Abstract: A collapsible trailer boat comprising a fore part and an after part jointed together in such a manner that they can be folded to form a trailer by pivotally moving the fore part in the longitudinal centerplane until it rests, with its bottom up, on the after part. The hull of the after part is shaped with a flat deadrise and has provision for mounting an outboard motor, a retractable undercarriage and a removable towbar. The fore part is detachable and adapted for use as a separate craft, the hull thereof being constructed in the form of a planing catamaran. To use the boat, the portion of the catamaran tunnel adjacent to the after part is closed with a removable hydroplate adapted to match the shape of the catameran bottom to the shape of the bottom of the after part and to form a transverse step at the junction of the fore and after parts, a gap being provided at the step for water to be drawn from the catamaran tunnel when the craft is planing.