It is a frequently described phenomenon with larger motor yachts: the discrepancy between the speed recorded during the shipyard sea trial and the reality experienced on board later on. What appears on paper to be a purely technical discrepancy turns out to be a sobering realisation for many owners of yachts 14 metres or longer – and a problem that regularly pushes traditional service providers to their limits. The good news is that there is a solution. The bad news is that very few people know about it.
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Port Hercule in Monaco in February. At one of the world’s most sought-after moorings, Stephan Bernhart is waiting for a 26-metre motor yacht. What follows is not an isolated incident, but a textbook example of a structural problem affecting larger semi-planing yachts. The yacht, a 61-metric-tonne vessel fitted with two Volvo Penta D13 1000 engines and two IPS 1350 units, had easily reached 26.5 knots whilst at the dealer’s. In real-world operation, with a full fuel tank and standard cruising equipment on board, performance plummeted. Top speed: just 22 knots. And the trend was downwards.
The reason is simple, but its consequences are serious. Between the often almost empty shipyard sea trial and everyday life on board, several tonnes of additional weight are added. Fuel and other consumables alone account for a large part of this: a diesel tank with a capacity of 1,500 to 10,000 litres weighs up to 8.5 tonnes when full, whilst the water tank adds a further 2 tonnes. Added to this are the tender and jet ski, weighing up to 2.5 tonnes, as well as equipment and provisions. For a 37-metre yacht, the total payload in real-world operation can amount to 15 tonnes or more – a load that was simply not present during the initial trials.
Added to this are external factors that further exacerbate the problem: fouling on the hull’s underwater surface results in a measurable loss of efficiency; propeller fouling significantly disrupts the flow characteristics; and the transition from salt water to fresh water alters the yacht’s entire hydrodynamic behaviour due to the lower density of the water. “The result is always the same,” explains Stephan Bernhart, founder of Yacht Performance, who has specialised in optimising marine propulsion systems for almost two decades: “Owners find their yacht agile and powerful when they take delivery – and a year later, they wonder what has happened to it.”
The physics behind the solution
The approach to solving this problem lies in the interaction between two disciplines: electronic engine management and propeller design. Modern turbo-diesel engines with fully electronic engine management – now standard on cruising yachts built from 2005 onwards – offer considerable reserves which, for various reasons, are not fully utilised by the manufacturer. ECU remapping – that is, reprogramming the engine control unit – allows these reserves to be tapped in a targeted and safe manner.
The underlying physical principle is as elegant as it is effective: greater power generates higher speed; this higher speed generates greater dynamic lift; the hull rises further out of the water; hydrodynamic drag decreases; and, as a result, specific fuel consumption also falls. The apparent contradiction of more power with lower fuel consumption is resolved as soon as one realises that a hull in planing mode requires considerably less energy than in displacement mode.
If a heavily laden yacht can be brought into planing condition more easily and sooner, specific fuel consumption falls despite an increase in rated power. “It’s not about pushing the boundaries, but about letting the engine operate where it is physically most efficient,” says Bernhart. “An engine that runs at 70 per cent load in the cruising range rather than at 90 per cent is not only more economical – it also reduces wear and tear in the long term.”
Multi-stage optimisation process
The methodological approach follows a clearly structured procedure. First on the agenda is a detailed analytical test drive, during which engine speeds, boost pressures, exhaust gas temperatures, speeds and acceleration values are systematically recorded under real-world conditions, with a full tank of fuel, a full load and under various load conditions. It is only on the basis of this measurement data that the actual work on the control unit begins.
During ECU remapping, the original engine software is read out and the relevant mapping curves – injection volume, target boost pressures and torque characteristics – are recalibrated. One absolute principle applies here: all of the manufacturer’s original safety limits remain unchanged. Maximum boost pressures, exhaust gas temperature limits and all protective thresholds are neither increased nor deactivated.
“That’s non-negotiable for me,” emphasises Bernhart. “Engine protection that no longer works reliably is no longer protection. We work exclusively within the manufacturer’s guidelines, making use of the leeway that the manufacturers have deliberately built in.” The engine diagnostics remain fully functional, and operation within the manufacturer’s service network is possible without restriction.
Just as important as remapping is the accompanying propeller adjustment. Software optimisation alone is not enough if the propeller set-up does not match the new performance profile. The precise calibration of the engine map and propeller geometry is the decisive synergistic factor and the point at which many well-intentioned individual measures fail, because they take account of only one of the two variables.
