Moving a Heavy Ship with Relatively Small Sails?

I recently read this article:

And frankly my gut reaction is that the sails shown are too small to move this heavy a vehicle. Why am I wrong?

I don’t have an answer, but here’s a similar ship that will travel at about 11 kts, which is essentially the same as the 10-12 kts claimed for that ship:

They are wingsails, aerodynamic rigid sails that provide far more power than an equivalent area of conventional sails. They operate on the same principle as aircraft wings.

The dimensions of the sail may surprise you if you’re familiar with sailing technology. In fact, it may seem too small. However, Evidence has no need for a big rig. In fact, Van Peteghem explains, “Given that it is twice as powerful as a traditional rig, you get the same level of performance for half the surface area.”

Also going to comment about they are wingsails and use different airflow/air pressure differences for propulsion than conventional sails. Look at the blade area of a modern wind turbine vs the old style dutch windmill to see the difference.

That may be true, but I’d like to use this opportunity to point out that (many, not all) conventional sails, too, operate on the same principle as aircraft wings. There’s a common perception that sails function by the boat being “pushed forward” by the wind blowing from behind. That is true only for some conventional sails, such as spinnakers. Most conventional sails, such as lateens and square rigs, work best with wind coming from aside rather than from behind (on beam reach, in nautical terms). The wind flows along the sail, and the profile of the sail creates a pressure differential between the two sides that propels the sail forward. It follows the same principles as the creation of lift by an aircraft wing.

True. But the effective area of those foils far exceeds that of conventional sails. Much of the cloth sail is used to form the shape of a foil without providing much lift. The term ‘lift’ is often used for sails and keels because it is the same function provided by aircraft wings even though used for horizontal movement.

I expect that a lot of sails spend their time in what would be called a stall on an airplane. Stalls don’t decrease the lift to zero, they just decrease it over the maximum due to the flow separation on the top of the wing–what would be the leeward side of a sail. An airfoil sail should be able to achieve true lift, with both surfaces contributing to the motion. A more normal sail design might be able to achieve lift some of the time, or over part of the surface, but with limits on the angle of attack. A square rig might gain efficiency with the wind coming from the side but it’s probably going to have fully separated flow on the leeward side, so not really achieving lift in the normal sense.

Another point is that you can tolerate low thrust, if you’re not in any particular hurry. Cars aren’t going to go bad if you don’t get them to their destination quick enough, so if using sails rather than diesel engines adds a week to the trip, that might be acceptable.

The tell-tales on the sail will tell you if the sail is stalling, so that you can adjust your sail. Lots of sails don’t have tell-tales, because any competent sailor will have some idea what the most efficient sail angle is. But from sailing with tell-tales, I would say that, unless it’s gusting, most of the time most of the sail is not stalling on either side. It’s just not an efficient airfoil shape.

Actually getting an efficient sail shape from a stitched piece of cloth is impossible, and the more stitching and shaping you do the more it costs, but given whatever you’ve got, you still try to run it without stalling any more than a tiny amount of the sail.

I defer to your experience. I guess my thinking is that you don’t always have a lot of choice in the matter, and if you’re simply headed downwind then the sail is going to be in drag mode. And probably the same for any heading not too far from downwind. But then, that’s not an efficient way to sail, so why do that? Sailboats don’t have the problem that they have to support their weight on their wing, so they have a lot more flexibility in managing their angle of attack.

But it definitely is costly to build a shipload of cars, then keep them from the hands of consumers for an extra week.

There’s also the point that a slower-moving ship (and its well-paid crew) can carry less freight in a year than a fast one.

The point being that time is money in much the same way fuel is.

Fuel is probably around $15,000 a day though. I agree that the economics of sail may well not stack up, overall, but saving fuel isn’t insignificant

Difficult, yes …

But the Americas Cup boats now racing in Auckland have achieved notably efficient sails by using two cloth surfaces - with some fairly fancy cloth and rather sophisticated schemes for actively tuning its shape.

Fuel is billed to the charter customer. Capital costs come out of the owners pocket and at the owners risk. Will the customers be willing to pay higher fixed costs to offset uncertain lower fuel costs? What if the price of fuel continues to go down?

Sure. Charterers take into account tonnes of fuel per day in their current chartering decisions. Owners already consider more fuel efficient designs when ordering newbuilds with an eye to gaining a competitive advantage in winning charter business.

So it’s all possible in theory.

I suspect that actually this sort of vessel is far more likely to be built for a particular trade, after taking into account prevailing winds, speed, etc and vertical integration is more common in that sort of trade ie more likely that owners are building something for their own use.

Looking at the picture in the OP reminds me of Flettner Rotor Ships, another case where the mechanism for the propulsive force looks woefully small (and vertical). Only in that case the mechanism itself is unusual:

When the sails are combined with conventional propulsion that ship could travel with remarkable fuel efficiency. I expect many more ships to add sails. They don’t have to sit adrift and wait for a breeze, and whenever the winds are favorable they’ll be saving on fuel without sacrificing travel time.

The elephants in the room though are (off the top of my head) reliability, maintenance, air draft, interaction with loading infrastructure and low crew numbers. There may be more.

Everything deteriorates at sea and needs a lot of maintenance. On deck mechanical systems are notorious. The opportunities for maintenance for a hard-working ship are limited. So it will need to be reliable, low maintenance, and not require many more crew to operate and keep in good repair.

Also, high masts will give trouble at a number of ports due to low air draft (from bridges). And a lot of loading and discharging infrastructure require, ideally, unimpeded access above the holds.

None of these issues are insurmountable but they will mean that owners will be cautious about leaping into the purchase of new and untested technology of this nature.

As mentioned above, what I can see happening is purpose-built sail assisted ships being used for a very specific trade, where it is known what the routes will be, what prevailing winds will be, and which (potentially customised) berths the vessel will be calling at.

Knowledge gained from that sort of limited exercise could then (as the technology matures) be seen as a lower risk proposition for more general trading vessels.

It’s clear that you haven’t even bothered to find out the basic facts.

The wingsails are retractable. They can be retracted at the touch of a button for bad weather and going under bridges. Since it’s a Roll-on/Roll-off vehicle carrier, not a container ship, it’s not loaded and unloaded by cranes above the holds.

This vessel hasn’t been designed by some tech whiz kid, but by a major shipping corporation, which has been in business for 160 years. Their subsidiary Wallenius Lines owns a fleet of 40 RoRo vessels, and they control far more vessels than that through partnerships.

We can assume they know what they are doing.

Here’s their web page about the wind-powered vessel, including a 45 min video:

I watched that 45 min video, and I have to say it’s very impressive.

These are serious, intelligent, capable business people, who are not playing games.

They are thinking through all the issues in great detail, and planning carefully. They have built a prototype, done wind tunnel tests, etc. They are are considering all the financial and practical implications, and drawing on a long, successful history in the shipping industry.