With Parliament on 14 October 1645 approving the disposal of six elderly ships, instructions were issued for the speedy building of other vessels in their place.

[1] Orders placed later in December with Master Shipwright Peter Pett (Snr) to build two frigates (Assurance and Nonsuch) at Deptford Dockyard and with his son Peter Pett (Jnr) to build one frigate (the Adventure) at Woolwich Dockyard.

The two Master Shipwrights were individually responsible for the respective designs for the three vessels and for supervising their construction.

[2] All three frigates were built as single-decked warships, with their main battery on the sole gundeck, with eleven pairs of gunports carrying a mixture of culverins and demi-culverins.

The latter omission was soon corrected, with a forecastle (as an elevated structure over the forward part of the gundeck, but not carrying any guns) being built to "add very much to their strength".