The chief strength of submarines in attack was their invisibility to merchant ships when partially or wholly submerged. Periscopes were especially hard to spot in rough seas and with the sun behind them, whilst the target merchant ship would be seen in sharp outline against the horizon.
The Admiralty recommended several measures to merchant ship captains to help protect their vessels. They could steer a zig-zag course to make themselves a more difficult target. They could paint their ships with dazzle camouflage to obscure their size and shape and make it more difficult for the enemy to estimate their speed and direction of travel.
Many of the merchant vessels were also fitted with deck guns and provided with naval gunners. Merchant ship captains were also encouraged to take more drastic offensive action by ramming submarines.