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.
The governing environment is benign most of the time (Hs<1.0 m) with occasional 3 m seas, and the target is a sustained 30+ knot service — a speed-length ratio well into the semi-planing/planing/multi-hull regime where conventional displacement-hull resistance theory no longer governs.
(a) Four most promising combinations. (1) Catamaran with water jets: the twin slender demihulls give low wave-making resistance at high Froude number and good transverse stability without ballast, and water jets avoid exposed running gear in shallow-draft lake operation while giving excellent low-speed manoeuvrability for frequent dockings. (2) Catamaran with azimuthing podded propellers: pods give 360° thrust vectoring (no rudders needed) and can be more efficient than jets at the lower end of the speed range, useful if the service also calls at exposed piers. (3) Semi-planing monohull with water jets: a proven, lower-capital-cost combination for a 30+ knot ferry in moderate seas, with a single hull simplifying berthing infrastructure. (4) SWATH with conventional propellers (shafted, deeply submerged): SWATH's submerged lower hulls give exceptional seakeeping through the occasional 3 m seas — valuable for passenger comfort and schedule reliability — and deep struts allow conventional open-water propellers to run efficiently below the free surface. Other options not in the matrix: a hydrofoil-assisted catamaran (foils to reduce wetted-hull resistance further at speed while retaining catamaran displacement stability at rest), and diesel-electric propulsion with any of the above (decouples engine placement from shaft geometry and eases redundancy).
(b) Largest technical challenges per combination. Catamaran/water-jet: jet-inlet cavitation and debris/ice ingestion in a lake environment, plus wash/wake-wash limits near shore approaches. Catamaran/pods: pod-strut structural fatigue from wave slamming between the demihulls (the "wet-deck slamming" problem common to fast catamarans), and pod maintenance requiring drydocking access. Semi-planing monohull/water-jet: resistance hump transition through the semi-planing speed range, and trim control (interceptors/trim tabs) needed to keep the jet inlet properly wetted across the speed range. SWATH/propellers: strut-borne vibration and the very tight metacentric-height margin inherent to the SWATH form (small waterplane area gives small $I_T$, hence small $BM$ and $GM$), demanding careful loading control.
(c) Additional factors for feasibility. Beyond the technical comparison: capital and life-cycle cost per seat-mile at the 30+ knot service speed (fuel burn rises steeply with speed for all four options); wake-wash regulations and shoreline erosion limits on a inland lake with residential shoreline (particularly relevant for a catamaran or planing hull at speed near shore); berthing/terminal infrastructure compatible with the chosen hull beam and draft; Transport Canada vessel-class and stability certification requirements for the passenger service (Small Vessel Regulations / high-speed craft code as applicable); crewing and maintenance-skill availability for the chosen propulsion type (pod and jet maintenance require specialist support not universally available inland); winter operability (ice conditions on Lake Ontario, relevant to jet-inlet icing and hull ice class); and market risk — ridership sensitivity to fare levels that must recover the higher capital/operating cost of a high-speed service.
