'Onboard' was a British naval sweep by destroyers of Vice Admiral Sir Arthur Power’s East Indies Fleet in the Andaman Sea (25/29 March 1945).

Undertaken by Force 70, and thus comprising Saumarez, Vigilant, Virago replacing Rapid which had been damaged in 'Transport', and Volage, the operation’s objective was to find and destroy any Japanese shipping that might be used to reinforce, resupply or evacuate the garrison of the Japanese-occupied Andaman islands group.

The ships of Force 70 departed Akyab in the Arakan western coastal region of Burma during 25 March and entered the Andaman Sea by the Preparis North Channel. At 10.47 on the following day, a Japanese convoy of two merchant vessels, escorted by two submarine chasers, was sighted, attacked and destroyed.

The 1,500-ton Risui Maru was stopped by gunfire from the destroyers and then sunk by a Consolidated Liberator bomber of the RAF, which had been homed onto the target by the force and joined in the attack. In making this attack, however, the aeroplane struck the merchant vessel’s mast and crashed, only two of the crew surviving and being rescued.

The second merchant vessel, the 400-ton Teshio Maru, was sunk by gunfire. Of the escorting submarine chasers, Ch-34 and Ch-63, one was sunk by gunfire and the other by a torpedo. Five Japanese officers, 45 men and seven women were rescued and taken prisoner. Only superficial damage was suffered by the British ships, which arrived at Trincomalee in Ceylon on 29 March.