Definition
A forward-facing tube senses dynamic pressure, which rises with the square of speed, and this is compared with static pressure to derive speed through the water. The square law makes the instrument insensitive and inaccurate at low speeds but responsive at high speeds, which is why it is more common on fast craft than cruising yachts. The small orifice is prone to blocking by silt or marine growth, giving a sudden under-reading.
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