Fuel-efficient motoring: the propeller is the biggest lever

Marina diesel around $2.20 a litre focused a lot of minds. Before you slow down or motor less, know this: the same boat at the same speed can burn meaningfully less fuel — the variable is how well the propeller loads the engine.

Engines have a happy place

A diesel is most efficient in a band of load and RPM. A fixed prop's single pitch means the engine only visits that band at one boat speed — everywhere else you're either lugging (over-pitched) or revving without progress (under-pitched). Both waste fuel; the second also glazes bores.

What re-pitching continuously buys

A self-pitching Autoprop holds the engine properly loaded across the range. The documented pattern: the same cruise speed at 300–400 fewer RPM, and up to 20% less fuel against fixed pitch (independent testing ranked it best in test for consumption at six knots; documented owners span 15–30%). Fewer revs also means less noise and less drivetrain wear — fuel is just the measurable part.

Run your own numbers

• A 40 hp cruiser burning ~4 L/h over 100 hours: ~400 L/year. 20% is 80 L — around $175 a season, more if you cruise seriously.
• A catamaran doubles it: two engines, two savings.
• A 75 hp passagemaker at 250 hours: the saving funds the haul-out.

The fuel calculator does this with your engine, your hours and your fuel price — defaults honest, every field editable, assumptions on the page.