Cezbor and Kielhauler are right. And yes, you can sometimes find fast travelling lone merchants in the early war years. I found a couple of them near Britain in '40 and '41 - a nippy wacky pakky type tanker (as I call them) and a whale factory ship, IIRC. My sonarman reported them as "fast".
Note that in rough seas ships will decelerate as they climb a wave and accelerate on their way down the other side of it. If your sonarman catches it during the deceleration then a ship doing 8kts can be reported as "slow" and a ship doing 13kts as "medium". But I don't know how you got 15kts reported as "medium"!
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