And if you know the length of the target you can time how long it takes to pass through a fixed periscope line. Fixed meaning it doesn't turn, and neither should your boat. If you point it at a bearing anything other than 0 or 180 degrees then your own speed will affect the result. So to avoid that you have to turn your bow or stern to just in front of the target and time the start and finnish of passing the line. Then divide length in yards by time in seconds. To convert that to knots double the result and add one tenth of it.
If you know length in meters then just double the value (not adding one tenth) to get knots.
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