11 · Design journal
Why 4.2 meters
The length was not chosen. It was the only number left after two constraints.
2026-09-02 · By Hugo Ramírez
Short answer. 4.2 meters was not the starting point of the design, it was what was left once two other decisions were made first: what tows behind an ordinary truck without turning towing into a project, and what still reads as one deliberate hull instead of a compromise between a dinghy and a full day boat.
01 · The towing constraint
A boat that needs a dedicated tow vehicle stops being an impulse decision on a Saturday morning and starts being a logistics project. The brief for the 420 assumed an ordinary truck already sitting in the driveway, not a truck bought specifically to make the boat possible. That put a hard ceiling on combined trailer and boat weight before a single line of the hull was drawn.
02 · The proportion constraint
Below a certain length, a RIB stops looking like a boat and starts looking like an accessory, a tender-shaped object with no presence of its own. Above a certain length on this deck plan, the same hull starts reading as oversized for what it actually does, a small boat pretending to be a bigger one. 4.2 meters is where the deck plan, the tube diameter and the console still read as one proportioned object rather than a scaled-up dinghy or a scaled-down day boat.
03 · What was left
Once towing weight and proportion were both fixed, the range of workable lengths was narrow, and 4.2 meters sat inside it with room for the deck width, tube diameter and console that make the layout work at four people. It is a length that was arrived at, not chosen for its own sake, and that is the honest version of the story.
Related reading
Built one at a time.
The ADB 420 is welded aluminum, numbered, and designed by the person who answers the phone.
See the 420
