The beam is the maximum width of a boat or ship, measured from the port side to the starboard side, typically at its widest point. It is a critical factor for stability, interior space, and handling, with wider beams generally providing more stability and interior volume, while narrower beams offer higher speeds.

Key Aspects of the Beam:
Stability: A wider beam reduces the tendency to capsize or rock, offering higher stability, especially in, for example, catamarans or wide-hulled vessels.

Space: A larger beam allows for a wider deck, larger cabin space, and more comfortable, functional layouts.

Performance: While stable, a wider beam can increase drag, potentially reducing top speed compared to a narrower, more hydrodynamic hull.

Measurement: Generally measured at the widest point of the hull, often described as the "waterline beam" when referring specifically to the width where the hull meets the water.