How do I choose the right inboard diesel engine for my boat?

Choose an inboard diesel to suit your boat's loaded displacement, intended cruising speed and propeller, and check that it fits with enough room for servicing. When replacing an engine, start with the existing installation and its performance: extra horsepower only helps if the hull, gearbox and propeller can turn it into useful thrust.

Work out the power requirement

Check whether the present engine reaches its specified full-load rpm with a clean hull and propeller. Poor speed can stem from fouling, excessive propeller pitch, restricted fuel supply or a tired engine. It does not automatically mean the original horsepower was inadequate.

For a conventional displacement sailing yacht, 4 to 6 hp per tonne of loaded displacement is a useful starting range, not a sizing rule. Include fuel, water, cruising stores and equipment, then compare similar boats and obtain a propeller calculation.

A Hallberg-Rassy 29 originally fitted with 18 hp will commonly suit a replacement of around 20 to 25 hp, depending on displacement and use. Moving towards 30 hp needs a clear reason. Check that the installation can accommodate both the engine and a suitable propeller.

Displacement hull speed in knots is roughly 1.34 times the square root of waterline length in feet. Beyond that region, extra power brings rapidly diminishing gains. Reserve power helps against wind and waves; it cannot overcome a tide running faster than your speed through the water.

For semi-displacement or planing motorboats, ignore the sailing-yacht guideline. Use the boatbuilder's power range, loaded performance data and the engine's duty rating, especially for sustained high-output running.

Measure the complete installation

Measure the engine compartment before comparing brochures. Use the installed dimensions, including the gearbox, mounts, exhaust elbow and belt guard. Allow room for engine movement and maintenance.

• Engine beds: record their spacing, height, condition and available mounting length.
• Shaft position: measure the coupling centreline and shaft angle. Gearbox output height and down-angle vary.
• Service access: check that you can remove the impeller, oil filter, fuel filter, belts and heat-exchanger end caps.
• Connections: check exhaust diameter, seawater inlet, fuel hoses, control cables, wiring and battery requirements.
• Weight: compare complete engine-and-gearbox weights, including fluids, particularly in smaller yachts.

Make a cardboard outline or use the manufacturer's installation drawing. Two engines based on the same industrial block can have quite different filter positions, pump access and exhaust arrangements after marinisation.

For a saildrive installation, obtain written confirmation that the engine, adaptor and drive are compatible. Matching horsepower alone proves nothing.