Definition
Beam is the maximum width of a vessel's hull, measured at its widest point. On a tender or chase boat the figure governs interior volume, garage fit, stability at rest, and the platform area available for swim-step and boarding work.
Background and use
Beam is one of the three primary hull dimensions, alongside length overall (LOA) and draft. For superyacht tenders it is the dimension that most often dictates whether a boat can be carried at all. Tender garages on production yachts are built to a specific internal width plus a clearance allowance for chocks, fenders, and handling gear. A 7m limousine tender with a 2.6m beam fits where a 7m chase boat with a 2.9m beam will not.
Beam-to-length ratio shapes the boat's character. Narrow hulls (ratio above 4:1) cut the water cleanly, accelerate well, and feel sporty but roll harder at rest. Wider hulls (ratio nearer 3:1) are more stable, carry more interior volume, and lift onto plane sooner under load, but they pay for it in wetted surface area and top-end drag. Catamaran tenders break this rule; their twin hulls give a beam at the deck that exceeds anything a monohull could carry, and the stability comes from hull separation rather than hull width.
Builders quote beam in two figures: maximum beam (BMAX) including any rubrails or sponsons, and waterline beam (BWL). Both matter. BMAX governs garage fit; BWL governs hydrodynamic behaviour.
Related considerations
- Confirm both BMAX and BWL when comparing tenders for the same garage; rubrail thickness alone can swing the figure 100mm.
- Beam at the transom, not amidships, often determines whether twin outboards or a wide jet drive will fit.
- Heeled beam matters for boarding; a narrow hull leans further when guests step on, which can make swim platform use awkward.
- Trailerable beam limits vary by jurisdiction; in the EU it is generally 2.55m without a special permit.
- Wider beam usually adds wetted surface and reduces top speed at the same horsepower.