A jack-up, 180 feet in length, has the center of flotation at 110 feet aft of frame zero. The draft at the bow is 11.0 feet and the draft at the stern is 13.0 feet. What is the true mean draft?
• Apparent mean draft vs. true mean draft when a vessel is trimmed by the stern • How the center of flotation (LCF) location affects the correction from apparent to true mean draft • Using trapezoidal/linear draft distribution between bow and stern for small trim angles
• First, compute the simple arithmetic mean of the bow and stern drafts. How does this "apparent" mean compare to the true mean when the LCF is not at midships? • Ask yourself: Is the center of flotation closer to the bow or the stern, and how does that shift the area of immersed section and therefore the mean draft? • Consider whether the true mean draft should be slightly closer to the deeper end (stern) or the shallower end (bow), and by about how much given the distances involved.
• Calculate the simple mean draft: (draft forward + draft aft) / 2, and compare it with the options • Determine the midship location (half of 180 ft) and compare it to the LCF at 110 ft aft of frame zero to see which side of midships the LCF lies • Check whether the correction from apparent to true mean draft would be positive or negative based on whether the center of flotation is aft or forward of midships
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