Wind Contracts Need Verified Savings

Wind contracts need a shared savings measure

According to Hellenic Shipping News, NAPA’s Wind Assisted Vessel Optimization Lead Mikko Vartiainen says wind-assisted propulsion needs stronger performance evidence now that BIMCO has a contractual tool for the sector. BIMCO adopted its Energy Saving Device Retrofit Addendum in autumn 2026. The addendum gives owners and charterers a framework to share retrofit costs, time and fuel savings under a time charter.

For wind-assisted propulsion systems (WAPS), owners usually fund installation while charterers receive the fuel savings. Vartiainen argues that both sides must agree on the saving before they can share it.

Wind propulsion has grown faster than verification

The International Windship Association reports that more than 100 commercial ships carry wind-propulsion equipment, totalling over six million deadweight tonnes, and roughly 100 more ships are on order. Retrofits typically save between 5% and 20% of fuel in service, depending on the trade route, prevailing winds and sail size relative to installed power.

Vartiainen identifies that range as a commercial problem. On a given voyage, the parties cannot determine how much of the saving came from sails, favourable weather or routing choices.

Regulatory exposure is increasing

CII, FuelEU Maritime and the EU ETS increase the value of these figures. FuelEU Maritime applies a multiplier to a vessel’s GHG intensity based on available effective wind power relative to propulsion power. The EU ETS covers ships of 5,000 GT and above carrying cargo or passengers for commercial purposes: 100% of emissions on voyages between EU/EEA ports and while at berth in them, and 50% on voyages between an EU/EEA port and a non-EU port. On those in-scope voyages, each tonne of CO2-equivalent that sails avoid reduces the allowances the shipping company must surrender. From 2026 emissions, methane and nitrous oxide are included and companies surrender allowances for 100% of in-scope emissions.

The IMO Net-Zero Framework remains under discussion and could combine a global fuel standard with emissions pricing. Vartiainen argues that fleet operators need a WAPS’s actual contribution to assess compliance exposure as well as fuel costs.

Routing tools can miss key interactions

Vartiainen says many weather-routing tools hold vessel performance fixed and add wind as extra thrust. They do not model wind together with waves, rudder angle, propeller and engine load.

According to Vartiainen, NAPA bases its performance models on a ship’s 3D hull form and hydrodynamics, then calibrates them with operational data. The models estimate fuel burn for the same voyage without sails. He says NAPA uses that counterfactual baseline to turn a claimed saving into one that parties can verify.

Limits remain

Vartiainen also acknowledges limits. Wind forecasts can shift quickly, so a route planned on a five-day forecast may lose its advantage as conditions change. Sail performance is also harder to predict in heavy seas than in settled weather. In his view, no model fully captures the gap between theoretical potential and the performance a schedule-bound crew can deliver.

Vartiainen says NAPA has developed a method that uses a vessel’s own signal data and a verified digital twin to measure WAPS performance during normal commercial operation rather than dedicated trials. Vartiainen states that the sector 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.