What are Bottom Speed Logs?

Bottom Logs are types of Speed Logs fixed to the bottom of the ship which are used to measure the speed of a ship. It can be retracted from the bridge itself when entering shallow waters. This is combined diagram showing all three types of bottom logs – Chernikeff   (Impeller), the EM and the Pitot (Pressure) logs

Normally one of the three is fitted on a ship in a tubular structure which is lowered and raised through the keel.The log is kept housed (raised) when alongside or in shallow waters and deliberately lowered when ship comes out in deeper waters. On being lowered, the log will sense the flow of water across it, measure it and provide display of ships speed through water.

The speed data is also fed into other equipment like the Radar, GMDSS and AIS.