Although an X-craft is a complete submarine in miniature, it does not fire torpedoes but carries two large mines which are dropped under the enemy ship at anchor in its harbour and time-fused to explode when the X-craft has retired to a safe distance. Each mine—or side-charge, as they are called—carries a heavy weight of explosive, but is fitted with internal buoyancy tanks so that its weight on the boat in water is negligible. Each side-charge is faired nicely to the hull to avoid catching nets or wires and well streamlined to keep its effect on the boat's performance to a minimum. In this article I will illustrate some of the navigational problems which arise in operating X-craft by giving an account of the attack we made on the Tirpitz in 1943