On 10 March, a convoy of small ships 'in ballast' was sighted off Tripoli. Conscious of the shortage of torpedoes he held fire. While on his fourth patrol, on 3 April 1941, he sighted a convoy but the ships were at extreme range. On 10 April, a second attack went wrong when his torpedoes' tracks were seen and the convoy took evasive action. "Upholder" was subjected to a sustained counter-attack and was then forced to crash-dive deep when the hydrophones detected a torpedo closing on the vessel. Rather than give up, Wanklyn decided to reconnoitre convoys. On the 12 April 1941 he sighted five large ships with three destroyers. He surfaced to broadcast their position to Malta headquarters, but an enemy aircraft spotted him and "Upholder" was forced to dive. Later, Wanklyn repeated the procedure which was acknowledged. He fired a star shell over the convoy to mark its position and the enemy fleet quickly reversed direction to port. It attracted the 14th Destroyer Flotilla, comprising , , and . The force did not make contact that night. Days later, during the Battle of the Tarigo Convoy, the same tactics allowed other submarines to lead the force to devastate an Italian convoy for the loss of one destroyer.