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 24 m × 8 m twin-screw tug required to develop 60 tonnes bollard pull is a substantial harbour/escort tug, and the main-engine selection cascades into almost every other system on board.
Power sizing. Bollard pull scales roughly with installed brake power via an empirical ratio of about 14–17 kg of bollard pull per kW for a well-matched conventional-propeller tug (somewhat better, 16–19 kg/kW, for a Z-drive/azimuthing tug with optimized nozzles); at a representative 16 kg/kW, a 60,000 kg (60 t) bollard pull target implies roughly $60{,}000/16\approx3750$ kW total installed power, split across the two shaft lines — a figure that immediately sets the class of engine (roughly 1,900 kW per shaft line) and the shaft/gearbox torque rating needed. This power estimate should be treated as an early sizing check, refined once the actual propeller/nozzle combination and hull resistance at bollard-pull (zero-speed, high-thrust) condition are modelled.
Number and redundancy. With two propellers already fixed by the requirement, the usual arrangement on a 24 m hull is one main engine per shaft: the twin-screw layout itself provides redundancy (an engine failure leaves half the power and full steering control on the other unit), and it is the simplest and most compact option. Two engines per shaft through a combining (“father–son”) gearbox add part-load flexibility and a further level of redundancy, but at the cost of gearbox complexity, weight and engine-room length that a 24 m hull can rarely spare — that arrangement is more typical of larger escort or offshore tugs where a partial power loss mid-escort is critical.
Engine type and speed. At about 1,900 kW per shaft on a 24 m hull, compact high-speed diesels (roughly 1,200–1,800 rpm, e.g. 16-cylinder V-engines of the kind fitted to most modern 24 m harbour tugs) are the common choice, because their small footprint and low weight suit the short engine room and help keep top-weight and trim under control. Medium-speed diesels (typically 720–1000 rpm) offer longer overhaul intervals and better tolerance of the sustained high-load duty cycle a tug actually sees (continuous near-bollard-pull operation while towing, unlike a transiting cargo ship's steady cruise load), but they are heavier and longer and are usually reserved for larger tugs; the choice therefore trades engine-room length and weight against maintenance intervals and rated continuous (not intermittent) power, and it sets the reduction-gear ratio needed to turn a large, slow, efficient propeller.
Propulsion configuration. The choice between conventional shaft-and-propeller (with a Kort nozzle, almost mandatory at this bollard-pull level to raise thrust per unit power at low speed) and azimuthing (Z-drive) units is itself an engine-selection driver: Z-drives integrate the engine, gearbox and steerable thruster into one package and give 360° thrust vectoring valuable for ship-handling and escort work, but constrain engine footprint/weight to what the drive unit accepts and complicate engine-room layout (angled shaft downtake) compared with a straight in-line shaft to a conventional propeller.
Fuel, emissions and range. Engine choice sets fuel type (marine diesel oil vs. dual-fuel/LNG options increasingly required in harbour service near populated waterfronts) and emissions-compliance path (IMO Tier III / ECA compliance may require selective catalytic reduction or exhaust-gas scrubbers, both of which need engine-room space and weight budget decided at engine-selection stage, not retrofitted later).
Engine-room integration. Engine weight, length and vibration/noise signature directly drive the machinery-space length (competing with bollard-bitt and towing-winch space forward, and accommodation aft, on a hull that is only 24 m overall), foundation/seating structural design (must react full bollard-pull thrust reaction into the hull girder), and cooling/exhaust routing (raw-water intake sizing scales with engine cooling load, and stack height/placement affects both funnel smoke clearance over the bridge and superstructure top-weight/stability).
Serviceability and support. A widely-supported engine make/model (parts and service network at the tug's home ports) reduces off-hire risk over the vessel's operating life — often as decisive commercially as any performance figure, especially for a tug expected to operate for decades with minimal downtime tolerance.
