Environmental factors influencing the speed log measurements, Litton (1998):

Water clarity

Measurement of the speed through water depends on acoustic reflection from solid particles in the water such as microorganisms or suspended dirt. In extremely clear water the quantity of scatters may be insufficient for adequate signal return.

Aeration

Aerated water under the transducer may reflect sound energy which could erroneously be interpreted as sea bottom returns. Sailing in heavy weather may be the source of this effect and so could non-laminar flow around the transducer. By placing the transducer near the bow the effect of non-laminar flow is reduced considerably.

Ship’s trim and list

Changes in the trim (affects fore/aft speed) and list (affects transverse speed) of the ship will affect the measured speed. (Example: 5° trim change gives 0.4% speed change, (Litton (1998)).

Current profile

Speed through water is measured relative to a water layer beneath the ship (> 3m). Sailing in strong tides and current, the direction and magnitude of the surface current can be different from the measured layer, which may lead to errors in the measured speed.

Eddy currents

Sailing in eddies in boundaries of ocean currents where the flow can be opposite or normal to the direction of the primary current and will affect the speed measurement.

Sea state

Following seas result in a variable change in the vessel’s speed. This produces a fluctuation in the measured speed.

Fouling of sensor

No effect of fouling of the sensor has affected the speed measurements

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