25-Nav-B3 Finite Element Analysis for Ship Structures · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, May 2016 — 98-Nav-B3, 3 hours, closed book, non-communicating calculator permitted (any five of the seven questions constitute a complete paper, all equal value; all seven answered below for full study coverage).
Reference texts: Tupper, Introduction to Naval Architecture, 5th ed.; Lewis (ed.), Principles of Naval Architecture (PNA), 3 vols.; International Code on Intact Stability (IMO IS Code), 2008; Canada Shipping Act / Transport Canada Marine Safety.
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) Environmental loads. A small commercial vessel sees: still-water and wave-induced hull-girder bending (hogging/sagging as the vessel rides successive wave crests/troughs); slamming loads from bow re-entry in head seas, and bottom/flare slamming on planing or semi-planing hulls; green-water/deck-wetness impact loads; local sea-pressure loads on the outer hull plating that vary with draft and panel location; racking/torsional loads in quartering seas; wind loads on superstructure and rigging; thermal loads from solar/ambient temperature cycling; corrosion and fatigue accumulation from cyclic wave loading over the vessel's service life; and, for a fishing vessel specifically, point loads from gear handling (winches, davits, trawl doors) and deck cargo/catch loading.
(b) Construction responses. Hull-girder bending is resisted by longitudinal strength members (deck and bottom plating, longitudinal stiffeners, and where fitted, a keel/girder system) sized to the section modulus required for the design wave bending moment. Slamming is addressed by increased local plating thickness and closer frame spacing in the bow-bottom "slamming zone," per class-society scantling rules. Local sea pressure is resisted by transverse framing (frames/floors) sized to span between longitudinals, with plating thickness set by local head-of-water plus a dynamic factor. Racking is resisted by transverse bulkheads and web frames that maintain the hull's cross-sectional shape. Corrosion/fatigue allowance is built in through a corrosion margin on plate thickness (or non-corroding material choice) and conservative stress concentration detailing (rounded hatch corners, soft-toe brackets) at high-cycle locations.
(c) Material advantages and disadvantages.
| Material | Advantages | Disadvantages |
|---|---|---|
| Steel | High strength & stiffness, low material cost per unit strength, well-understood welding/repair, high impact/abrasion resistance (fishing gear handling) | Heaviest of the three (reduces payload/speed), susceptible to corrosion (needs coatings/cathodic protection and a corrosion margin), harder to form complex curved hull shapes |
| Aluminum | About one-third the density of steel for a useful strength — higher speed/payload for the same power, naturally corrosion-resistant in most services, easy to extrude into stiffened panel shapes | Lower fatigue strength / no clear fatigue limit, more expensive per tonne than steel, loses strength rapidly in fire, galvanic-corrosion risk where it contacts steel fittings, requires more skilled (and controlled-atmosphere) welding |
| Composites (GRP/FRP) | Lightest option, no corrosion, excellent fatigue life, complex hull shapes moulded easily, good impact/slamming energy absorption, low maintenance | Higher material and tooling cost (especially for one-off/low-volume small-craft builds), harder to repair in the field, more difficult to non-destructively inspect for hidden delamination, fire performance requires added resin/coating systems, end-of-life recycling is difficult |
(d) Most ‘production friendly’. For small commercial ship series-production, steel is generally the most production-friendly of the three: hull panels can be flat or singly-curved with straightforward CNC plate-cutting and standard fillet/butt welding, no special curing environment or highly skilled composite-lamination trades are needed, and repair after in-service damage (a frequent occurrence for small commercial/fishing vessels) can be done with ordinary welding at almost any yard worldwide. Aluminum requires tighter process control (shielded-gas welding, distortion control on thin panels) and composites require mould tooling and controlled cure conditions that only pay off at higher production volumes or where the lightweight/no-corrosion benefit is decisive (e.g., a fast planing hull). For a one-off or small-series small commercial vessel where simplicity of build and repairability dominate, steel remains the practical default.
