There are various methods, I use:
1. if the contact was reported by radio, the report includes the speed or speed range;
2. if you have enough time use the 3 minute rule to get an estimated speed;
3. if you are short on time, guestimate a speed, I sually start with 8 kts for merchant/convoys; 15 kts for warships;
4. plug in estimated speed in Position Keeper and refine to final speed;
5. if all else fails, setup for a snapshot as the target crosses you bow.
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