The development of the Mark I during Spring 1942 led to preliminary tests in August. The following is a summary of key changes from the 20′ prototype tested earlier in the year :
- Adoption of buoyant ‘counter’ stern as used in Uffa’s canoes.
- lengthened to 23’2″.
- waterline beam and freeboard increased through use of tumblehome to raise sheer and bring gunwhales back in contact with aircraft fuselage higher up. This also had the effect of giving the boat a strange sheerline higher amidships than fore and aft – less noticeable once the turtleback buoyancy fore and aft inflated.
- two engines, off centre, which increased prop-ground clearance. This decision may have been partly influenced by what engines were available.
These tests were successful and the basic form of the Airborne Lifeboat was agreed. Work then continued to finalise design and supply of the equipment, fittings and gear required to make launch and use a relibale, repeatable exercise.