Case study · Marine engineering · Wind propulsion retrofit
In 2020, Bound4Blue needed to put their first commercial rigid wingsail on a real ship. The hull they were given was Naumon — La Fura dels Baus' floating theatre, born as the coastal cargo Elvi Kull. The structural engineering that turned an aft deck into a class-approved wingsail foundation was willbö's.
A retrofit of this kind is a structural problem before it is a wind-propulsion one. A rigid wingsail introduces sustained heeling moments and concentrated point loads on a hull that was never designed to carry a sail. Every load case had to be answered against the existing steel — bending, shear, fatigue, local connections — and the result had to clear DNV before the mast went up.
Before willbö, before the eSAIL, before La Fura: the project's first move was sourcing a coastal cargo robust enough to carry a wingsail and characterful enough to become a stage. The vessel that fit both briefs was the Elvi Kull — later rebranded Naumon. The structural retrofit followed the ship.
What willbö did
A wingsail's load case is not a sail's. We mapped the design conditions for the eSAIL together with Bound4Blue: aerodynamic moments, gust scenarios, transit and parked configurations, abatement-system loads — and identified the critical situations the hull had to survive.
A wingsail bends the ship. We re-ran the global strength check on Naumon's hull girder under the new loads — bending moments and shear forces along the length — to confirm the existing scantlings still cleared DNV's longitudinal-strength criteria with the wingsail in service.
The wingsail meets the ship at a handful of structural connections — and those are where retrofits fail. We designed the local detail at every interface: mast footing, boom attachment, deck doublers, standing-rigging chainplates and the maneuvering-system reactions.
Structural analysis · SESAM GeniE
Every load path that a wingsail introduces — bending the hull, pulling on the deck, concentrating force at the footing — was modelled, checked and reinforced where the rules required. Three views from the structural model:
Timeline · vessel hunt to delivery
Bound4Blue's eSAIL family is now installed on bulk carriers and tankers across the world fleet. The structural blueprint that made class-approval routine for those later units was written, in large part, on the deck of Naumon. willbö wrote it.
What this proves for your project
Owners often picture wind propulsion as a deck installation. The honest engineering answer is that you cannot answer a footing question without first answering a longitudinal-strength question. willbö runs the global check first — bending, shear, section modulus — and only then does the local detail.
A retrofit that adds steel everywhere is cheap engineering, expensive shipbuilding, and a permanent weight penalty for the owner. We use the FEA — global and local — to show DNV exactly where steel is needed and exactly where the existing structure already clears the rule. The package gets approved on evidence, not on conservatism.
Half the friction in a wingsail retrofit is agreeing the load envelope with the OEM and the class society before any steel is drawn. We bring that conversation forward — define loads with the OEM, format them for class, and let the structural design fall out of an envelope everyone has already signed off on.
Specs · Naumon retrofit
| Item | Value | Note |
|---|---|---|
| Vessel | Naumon | ex-Elvi Kull · coastal dry cargo · theatre conversion |
| Owner | La Fura dels Baus | Catalan performance company · floating-theatre programme |
| Approx. dimensions | ~60 m LOA · ~12 m beam | Coastal cargo profile · indicative |
| Project type | Wind-propulsion retrofit | Rigid wingsail (eSAIL) + abatement system |
| Client | Bound4Blue | eSAIL OEM · structural integration partner |
| Class | DNV | Structural class approval package issued + cleared |
| Software | SESAM GeniE | Global hull-girder + local FEA at footings & chainplates |
| willbö scope | Loads · global · local · class package | Load cases · hull girder · footings, rigging, deck · DNV dossier |
| Year | 2020 | Among the earliest commercial rigid-wingsail retrofits |
More case studies
Naumon is the wingsail story. CN296 Arpaarti Arctica is the ice-class story. The BOLT waterpark on Carnival Mardi Gras is the structural-package-on-a-deadline story. SH Vega, Evrima, Juan Pablo II — more case studies coming to this format.
See all projects →Wingsails, Flettners, suction sails — bring the OEM and the hull, we'll handle the structure and the class dossier.
julian.fontela@willbo.es →