The Loaded-Deck Bound: How Catch Stowed on Deck Consumes the Stability of Small Fishing Boats, and a Loading Rule Crews Can Apply Without Instruments

A companion study in this series computed capsize budgets for a representative small fishing boat and showed that shipped water and catch stowed high together nearly erase its stability. This paper isolates one mechanism -- catch carried on deck rather than low in the hold -- and converts it into a bound and a field rule. A parametric hydrostatic model of a representative 7.0 m fishing boat (unloaded displacement 1,100 kg with four crew and no catch) is used to compute the full righting-arm curve as catch is added at deck height and at hold height. At 400 kg of catch, deck stowage leaves GM of 0.038 m against 0.304 m for hold stowage, and a usable capsize energy of 19.6 percent of the unloaded value against 88.9 percent. Against three pre-stated criteria (GM at least 0.15 m; vanishing angle at least 40 degrees; usable capsize energy at least 50 percent of unloaded), the critical deck-catch mass is 244 kg, while hold catch does not breach the criteria until 1,399 kg. Across boats of 6 to 9 m the critical deck fraction runs from 12.9 to 35.5 percent of unloaded displacement, giving a single conservative rule (SIMULATED, NOT MEASURED): catch on deck must not exceed 10 percent of unloaded displacement. Free surface of melt-water and slurry in boxed catch lowers the critical mass by 5.5 percent; loose catch in open deck bins removes a further 0.100 m of GM at 400 kg. The rule covers still-water loading only, not waves, wind or icing.