The Siku model is advertised in a 1/55 scale, which is the scale of the towing car. Boat and trailer information listed at the back of the box seems to advocate a 1/52 scale (except for the boat width 1/55 and hectic height scales of respectively 1/75 and 1/96).
The Audi car was replaced by a 1/50 van with real dimensions of both boat and trailer searched and found, to simply add a new 1/50 scale combination:







Renault Trafic 4x2/2 van (modified(*) Solido model) and Harbeck Fahrzeugbau BT 3500 MALM6-Confort 2-axle boat trailer (modified(*) Siku model), loaded with a Performance Marine 807 in-board 2-engine racing boat (Siku model),





Notes (*): the towing eye on the Siku trailer was replaced by a camper-type towing device with a corresponding hook added at rear of the Solido van.

Regards.
Guy