Rules such as “horsepower per tonne” are only rough screening tools. Final sizing must consider displacement in cruising trim, waterline length, hull form, windage, propeller aperture, reduction ratio, expected sea state, electrical loads, intended duty and the manufacturer’s rating.
A sound sizing process
- Establish actual loaded displacement, not an optimistic brochure figure.
- Define the job: harbour auxiliary, coastal cruising, ocean passagemaking, commercial duty or high-speed propulsion.
- Review the original designer or builder’s power and weight limits.
- Determine the largest efficient propeller the hull can accommodate with suitable clearance.
- Match engine curve, gearbox ratio and propeller demand.
- Check physical access, service clearances, exhaust height, cooling flow, ventilation, mounts and weight distribution.
- Model alternator and hydraulic loads where fitted.
- Confirm the boat can achieve the engine maker’s required loaded rpm range.
A common rough range quoted for displacement yachts is around 3 to 5 hp per tonne, but light modern hulls, heavy long-keel cruisers and high-windage catamarans can fall outside it. Treat it as a conversation starter, never a purchase specification.
Twin engines offer redundancy and manoeuvrability, but only if their systems are genuinely independent enough to survive one fault. Shared tanks, battery switches, exhaust vulnerabilities or inaccessible seacocks can undermine that redundancy.
Before changing power materially, consider structural engineering, shaft or drive torque limits, propeller clearance, steering effects, the Australian Builders Plate where applicable, insurer requirements and registration details.