How much should an anti-drift system cost? RISE did the maths

A modern cargo ship with six towering wing sails sounds simple enough: catch the wind, cut the fuel bill. But wind doesn't only push a ship forward. It also pushes it sideways. Left unchecked, that sideways drift forces a ship to use more rudder or throttle back its sails, quietly eating into the very savings the sails were meant to deliver.

When the French shipping company Zéphyr & Borée set out to develop a concept for the first wind-powered transatlantic container service, they faced a deceptively simple question: which solution offers the best balance between performance and cost?

A 1000-year-old problem – and very modern solutions

Sideways drift, known to sailors as leeway, is nothing new — retractable leeboards that "hold the ships, so that they are neither driven sideways, nor overturn" were already described in a Chinese text from the year 759. What is new is the price tag. Today's anti-drift devices range from a simple fixed keel to sophisticated, actively controlled daggerboards, and the more effective a device is, the more it typically costs to build, install, and maintain. For a shipowner deciding which anti-drift solution to integrate into a newbuild, hydrodynamic performance is only half the story.

RISE puts the options to the test

This is where RISE came in. Zéphyr & Borée, together with manoeuvring systems and energy-saving device specialist Becker Marine Systems, asked RISE to independently evaluate three candidate solutions for a new 2700 TEU container ship concept: a fixed centreline box keel, a retractable daggerboard with a fixed angle, and a more advanced retractable daggerboard whose angle adjusts itself in real time.

Using virtual captive-test simulations of the flow around the hull, ship performance modelling, and full voyage simulations across a year of real North Atlantic weather data, RISE's naval architects put all three appendages through the same rigorous test:

– As an independent research institute, we do not sell hardware. Instead, we provide impartial third-party advice to shipowners, equipment manufacturers, and shipyards, helping them identify case-specific solutions that are both technically sound and economically viable. This case study for Zéphyr & Borée and Becker Marine Systems was a textbook example of how RISE can create concrete value for customers

Frederik Gerhardt, Lead Naval Architect, RISE

The results: a clear gap in performance

The differences turned out to be substantial. For the investigated vessel and route, the advanced, angle-adjusting daggerboard cut annual fuel consumption by up to 163 tonnes per year compared with a ship fitted with no anti-drift device at all. The simpler, fixed-angle daggerboard came close behind, at 132 tonnes. The fixed box keel, cheaper and simpler to build into the hull, delivered only 8 tonnes of savings, and on some legs of the route, where the wind blew from astern, it actually increased fuel consumption slightly, since its fixed underwater area still had to be dragged through the water even when it wasn't needed.

The anti-drift systems also significantly improved the vessel's ability to maintain efficient operation under sail. Without any device, the ship needed to reduce sail power in strong winds roughly 10% of the time to keep the rudder within a safe operating angle. With the advanced daggerboard fitted, which fell to about 0.2%, the ship could almost always keep its sails fully powered.

Bang for the buck

Better performance almost always costs more, so RISE translated the fuel savings into a very practical number: how large a share of the ship's price can each device add and still pay for itself? Assuming a 15-year repayment period, the answer varied sharply. The box keel could add barely 0.1% to the vessel's price before the investment turned unprofitable. The fixed-angle daggerboard could add roughly 1.35%, and the advanced, angle-adjusting daggerboard as much as 1.65%, because its far larger fuel savings gave it much more room to absorb a higher price tag.

The conclusion wasn't that one device is simply "best." The fixed box keel is the cheapest and mechanically simplest option, but its performance gains are modest enough that it only makes financial sense if it is priced very low. The advanced daggerboard delivers the biggest savings by far, but only pays off if its own cost stays proportionate. The fixed-angle daggerboard, in the middle, offered a good balance of solid performance and a realistic chance of paying for itself, a genuine case of "bang for the buck".

RISE analysis gave us a clear, independent picture of how each system performs and at what price — exactly the kind of evidence we needed to choose the solution that delivers the best balance of efficiency and cost.

Amaury Bolvin, Managing Director, Zéphyr & Borée

For Becker Marine Systems, whose daggerboards demonstrated strong performance compared with the fixed box keel, having the results checked by an outside party mattered as much as the results themselves.

The independent assessment by RISE provides valuable evidence of the performance potential of our daggerboard solutions. It gives shipowners a sound technical basis for evaluating the balance between hydrodynamic performance and investment.

Sascha Kukowski, Head of Sales, Becker Marine Systems

Why it matters beyond one ship

Zéphyr & Borée's ship, designed to connect Antwerp, Le Havre, Philadelphia, and Charleston with roughly half the greenhouse gas emissions of a conventional container vessel, is just one of many wind-assisted vessels that are being developed. The above questions about the appendage are ones that each and every wind-powered newbuild will have to answer. With EU rules under the Fit for 55 package now putting a price on every extra tonne of fuel burned, the gap between a device that merely works and one that is actually worth its price is only going to matter more.