A contract for sharing savings, however, only works if both parties can agree on what the savings were. That is where wind-assisted shipping still has a gap.
Wind propulsion has moved well past the trial stage. According to the International Windship Association, more than 100 commercial ships now sail with wind propulsion fitted, totalling over six million deadweight tonnes, and around 100 more are on order. Retrofits are typically saving between 5% – 20% of fuel in service, with the variation dependent on trade route, prevailing winds and the size of the sails relative to the ship’s installed power. That spread is the commercial problem in a single figure: on any given voyage, how much of the saving came from the sails, and how much from good weather or a well-chosen route?
Those numbers matter more each year as CII, FuelEU Maritime and the EU ETS raise the bar on efficiency and emissions. FuelEU Maritime already rewards wind directly, applying a multiplier to a vessel’s GHG intensity that is banded according to available effective wind power relative to propulsion power. The IMO Net-Zero Framework, which is still under discussion at the time of writing, would pair a fuel standard with emissions pricing and could have a similar impact worldwide. Knowing how much a WAPS contributes to a ship’s emissions profile is therefore part of knowing a fleet’s compliance exposure.
Routing is a large part of the answer. For a conventional ship, wind is mostly an obstacle that adds resistance, rough seas and fuel burn. For a ship with rotor sails or wing-sails, it becomes a possible source of thrust, which can justify a longer track to reach a better wind speed and angle, or a slower passage to get more out of the sails. The best route will vary with the optimization objective, such as fuel saving, speed, arrival time, cost or profit, as well as with the weather, the ship and its wind system.
Many weather routing tools were never built to do this. They treat the vessel’s performance as fixed and add wind as a simple extra push, rather than modelling how it interacts with waves, rudder angle, propeller and engine load at the same time. A credible approach maps wind along the whole route before departure, reassesses it against swell, wind waves and current, and states savings in dollars rather than percentages, so operators can see the financial case for a routing decision before committing to it.
NAPA’s performance models start from the ship’s 3D hull form and hydrodynamics and are calibrated with operational data. That allows them to estimate what the vessel would have burned on the same voyage, in the same conditions, with no sails. This counterfactual baseline is what turns a claimed saving into a verifiable one – and it is the figure an owner and a charterer need in front of them when they divide the savings under a charter. The same model supports pre-installation simulation, routing in service and post-installation validation.
Mikko Vartiainen, Wind Assisted Vessel Optimization Lead, NAPA
The same evidence matters for compliance. FuelEU Maritime already rewards wind directly, applying a multiplier to a vessel’s GHG intensity that is banded according to available effective wind power relative to propulsion power. Under the EU ETS, every tonne of CO2 the sails avoid is an allowance the company does not have to surrender. The IMO Net-Zero Framework, which is still under discussion at the time of writing, would pair a fuel standard with emissions pricing and could have a similar impact worldwide. Knowing how much a WAPS contributes to a ship’s emissions profile is therefore part of knowing a fleet’s compliance exposure, not only its fuel bill.
It also helps with the split incentive. Under time charters, the owner usually pays for the installation, and the charterer keeps the fuel savings, which is widely seen as the biggest structural obstacle to wider uptake. Voyage-level data that isolates the WAPS contribution from gains due to routing or weather gives both parties evidence to negotiate how savings are divided.
Verification has limits, wind forecasts can shift quickly, so a route planned on a five-day forecast may lose its advantage as conditions change. Predicting sail performance in heavy seas is harder than in settled weather. And no model closes the distance between what a ship could theoretically achieve and what a crew, bound by schedule and charterer instructions, can realistically deliver.
NAPA has developed a method for measuring what a WAPS has delivered using the vessel’s own signal data and a verified digital twin, during normal commercial operation rather than dedicated trials. The industry still lacks an agreed standard for in-service verification. BIMCO’s addendum gives owners and charterers a way to write a savings split into a charter. With one, ship and shore teams can work from the same picture, charterparties can be negotiated with confidence, and owners and operators can be sure their vessels meet regulatory requirements. Source: By Mikko Vartiainen, Wind Assisted Vessel Optimization Lead, NAPA