Updating the Sailing Cargo Ship

The IWSA recently put together a study of a proposed commercial sailing vessel, the Fairtransport Ecoliner, a four-masted cargo ship carrying 43,060-ft² of sail. The ship would use trade winds and ocean current to get the same average port-to-port cruising times as oil-burning ships. Designers included Dykstra Naval Architects and the Rotterdam Naval College with some funding from the Wolfson Foundation of UK.

The Ecoliner is a fine design, but a smaller wind-powered cargo ship is more likely to be built. After all, when developing a new class of vessel such as this, it is better to start smaller because funding may be easier to obtain, giving designers a better chance of actually getting to the building phase.

Now consider the income the industry earns on shipping containers. If the route is used to ship machinery from Taiwan to Los Angles, the ship can charge about $3,560 to $4,000 per 40-ft container for delivery over this route. Our proposed sailing freighter carries 104 containers, 52 in the hull and 52 on deck. If this vessel were fully-loaded, it would earn about $391,000 per trip. Assuming a shipping firm can sustain a minimum of six such trips per year, it will earn $2.35 million per year, or $11.8 million over five years with a ship of capacity.

Wind conditions along trade-wind sailing routes are fairly reliable, so the biggest variable in the time it takes to complete an entire route are port turn-around times. Smaller vessels have shorter port turn-arounds than giant container ships and are back at sea sooner. This partially makes up for the smaller ships’ slower travel rates.

Sailing vessels following trade-wind routes must also travel farther than the direct paths taken by powered vessels. On the China to the U.S. west coast route, the difference is about 7,200 nautical miles for the trade wind route versus 6,300 for the direct route. Speeds for sailing ships are limited to 9 to 12 knots if they are heavily loaded. Powered vessels do 15 to 19 knots. These numbers indicate a good economic return on investment over the 30 year life of a $24 million sailing vessel (See Adding It All Up to the right).

Updating the Cargo Ship

The use of modern materials and design techniques to update earlier concepts is a major driver in the revival of commercial sail. For example, one design breakthrough was the combination of rotating carbon-fiber masts and practically robotic sails. They were both used on the steel-hulled Maltese Falcon, a ship built in 2006 that can be sailed by a single person. The sails and masts on this full-rigged yacht follow the Dynaship concept invented in the 1960's by German hydraulic engineer Wilhelm Pröiss.

This sailing rig is also the same general concept used in the Ecoliner and for our proposed ship’s three masts. But we combined it with a centuries-old idea developed by the Polynesians: the tripod mast. Carbon-fiber masts that rotate would be too costly for a commercial vessel. And unloading containers near large rotating masts would expose delicate mechanisms to damage.

The tripod mast we propose (see Tripod Mast Concept) spreads loads across a wider area of the hull and deck and lets it be made of conventional steel. It also moves the rotating machinery for the sails up and away from container-handling areas while still letting the sails rotate 180°, from full port to full starboard. Even though the mast is 43-ft taller than the Coast Guard Eagle’s mast, our design does not need the  miles of standing and running rigging which would make it impossible to have a modern deck load of containers.

The movable parts of the masts are light, strong carbon-fiber yard arms which carry the sails. They reduce weight aloft yet withstand large deflections during wind gusts without breaking. Carbon fiber’s strength and stiffness eliminates the sheets and braces, that is, the lines running from the ends of the old style yard arms down to the deck on traditional sailing ships.

The yard arms mount to circular tracks so motors can rotate them about the mast. Wind sensors at various heights tune each motorized yard arm to the proper angle for the sails to best catch the wind. This is important because wind direction can vary over the height of a tall mast. The overall design also lets one crew member handle the entire ship. Traditional sailing ships required a dozen hands per mast to trim the yards.

Each sail is made of conventional Dacron sail cloth. Strong wire rope sewn into the outboard edges (or leeches) helps the sails resist movement of the yards and prevents them from tearing under high loads.

One major difference between the Maltese Falcon’s mast and ours is that the Maltese Falcon‘s has a C-shaped section hollowed out along its forward side that runs it length. It is used to house the furled sails.

Each sail attaches to small motorized shuttles running along its upper and lower yard arms. The shuttles move in unison to pull the sails toward (furling) or away (unfurling) from the mast, much like a motorized window curtain on a track. When the sails are furled or stowed, they are tucked into the hollow section of the mast. However this design greatly reduces the strength of the mast which led to the requirement of using carbon fiber for Maltese Falcon’s masts.

The sails on our three-masted design are also furled and unfurled horizontally by motorized shuttles. But there is no storage area inside the mast. Instead, when they are furled, they are exposed to the sun and weather. To protect them, sacrificial sun-screen Acrylic panels are sewn over the leeches of each sail. These panels are the last portion of the sails pulled in and are left exposed, thereby protecting the Dacron underneath.

Problems left to solve

A preliminary study of any design project determines the possible problems designers may encounter during design, and our study has uncovered several. For example, even with a tripod steel mast, the side-to-side (or thwartship) loads it places on the hull and deck are large. Finite element analysis must be used to verify the design integrity of the hull and its ability to resist loads from wind pressure on the sails superimposed over hull-twisting loads due to wave action.

Another set of problems is that the deck of sailing ships must have large hatch openings for loading and unloading containers into the hull below decks. This weakens the hull right where strength is needed the most. Another potential problem is the tripod mast blocks easy loading and unloading of some containers stored on deck.

These obstacles call for developing ways to shift containers fore and aft to aid in loading/unloading. Adding a mast-mounted crane as in the Ecoliner is one solution. Another possible solution is to install deck railways to help shift cargo.

For those steering and commanding a sailing ship, visibility of the sails and workings of the ship is critical. The Ecoliner concept, for example, has a forward-mounted bridge that gives the crew a good view forward but not at the masts and sails behind it. However, on large sailing ships, someone should be constantly monitoring the sails. Like most sailing vessels, our design places the bridge toward the rear where it is easier to see the workings of the ship. Remote sensors on the masts would help monitor the sails and equipment, but most sailors are taught not to rely solely on electronics that can fail at sea.

Marine designers who study past designs of sailing vessels note that the older designs carried much more sail area than vessels designed today. One reason for this is the crews of that time knew how to handle large sailing vessels. So the knowledge base required to handle large sailing vessels at the grassroots crew level will have to be rebuilt for commercial sail to come back.