Dev Log #4.5 - The means of wind propulsion

This week we explore the shape language and technologies of ship propulsion.

In this excellent white-paper, the Windship association identified the main technologies that are expeded to lead Decarbonization in the maritime sector in the next two decades.

De-carboni - what? Removing the carbon dioxine fram the operations part of a ship’s lifecycle. In other words, making ships less polluting.

Visually it shows up in we grey smoke coming tram the engine’s chimney, and as colorful patterns of oil in suspension over water when ships discharge their bilgewater.

To fix this, the international maritime organization (IMO) I created a sulphur emission regulation and carbon emission reduction targets that incrase in ambition gradually. It’s still a change that happens quite fast from a shipowner’s perspective.

Ship operators can either comply by by retrofiting sails on the verssel to gain 20-40% emmission efficiency. Reducing speed is another passibility (called “slow steaming”).

The short term fix of buying filters and scrubbers to barely meet sox (sulphur dioxide) standards is over now.

-The operator must comply or become pant of what has been called the “shadow fleet”, These are ships that falsify records and hack ship registration numbers and do ship-to-ship transfers of merchandise to pollute in plain sight.

We’ll do a deep dive on the Shadow fleet and how it impacts story + gameplay in another dev-log.

For now, here is the list of ship technologies for wind propulsion that we want to represent in the game.

Flettner rotor

One of the lesser known technologies both in shape and function, fettner rotors are powered be the ships auxiliary engine. The rotation of the tube creates thrust thanks to the magnus effect.

“The Magnus effect is a particular manifestation of Bernoulli’s theorem: fluid pressure decreases at points where the speed of the fluid increases” Encyclopedia Britannica, (who also tells us this the same principle as a spinning ball or artillery shell).

That also means this type of wind assist needs power to produce the rotation in the first place, so perhaps it would be best combined with onboard power grenation through wind and solar. the tricky thing is deck space usage.

With a diesel engine, Fletnner rotors deliver + 20% efficiency. Using Flettner rotors with an electric engine and batteries would be efficient, but the lower energy density of batteries compared to fuel means a reduced range.

This is a well proven technology with modest efficiency gains, that is being deployed today on dozens of ships.

Rigid sail : fixed profile

Rigid sails exist since before the Shin Adku maru, a 70’s Japanese tanker that saved fuel during the 70’s oil crisis.

The mechanics of soft sails are conserved, but rotation happens through the mast itself rather than the boom. Rigid frames can be combined with cotton to create a more lightweight structure.

Some companies opt for marine steel and composites, using known processes from offshore wind.

A few french startups are now installing this type of module, designed to fit inside a 40ft ISO container if I recall correctly.

In our game, this is where having a Composite Workshop comes in handy.

These sails can retract in a telescopic fashion to clear low bridges.

RIdig sails with a fixed profile are less expensive their multi-wing counterparts, but cannot reach the peak efficiency.

Rigid sails: multi-wing

These rigid sails are made of multiple articulated wing profiles controlled through hydrolics.

The geometry of the wing reacts to incoming wind direction, speed, and the ship’s orientation in order to get the optimal thrust at any given moment.

Further improvement in computational dynamics (CFD) and simulations should be able to increase the efficiency of a sail of this type through a software patch where engineers tweaked the values.

This type of sail is the most complex and expensive of the bunch, and could have a number of vulnerabilities: EMP blasts, hacking, software bugs, vendor lock, subscription costs, and good ol’ rust from the marine environment.

Soft sails

Used since thousands of years and having proven itself many times over, nothing beats the simplicity of a big stick (the mast) with a sheet of something attached.

At the heigh of the first age of sail, the leading rig used (by tonnage transported per year, according to records) was by far the Schooner rig.

Recent innovations in sails are nylon and replacing wooden masts with steel or composites such as fiberglass and carbon fiber with a thin metal alloy for core.

The Sails can be made from ecological materials, such as linen fibers to protect from rotting, a red ochre color is traditionally applied.

The mixture is often a mix of oak bark, the same kind used for tanning leather. The main active ingredients that helps the preservation is Quercitannic acid.

The main disadvantage of soft sails is that it requires a larger well trained crew to operate the rigging.

in the 1960s German engineer called Wilhelm PrölB came up with a modern sail plan inspired by clipper ships where the entire rig can be controlled by a single crew member from a control panel. Famous ships with this rig called DynaRig include the Maltese falcon and Black pearl.

The Maltese Falcon can control the exact amount of sails directly from the bridge.

However, this rig is somewhat proprietary and guarded, highlighting the need for open source freighter designs, as presented by Steven Woods at Penn State University’s “Sustainability in ship design and operations” conference.

He presented to the audience a paper called “The need for a scalable open source sail freighter design”.

We also see copyrighted/ licensed designs as something that slowed down innovation by letting a few companies claim relatively broad domains such as “a computer -controlled sail rigging system”, etc. The means of de-carbonizing must be free and widely distributed.

An example of soft sail rig that combines low cost, soft sails, and modern materials include the Grain de Sail towit II.

Honora had the opportunity to watch a conference on the behind the scene details that informed design choices on that vessel.

Inflatable sails

Michelin Mostly Sells car tires, but in hopes of diversifying their brand into “we sell inflatables of all types”, they started working on a new concept of sails.

Like many of the smaller wind-assist system that can be retrofitted onto exiting chips, the inflatable sail fits inside the footprint of a ISO 40ft container.

There a many unknowns in this technology, but one big advantage: bridge clearing.

The only limit to height is the strength of materials.

Bridge clearing

Container ship Dali, Baltimore USA, 2024.

This might not explicitly show up in the game, but the common problem with going for taller and taller masts is the relatively low height of bridges.

Ship designer have three choices:

• accepting that the vessel will not be able to go anywhere in the word, and may even choose less optimal routes. This is rare.
• designing the sail system to fold horizontally, adding coast and complexity (ex: BarTech WindWings)
• Limiting the mast height to all the bridges that may be crossed by this ship over the next 10 years.

Good candidate for folding system: flettner rotors.

Kite propulsion

Like inflatable sails, Kites totally solve the Bridge clearing problem, but add a greater amount of complexity. The Kite can retract and deploy automatically.

To achieve the of best utilization of available energy possible, kite-surfers long found out that you need to do figure 8 shape by turning the kite within a narrow window where the kite is perpendicular to the wind (back against the wind).

Intuitively, we can see that there is more complexity here than just a ship being “pulled” by a lazy kite. It’s quite active; The window of greatest wind needs to be predicted by software control. The ship’s velocity and cable attachment point moving up and down needs to be accounted for.

As you can see, the Kite’s simplicity is deceiving, we essentially need to the same work as teaching a robot how to kitesurf. The software complexity is much greater than on rigid sails.

Improvements in software and designs could take this technology from a 20% fuel usage reduction to larger and more numerous kites for a 50% reduction.

Managing three giant metal cables doing figure 8 motions in the sky seems like an impossible feat, until you see ridiculous projects that almost worked, such as that Google-funded plane using the same window of greatest wind principle to get more energy than a wind turbine by having the axis of rotation be the cable instead of a mast.

We could imagine a world where this tech leads to a 50% fuel reduction on ships that cannot spare the deck space required for the other wind propulsion types.

Thanks to our Patreon supporters for supporting us in making a game about sustainable ships and ocean health.

Tune into the next dev-log for an update on buildings. Here’s a sneak peek:

• Shipyard Devlog
• WASP