general idea for the out of roundness of the cylinder but will not tell you anything about the circumferential surface. The formula Hugh is working with calculates the chord length of a template. The template itself is just a segment of a circle with the exact design diameter of the hull. The template is then used to compare the surface of the hull against an exact circle segment which will show localized out of roundness, dents, etc. As an example, if you were trying to measure the flatness of an eight foot counter top your template might be the edge of a ruler. You would not necessarily require a yardstick to measure the flatness, and in fact a yardstick might be too long of a span to make your measurements of out-of-flatness. In this example, the formula would tell you whether you needed a ruler, yardstick, or some other "chord length". Imagine the testing difference this way. If you took a ball-peen hammer and intentionally dented the longitudinal center of your hull, the first test checking the diameter of the cylinder at the ends likely would not indicate any out-of-roundness since the dent is localized in the middle of the cylinder and doesn't impact the hull at the ends. However, the second test using the template would definitely show the dent (out-of-roundness) assuming you placed the template over the local area where the dent existed. We can (maybe should) assume that the rollers used to roll a hull may not be perfectly aligned and therefore not produce a hull that is uniformly rolled along its longitudinal axis even if the ends of they cylinder are within tolerance. The second test with the template will identify such out of roundness. I'm sure Hugh or Sean can correct me if any of this is wrong. Jon On 11/3/2010 11:41 AM, Brian Cox wrote:
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