The Mark 3 had an additional midsection that gave it a length of and a displacement of 640 tons. Even with this extra weight, the vessel was slightly faster than the Mark 1. The Mk.3 was accepted on 8 April 1941, and was prefabricated in five sections. The Mark 4 was slightly shorter and lighter than the Mk.3, but had a much wider beam () and was intended for cross channel operations as opposed to seagoing use. When tested in early assault operations, like the ill-fated Canadian commando raid on Dieppe in 1942, the lack of manoeuvring ability led to the preference for a shorter overall length in future variants, most of which were built in the United States.