A Sponson Retrofit for Small Open Fishing Boats: Righting-Moment and Capsize Budgets from a Parametric Stability Model

Small open fishing boats capsize in service worldwide, often with loss of the whole crew. This paper designs a bolt-on sponson (side-flotation) retrofit for a representative 7.0 m open fishing boat and evaluates it with a parametric hydrostatic model that integrates the heeled, waterline-clipped midship section of the hull and of the sponsons. In nominal loading, a baseline sponson of 0.30 m width × 0.40 m height × 3.20 m length per side raises the maximum righting arm from 0.441 m to 0.590 m, raises the energy required to reach the downflooding angle from 3.39 kJ to 5.48 kJ, and delays downflooding from 43.5° to 48.5° of heel. Sensitivity calculations show that shipped water with a free surface is the single most damaging load case examined, and that in a combined worst case (catch carried on deck, crew crowded to one side, and shipped water aboard) the unmodified boat is effectively without stability reserve: it settles at a 13.5° loll with a maximum righting arm of 0.005 m and a vanishing-stability angle of 21.6°. The baseline sponson restores a positive range to 43.7° in that case but does not meet the acceptance criterion adopted here (vanishing-stability angle at least 50° and maximum righting arm at least 0.25 m in the worst case). A size and position sweep shows that the smallest configuration meeting the criterion is 0.50 m wide × 0.40 m high × 3.20 m long per side (0.640 m³ per side), centred 0.52 m above the keel; it gives a worst-case maximum righting arm of 0.311 m, a vanishing-stability angle of 51.0°, and a nominal maximum righting arm of 0.734 m. Because the sponsons sit 0.107 m clear of the water at rest in nominal trim, the calm-water speed penalty is approximately zero; an upper-bound estimate with both sponsons fully immersed gives about 5.6% speed loss at constant power. Downflooding, not hydrostatic capsize, is the limiting event in every case examined, and the model does not cover swamping in steep seas, wave dynamics, or attachment strength.