Like many H28 owners we enjoy our cruising with a mix of using sail-power and engine power. Sailing when the breezes are favorable and motoring with that iron spinnaker when the conditions are 'agin' us. During our summer cruising we can be out for 2 to 3 weeks, covering a lot of territory or water, so getting the most out of each litre of diesel can become important.
The charts below were established from our H28 boat and engine data along with some sea trial / boat usage measurements. It has given us useful information so that we can target a reasonable fuel economy whilst achieving a reasonable boat speed under power.
Our Yanmar engine spec's show max power is attained at about 3,300 rpm through to 3,600 rpm. Fuel usage at these engine speeds is very high at over 4 l/hr and Sandbourne's measured boat speed through the water peaks out at about 7 knots. Pushing the engine harder than 3,000 rpm just uses more fuel and digs a hole in the water as the boat speed has reached the maximum for the H28 hull shape.
At around 2,500 rpm fuel usage drops to about 2.2 l/hr and engine power is 18.5 hp so close to 90% of maximum. At this engine speed we can still achieve a good boat speed of around 6.5 knots (flat water, no current or wind and a good clean hull).
The graph Fuel Usage vs Baot Speed really highlights how fuel usage really increases significantly when you push for higher boat speed with the engine power alone. On our H28, boat speeds under engine power, in the 4 to 6 knot range clearly produce a sensible outcome. In this range, fuel usage is between 1 and 2 litres / hr. And if we choose to motor-sail, with the engine just ticking over to maintain boat speed or heading, then fuel usage gets down to around 1litre / hr which is great, particularly if a favorable breeze helps you achieve boat speeds of 6 to 7 knots.
Bob & Judy - “Sandbourne”