The definitive boat for production appeared in 1929, six inches (15 cm) longer and with the same 8 ft 10 in (2.7 m) beam as the second and third 35ft types.
These gave many stations their first motor lifeboats, but experience proved that stability was lacking and from ON 763 beam was increased by 5 inches (13 cm).
In common with the similar but non self-righting Liverpool-class, post war production switched to twin-engined versions, but only five were built.
The twin-engined boats had even shorter lives, being replaced at three stations by Liverpools and at the other two by 37 ft (11.3 m) Oakleys after less than fifteen years service.
The challenge of producing a motor lifeboat light enough to be manhandled for carriage launching resulted in a boat weighing around 5+1⁄2 long tons (5.6 t).