CargoProa, TankerProa, & the Fundamentals of Sailboat Maneuvering

By June 7, 2016 Article, Technology No Comments

^ CargoProa (Golden Colored) attached to a Cargo Ship – Image Courtesy of CargoProa at

Unparalleled Harnessing of Wind Power

In what could culminate into a fantastic invention, the Fair Winds Trading Company is building the PraoCargo, a modern version of the proa, an ancient, multi-hilled cargo ship of the South Pacific.

A proa is a sailboat with two parallel hulls usually of unequal lengths. Also known as perahu, prau, and prahu, it sails with one hull to the windward side and the other to the leeward. It therefore needs to shunt to reverse direction when tacking (as we shall see later).

Variants of the proa were also commonly used in the numerous islands of Malaysia, Indonesia, and Philippines. However, the English term proa particularly refers to the South Pacific proa as depicted in the journals of HMS Centurion, a British ship.

Proas of the Marianas Islands are the ones that Americans are most familiar with. When complete, the company will employ the ship to carry goods between Argyll in Scotland and its sustainable development project in West Africa.

Alain Guillard, designer and naval architect of the vessel, has already constructed a 12-meter prototype of the wind-powered ship. He is testing the model in the Gulf de Morbihan, France by ferrying gravel in it.

PraoCargo is based on the patented CargoProa design. U.S. innovator Dr. Frank J Berte conceived the CargoProa design for container ships and the TankerProa design for tankers and bulk carriers.

Downwind Testing of CargoProa’s Scale Model   Image Courtesy of CargoProa at

Downwind Testing of CargoProa’s Scale Model
Image Courtesy of CargoProa at

Both designs can slash fuel costs completely. Yes, you read it right – as much as 100% cut in fuel costs. Emissions will simply tank. Then, they minimize ballast water contamination. And, they use the existing assets of container ships and tankers. What more can you ask for?

Now, the objective of the Fair Winds Trading Company is not merely to build a zero-emission ship. It also seeks to empower coastal communities in least developed and developing island nations. How – by assisting them to set up their own maritime trade routes.

Maritime trade is the backbone of the global economy. With 71% of the world covered in water, shipping is an effective means of reaching out to the remotest corner in the world. So much so that over 90% of the globally traded merchandise is carried by ships.

Proa Design @ Container Ships & Tankers

Stability, load-to-size ratio, and multi access capabilities are the principal design merits of the PraoCargo. At 60 m length, the ship will have a shallow design draft of 4-6 m. The exact value of draft will depend on the rudder position.

Weathervane sails will tap wind from all directions. A rigid space frame stamps out rigging. Trimming sails is the adjustment of sails of a sailboat so they can use the wind most effectively.

Sailboat rigging refers to all those miscellaneous wires, lines, and rods that support the rig and maneuver the sails. Needless to say, rigging is complex equipment that sailboats cannot do without. That was before the PraoCargo.

Tankers have to Uninstall TankerProa when Entering Ports Image Courtesy of CargoProa at

Tankers have to Uninstall TankerProa when Entering Ports
Image Courtesy of CargoProa at

All this empowers the PraoCargo to sail at speeds comparable to similar diesel-propelled ships. Its average speed is 13 knots and the vessel can clock a maximum of 25 knots when necessary. An electric motor drive system provides back up propulsion.

Designers have made the PraoCargo capable of maneuvering under sail in shallow water while restricting its heeling to the minimum. Heeling is the leaning or tipping of a sailboat under the influence of wind.

With its ability to harness the might of the wind, the TankerProa temporarily transforms a tanker into a pure sailing vessel for over 95% of the voyage. The tanker won’t need even a drop of diesel when using the TankerProa along trade wind routes.

TankerProa will be armed with masts that scale heights of up to 400 feet. Plus, their sails will possess an area of four times as much as that of the PraoCargo. At such elevations, wind speed is twice as that at the ocean surface.

However, the contraption cannot enter ports. Operators have to uninstall / decouple it when entering a port. And they have to re-install it when leaving the port. The coupling / decoupling point is located only a short distance from the port.

It can move its masts to generate the maximum possible wind thrust for downward sailing and broad reach sailing. That apart, the apparatus can tap solar and wave energy necessary for propulsion as well as for the control of rudder, sails, and dagger boards.

TankerProa can drive unloaded plus loaded vessels. The structural frame provides the outrigger and the necessary sails, rudder, and other equipment that convert the tanker to a viable wind-powered vessel.

