From 1958-1970, the transom of all Boston Whaler's 13' hull were designed to fit a 15" short shaft outboard motor. However in 1971, the transom design called for a sink-like motor well (second photo). Our Transdaptor 13, designed by Erik Kent, allows for your 13' Whaler to carry up to a 20" shaft motor without permanently jeopardizing the integrity of your boat.