Page 28: of Maritime Reporter Magazine (October 2001)

most probable height mean height highest third significant height

Fig. 3 Wave Height torically observed qualita- tively. The modal period is determined by finding the average value of the wave periods.

Other factors that affect waves are the fetch and the water depth. Fetch is the distance of open water available for a wave system to develop. A protected bay has little fetch and waves cannot fully develop, regardless of the wind . strength. Water depth can produce larg- er waves, especially when the depth of water is less than half the wavelength. A good example of this occurs at harbor

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Circle 323 on Reader Service Card or visit www.maritimereporterinfo.com entrances, where a bar or local shallow- ing can develop. Such a bar can produce larger than ordinary waves as the wave energy is compressed by the rising ocean bottom.

In extreme seas two other characteris- tics come into play. These are deck wet- ness and slamming. Deck wetness refers to the presence of green water on deck, not just spray. Having waves board the vessel clearly limits the crew's ability to handle equipment or to safely move around. Deck wetness is a criteria that can be used for limiting operation.

Slamming is more serious. A vessel slams when the bow area is struck by or comes down on a wave. Slamming is characterized by zones of high pressure on the hull and associated shaking of the vessel. When slamming occurs, the operator must reduce speed in order to prevent structural damage to the hull.

Mariners have long known that if the wind begins to blow on an open, calm sea, after a while waves will develop and build until equilibrium condition is reached. This phenomena was catego- rized by the 19th century English sailor,

Admiral, Sir Francis Beaufort, KCB.

The Beaufort scale matches wind veloc- ities to wave conditions or Sea States.

This terminology has been adopted to define design conditions for vessels.

For the vessel designer, sea kindliness or ride quality must be expressed as a set of standards. Because of the varying nature of winds, operating areas, sea- sons of the year, resistance to motion sickness, etc., this usually means that statistics must be used. To say that a vessel has to work through Sea State 3 and survive a Sea State 5 is not precise.

Before defining the governing sea state one must consider where the vessel is to operate, what kinds of seas are prevalent at what times of the year, and what type of work will be done with the vessel.

For example, a research vessel handling plankton nets over the side can operate in higher sea states than one that will be handling an ROV. Therefore, it is important that the designer understand the vessel's mission and its limitations.

Having looked at the environment that provides the energy input to cause ves- sel motion, we next look at the vessel's response. The vessel system can be modeled as a linear mass spring system with a dampener. (See Fig. 4.) The ves- sel is the mass, the spring is buoyancy to restore the vessel to its equilibrium posi- tion as the waves pass under it, and the dampener is the sum of friction, turbu- lence, and drag. The equation of a lin- ear system takes the form of F(t) = mg + mA + cV + kD where:

F(t) = force varying over time m = mass g = acceleration due to gravity

A = acceleration of the vessel c = damping coefficient

V = velocity of the vessel ifflMK mMWWMMM^

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