Beam

What it means

Beam is the width of the boat at its widest point. On a catamaran it is measured across both hulls, which is why a catamaran's beam is so much greater than a monohull's of the same length.

What beam buys and costs

A wider hull is a more stable platform and a roomier one: cockpit seating, a proper galley, a walk-around deck. It also costs more to berth, since many marinas price by area or refuse anything over a set width, and it is the dimension that decides whether a boat can be trailered on public roads without a special permit. For an electric boat there is a further cost: beam is wetted surface, and wetted surface is drag.

Why it matters

Beam is where the trade between comfort and efficiency is most visible. A narrow hull slips through the water on less power and gives a longer range from the same battery; a wide one gives you somewhere to spend the afternoon. Neither is the right answer on its own.