25-Nav-B3 Finite Element Analysis for Ship Structures · May 2016

Question 6 of 7: Ship Types and Regulations

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Question 6: Ship Types and Regulations (20 marks: a–6, b–7, c–7)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Unique operating features. Small fishing boats routinely operate with a large, variable deck load (the catch) that can arrive suddenly and asymmetrically; they carry gear (nets, trawl doors, pots, cranes/davits) mounted high and often outboard, actively working against the vessel's stability while hauling; fuel and catch-hold consumables are consumed/loaded through the trip, giving a continuously varying loading condition (unlike a cargo ship's discrete loaded/ballast conditions); ice accretion in cold-water fisheries adds significant, unpredictable topside weight; crews are typically small and often owner-operator, so there is little redundancy in seamanship judgment about loading limits; and the vessels are frequently older, modified informally over their service life (added gear, extended wheelhouses) without a corresponding re-assessment of stability — a pattern regulators specifically worry about.

(b) Why stability is challenging. Fishing vessels combine several stability-eroding features simultaneously: a relatively small hull (limited righting-arm reserve to begin with), high gear/gantry weights that raise $KG$, free-surface effects from partially-filled fish holds and fuel/water tanks that reduce effective $GM$, and dynamic heeling moments from hauling gear over the side or from a sudden catch surge on deck — all superimposed on a vessel design that is often modified informally (per part a) without formal restability. The combination means the margin between normal operating condition and a capsize condition can be thin and, critically, is not constant — it changes trip to trip and even hour to hour as gear is worked and the catch/hold loading changes.

(c) Does the proposed regulation have merit? The proposed rule — a maximum 10° heel under a constant 5 tonne·m heeling moment, combined with a minimum 1.5 m freeboard — has real merit as a simple, field-checkable criterion: $\theta_{heel}\le10^\circ$ under moment $M_h$ implies (for small angles) $GM\gtrsim\dfrac{M_h}{\Delta\tan10^\circ}$, so the rule indirectly enforces a minimum $GM$ appropriate to the vessel's actual displacement without requiring a full GZ-curve computation that a small-boat owner/operator could not realistically perform dockside. Pairing it with a minimum freeboard is sound because freeboard governs the angle of deck-edge immersion and hence the point where the righting arm curve starts to fall away — a vessel could satisfy a small-angle $GM$ criterion alone and still have inadequate range of stability if freeboard is too low. However, the approach has real limits: it is a small-angle, static criterion and says nothing about dynamic stability (the area under the GZ curve, which governs energy absorption in a beam sea or during a sudden gear-haul moment), it does not address free-surface effects in partially-full holds/tanks that can silently erode the margin the rule assumes, and a single fixed heeling moment (5 tonne·m) does not scale with vessel size — the same absolute moment is a much more severe test for a small skiff than for a larger stern trawler, so any adopted regulation should scale the reference heeling moment (and possibly the freeboard) to vessel displacement or length rather than using one fixed pair of numbers fleet-wide. On balance: a useful, practical minimum-standard check for a class of vessels that is otherwise hard to regulate, but it should supplement (not replace) a proper GZ-curve-based stability assessment for larger or higher-risk vessels in the fleet.