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Measurements

This section explains the various measurements with position of the hull and the free surface of the water.

 

• Waterline: The line where the hull meets the surface of the water. It is also know as the International Load line or the Plimsoll line.

Draft: The draft of a ship or boat is the distance between the waterline and the lowest point of the vessel, the keel. The measurement is generally made as vertical as possible. It determines the minimum water depth to ensure safe passage of the vessel. The draft of a ship is generally shown using multiple lines on the hull of the ship. Sinkage : The amount or degree of sinking of the vessel. It is the change in draft a vessel obtains when moving through the water.

Figure 8.8 - Draft lines on a ship hull

• Trim: The ship trim is the difference between the forward draft and the aft draft. It is an offset or deviation from the longitudinal axis due to pitching of the ship. The trim is illustrated as shown below,

Figure 8.9 - Ship Trim2Source :http://slideplayer.com/slide/5699506/

 

• Drag Force: A ship traveling over a body of water leaves behind, a train of gravity waves. The waves are produced at the ship's expense. Therefore the ship consequently experiences a drag force () which is expressed in . Drag coefficient is the ratio of the drag force to the force produced by the dynamic pressure times the wetted area.

8.1

• Froude Number: The Froude number, , is a dimensionless value that describes different flow regimes of open channel flow. The Froude number is a ratio of inertial and gravitational forces. The Froude number is a measurement of bulk flow characteristics such as waves, sand bedforms, etc.,

8.2

where is the water velocity, is the hydraulic depth and is the acceleration due to gravity.

when:

	Critical flow
	Super-critical flow (fast rapid flow)
	Sub-critical flow (slow / tranquil flow)