With few exceptions (and those are unusual shapes), every vessel moving through the
water generates at least two sets of waves, divergent waves which move out from the bow
and transverse waves which move out from the stern. They are illustrated in the Figure
below. These patterns are easily observed when viewed from above in an airplane or
standing on a bridge.
A vessel generated wave that has traveled approximately one ship length from the point of generation will assume a form close enough to that of the sinusoidal wave, that we can use classic wave theory to quantify and characterize the wash generated by various hull forms and specific vessels.
This wave is defined by the following basic characteristics, illustrated in in the
following Figure:
The relative damage to the beach, to a passing vessel, or to a shore structure is a function of the amount of energy expended by the wave front upon impact. This function is not easily measured directly but we have the measurable quantities from which energy per linear meter of wave front can be calculated The energy is the sum of the potential and kinetic energy and is given by:
where g is acceleration due to gravity and g is
the density of water. Using the relationship for wavelength as a function of period for
open ocean waves of 
Then
with energy expressed in joules/meter of wave front when wave height is in meters and period is expressed in seconds.
Wave heights and periods of vessel wash are measured using a submerged instrument
package that measures pressure 4 times per second and records data to a HEX file in
CMOS memory in the package. The instrument package is anchored to the bottom, typically
in 200 feet of water, and suspended from a buoy that is held 6 to 10 feet below the
surface of the water by a taut line to the anchor. A marker buoy on the surface is used
for location and recovery. This test setup is depicted in the following illustration:
After data download, custom software converts the pressure readings to wave heights as
a function of time, enabling measurements and plots to be made of wave patterns passing
over the buoy.
To gather data, the test vessel passes the deployed instruments at a convenient distance at various specified speeds. The actual distance is measured using a laser rangefinder accurate to 1.0 meter. All data that is consistent and repeatable is analyzed. Each run's data is normalized to a distance off centerline of travel of 300 meters (~1000 feet) and, where possible, any multiple run data for each speed is averaged. Plots are then developed of wash height vs. speed, wash period vs. speed and wash energy per meter of wave front vs. speed.
Based on theoretical, model and empirical data collected in several past studies, predictions of the wake impact on a given waterway can be provided. Using the waterway profile (for example :
and vessel data, either in the form of a specific design, a proposed design, or a range of designs, the consultants will provide the impact on a candidate beach in terms of wave height, wave energy, and wave period. Working with coastal engineers, this information can be translated to specific predictions of environmental impact.
Connecting two or more specific ports by water transportation involves far more than “Distance ÷Time=Speed required”. Experience in route planning will include the following factors in establishing a route plan:
The product of the evaluation is a range of options for vessel type, size speed, and accommodations, as well as recommendations for terminals and support structure where required. It includes recommended optimums and identification of possible impediments to success.
In recent years wake wash, noise, and environmental pollution have become design criteria in the same way that speed, deadweight capacity (payload), passenger accommodations, and maneuverability have traditionally been design criteria. The balance of all these criteria is a daunting task for the naval architect, working with the owner. Using a considerable data base of environmental and design data, the consultants will assist in the process of showing the penalty paid in performance of the environmental factors by changes in other criteria.