Wearability Budgets for Working Lifejackets: Thermal, Mass and Mobility Costs as Design Constraints on Devices People Will Actually Wear

A lifejacket protects only a person who is wearing it, and working fishers commonly give heat, bulk and interference with work among their reasons for not wearing one. This paper treats wearability as an engineering budget and originates a quantitative design envelope for working lifejackets. Using a parallel-path heat-balance model (covered and uncovered body areas in parallel) for an 80 kg reference worker (body surface area 1.956 m²), four representative device types — an offshore foam vest, a thinner open-sided foam work vest, an uninflated inflatable yoke, and an inflatable belt pack — are costed in five climate classes (15–35 °C) at three work rates (metabolic rates of 200, 300 and 420 W). The foam vest adds only 0.296 clo of effective whole-body insulation, but raises whole-body evaporative resistance by 27.3%; in Warm conditions (28 °C, 65% relative humidity) at heavy work it raises required skin wettedness from 1.10 to 1.46, raises heat storage from 29 W to 102 W, and reduces the modelled time for core temperature to rise by 1.5 K from 241 minutes to 68 minutes. In Hot conditions (32 °C, 70%) at moderate work it reduces that time from 187 minutes to 80 minutes; the inflatable yoke reduces it to 132 minutes and the belt pack to 159 minutes. Carried mass is, by contrast, a minor thermal cost: the heaviest device (0.95 kg) adds 3.2 W of metabolic heat at moderate work, while its bulk cost is large — a mean torso thickness of 28 mm corresponds to a chest-circumference increase of about 176 mm and 32 mm of added front depth. Inverting the model at a design threshold of required wettedness no greater than 0.85, with 30% body coverage, gives maximum local added insulation values of 0.91 clo (Temperate, heavy work), 0.65 clo (Warm, moderate work) and 0.54 clo (Hot, light work) for impermeable foam-like construction, roughly doubling for vapour-permeable construction, and a zero budget — the unaided worker already exceeds the threshold — for heavy work in Warm conditions and for all work rates in Tropical conditions (35 °C, 80%). For foam-vest construction the coverage budget at the same threshold falls to 19% of body surface in Warm conditions at moderate work, below the 32% coverage of the foam vest modelled. The resulting design rule is that, in warm climates, thermal wearability is governed by evaporative blockage and coverage, not by mass, and that no insulation budget exists in the hottest class: there, work-rate and exposure management, not device choice, is the binding constraint. The model's limits — fixed skin temperature, no solar load, no acclimatisation or drinking, and a screening-level storage calculation in place of a full predicted-heat-strain simulation — are stated in full.