When the figures speak for themselves
The 26-metre semi-planing yacht from Monaco illustrates this relationship impressively. In an initial attempt to improve performance, the owner had switched to smaller Q1 propellers, hoping that this would allow the engines to rev higher and gain more speed. The result was the opposite: the top speed dropped from an already disappointing 22 to 20 knots, even though the engine speed increased slightly. The smaller propeller caused the thrust to deteriorate, as less power and torque were transferred to the water.
Following the ECU optimisation, which involved newly calibrated maps for boost pressure, fuel injection volume and torque characteristics, the first test run yielded results that surprised even the experienced crew: 2,450 rpm and 23.5 knots, with the Q1 propellers – which were actually too small – with a full tank and a full load. The acceleration characteristics had changed fundamentally: it now took 13 seconds to go from 1,500 to 2,300 rpm, compared with 25 seconds previously. The crew could physically feel the acceleration. After subsequently switching back to the larger Q2 propellers, which now harmonised perfectly with the new engine profile, the full picture of the optimisation became apparent.
| Parameters | Original (Q1, loaded) | after optimisation (Q2, loaded) |
| Engine power | 2 x 1,000 PS/3,500 Nm | 2 x 1,150 PS/3,750 Nm |
| Maximum speed | 20 knots @ 2,300 rpm | 26 knots @ 2,440 rpm |
| Acceleration (1,500–2,300 rpm) | 25 seconds | 15 seconds |
| Fuel consumption at 16 knots | 275 l/h | 230 l/h |
| Fuel consumption at 21 knots | 380 l/h | 340 l/h |
| Cruising speed at 20 knots | 2,300 rpm (100 per cent load) | 2,100 rpm (85 per cent load) |
The fuel saving at a cruising speed of 16 knots is 45 litres per hour, which is a significant figure in economic terms on a long blue-water voyage. Equally noteworthy: the yacht now achieves a cruising speed of 20 knots at 85 per cent engine load rather than 100 per cent, which reduces wear and tear and contributes significantly to smoother operation.
More than just top speed
It would be a mistake to reduce the optimisation of marine propulsion systems to the pursuit of maximum speed. The more relevant question for most owners in the large yacht segment is: How does the yacht perform at typical cruising speeds, in rough seas, against the current and wind, and after weeks in harbour without underwater maintenance?
“Anyone who owns a yacht of this size doesn’t sail at full throttle every day,” says Bernhart. “But they want to feel that there are enough reserves so that they aren’t operating at the limit just to maintain an acceptable cruising speed. That’s the real gain in comfort.”
In practice, successful tuning means that the yacht glides effortlessly at 80 to 90 per cent load, where it previously reached its limits at full throttle. The engine runs significantly more smoothly and harmoniously, wear and tear is reduced, on-board noise is minimised, and power reserves are preserved for when they are genuinely needed.
Technical requirements and limitations
This method is not universally applicable. Older mechanical or partially electronic systems do not allow for the required level of precision control. Equally important: optimisation is no substitute for overdue maintenance. An engine with worn fuel injectors, a blocked intercooler or poor maintenance of the undercarriage cannot be saved by a new map.
“We can get the most out of an engine that is technically possible,” said Bernhart. “But we cannot defy the laws of physics. Anyone arriving with a fouled ship and faulty nozzles must first clear the service backlog.”
The potential lies in the system
The so-called ‘performance paradox’ – whereby a yacht becomes heavier, slower and more fuel-hungry after delivery – is not a quality issue. “It is the result of an industry reality in which test runs take place under idealised conditions and engine performance curves are designed for universal applicability rather than for individual boat weights,” explains engine expert Bernhart.
The technical means to solve this problem do exist. The electronic engine management systems of modern turbo-diesel engines offer untapped potential when utilised by experienced specialists and consistently within the manufacturer’s specified limits. When combined with careful propeller optimisation, this creates synergies that translate into clear, measurable results: lower fuel consumption, higher speed and smoother engine operation.
For yacht owners who feel that their vessel ‘sails differently’ than it did two years ago, it is worth carrying out a detailed analysis before considering new hardware, larger engines or costly hull modifications as a solution. Sometimes the answer lies in the steering control unit.
Book recommendations
Stress-free engine maintenance provides a comprehensive and practical guide to on-board maintenance and troubleshooting. As well as engines, the book also focuses on electronics, gearboxes, and fuel, cooling and air systems.
Perfect boat electrics focuses on the electrical systems of modern boats. Power management and battery systems are just as much a part of the overall technical picture as the electronic engine control system, which is adjusted through ECU remapping. The book demonstrates how important the interaction between the individual systems is for reliable and efficient operation.
More speed, less engine load and lower fuel consumption: does that sound like sensible optimisation or too much technology? Have your say.
Martin Hager
Editor in Chief YACHT