Equipment Guide · Echo Sounder & Doppler Log
Seawater is opaque to radio and light but carries sound superbly — so the ship answers her two oldest questions, "how much water under me?" and "how fast am I really moving?", by shouting into the sea and listening. The echo sounder times the echo; the Doppler log reads how the echo's pitch has changed. Tap a step or press "Play" to follow the ping down to the seabed and back, see why the displayed depth is not the under-keel clearance, and why the log carries two different speeds — then take the simulator alongside and feel what a hidden two-knot tide does to a berthing approach.
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Picture the approach: engines stopped, a 2-knot fair tide under the hull, and the log on water track. Ship, water and tide are all moving together, so the repeater honestly reads 0.0 kn — through the water she really is stopped. Over the ground she is still closing the berth at 2 knots, and at 100,000 tonnes that is ½ × M × v² ≈ 53 MJ of kinetic energy — an order of magnitude beyond what any fender line is designed to absorb. Nothing has failed and nobody has lied: the instrument answered a different question than the one being asked. The defence costs one glance — read the track flag next to the number, and for anything near a berth accept only BT (or another ground reference).
Press Start to begin the approach — then try stopping the engine in a fair tide, or take the depth past 200 m and watch the log fall back to water track on its own.
About the simulator
You are 500 m off the berth. The telegraph orders speed through the water; the tide adds itself on top, and only a ground speed shows the sum. The repeater displays whichever track you select — and if the seabed drops beyond ~200 m the bottom echo is lost and it falls back to water track on its own. Take the way off early and land at 0.35 kn or less: fender energy grows with the square of the contact speed.
Contact bands at the fender — safe ≤ 0.35 · firm 0.35–0.7 · damage > 0.7 kn. Energy = ½Mv² — doubling the speed quadruples the blow.
The ship answers the telegraph slowly (she is 100,000 t); the tide never answers it at all. Watch the gap between the repeater and SOG grow the moment the log is on water track — then try stopping the engine in a fair tide and see what still happens at the wall.
Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.