How do I calculate draft?

Written by melissa wilson
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How do I calculate draft?
Loaded barge with tug boat (Photos.com/Photos.com/Getty Images)

If you need to know how far a boat or barge will sink when a weight is added to the deck, then you'll need to be able to calculate the draft of the vessel. This is one of the simplest and most widely used calculations on water. For example, if you add 100,000 pounds (45,450 Kilos) of weight to a vessel, it will in turn displace 100,000 pounds (45,450 Kilos) of water. This calculation is widely used for rectangular barges.

Skill level:
Easy

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Things you need

  • Tape measure
  • Calculator
  • The weight of the vessel
  • Pencil
  • Paper

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Instructions

  1. 1

    Measure the length of the deck. Fix one end of the tape measure to the deck of the barge, and measure the length from end to end.

  2. 2

    Measure the width of the deck. Fix one end of the tape measure to the deck of the barge, and measure the width from side to side.

  3. 3

    Write both of these numbers on your paper.

  4. 4

    Calculate the area of the barge deck. Multiply the length and the width measurements together and write down the product.

  5. 5

    Divide the weight of the object you want to add to the vessel by 64 (pounds per cubic feet, the unit weight of salt water). This will be the total volume of water replaced by the vessel as it sinks from the weight of the object in cubic feet.

  6. 6

    Divide the total volume of water replaced by the surface area of the barge calculated previously. This will be the distance in feet that the vessel will sink due to the added weight.

Tips and warnings

  • Sixty-four pounds per cubic foot is the unit weight of salt water and is a safe number to use even if in fresh water. The actual unit weight of fresh water is 62.4 pounds per cubic foot.

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