Broad Reach Testing of CargoProa’s Scale Model   Image Courtesy of CargoProa at

Broad Reach Testing of CargoProa’s Scale Model
Image Courtesy of CargoProa at

It ensures that unloaded vessels do not need ballast water – its outrigger provides stability instead. This also cuts down sizably the tonnage of the unloaded vessel. Another reason why TankerProa cuts emissions.

Unloaded vessels however have to take in ballast water when leaving port for the point where they can install the TankerProa. After coupling with TankerProa, they can de-ballast in a meager 90 minutes.

Because the installation / un-installation point is near the port, ballast water transfers occur at close locations. This minimizes the transfer of aquatic invasive species – an issue that has assumed pandemic proportions over the years and has considerably impaired the ecology of our oceans.

According to the Maritime Executive Magazine, increasingly stringent standards for the processing of ballast water before dumping it will escalate tanker operations costs to an astounding $1-5 million per vessel. TankerProa will root out this expense.

Testing of a 10-meter TankerProa prototype on a 20,000 pound test tanker vessel of the Massachusetts Maritime Model was successful. The prototype propelled the tanker at about 3 knots in a 10-knot wind – comparable speeds to those provided by the test tanker’s diesel engine.

Even in a 20-knot wind with the sails deployed totally and waves washing over to the deck of the test tanker, the TankerProa remained attached to the test tanker. Most importantly, both remained stable. Dr. Berte is looking to build a full sized TankerProa.

TankerProa being attached to a Tanker Vessel   Image Courtesy of TankerProa at

TankerProa being attached to a Tanker Vessel
Image Courtesy of TankerProa at

Installing TankerProa on existing vessels is a low-cost operation. The mechanism is scalable, fitting easily and quickly on a wide variety of tankers. And by cutting down fuel use, it minimizes emissions and fuel expenses.

Basics of Sailboat Maneuvering

In order to understand the working of a proa boat, we must first familiarize ourselves with the essential parts of a sailboat and their functions. A common sailboat is made of eight basic components:

  • Hull is the shell of the boat that houses all the internal parts of the boat
  • Mainsail is like the main engine of the sailboat. It harnesses wind energy to drive the sailboat
Basic Parts of a Sailboat

Basic Parts of a Sailboat

Basic Parts of a Sailboat

  • Jib is similar to the auxiliary engine of the sailboat. It is a fixed, smaller, and triangular sail that generates additional thrust
  • Mast is the tall, vertical pole that secures the vertical side of the mainsail
  • Boom is the horizontal bar that runs parallel to the deck of the boat. It secures the bottom horizontal side of the mainsail. Sailors move it to adjust the orientation of the mainsail in accordance with wind direction i.e. to trim
  • Tiller is the steering wheel of the sailboat that the operator uses to change the boat’s direction
  • Rudder is equivalent to the sailboat’s tire. When you pull the tiller to, say, the left side, the rudder changes its direction and thereby the direction of the boat
  • Keel is the long, narrow board projecting out of the bottom of the hull. It stabilizes the boat by balancing it underwater and preventing it from tipping over. The keel also holds ballast

Sailing downwind or with the wind is easy. The wind pushes the sails. Newton’s Third Law of Motion ensures that sails push back the wind. Through this, they decelerate or slow down the wind. As we shall see, a sailboat going straight downwind can never sail faster than the wind.

Vector Diagram: Forces Acting on a Sailboat when Tacking

Vector Diagram: Forces Acting on a Sailboat when Tacking

You cannot sail directly into the wind. That is simply impossible. A motor boat can do that. Not a sail boat. The wind will only make the sails flap. But sail boats can move upwind at an angle of 40 degrees or so. Let us see how.

In order to flow around the sails, the wind has to deviate. Lets us say, vi and vf are the initial and final velocities of the wind respectively. Their directions, with reference to the orientation of the boat, are as shown in the vector diagram.

Whenever the magnitude or direction of a body’s velocity changes, the body accelerates or decelerates. The said change in velocity accelerates the air. And, force is the product of mass and acceleration. In this case the mass of air is ma and air acceleration is aa.

Air therefore exerts a sideward force on the sails, say Fw. This is equal in magnitude but opposite in direction to the force that the sails apply on the wind, say Fa. This equal-opposite equation is on account of Newton’s Third Law.

Now, most of Fw is sideward and it gets more sideward as you sail more against the wind. But it does have a small forward component (Ft). And it is this forward element that moves the boat forward, against the wind.

Righting Moment & the Weight-Buoyancy Balance

Righting Moment & the Weight-Buoyancy Balance

When sailing into the wind, a boat heels i.e. tilts sideways. The keel prevents the boat from tipping over when heeling. The force of air acts on the sails while the force of water (Fk) acts on the keel in a direction opposite to that of the force of air.

Because water is denser than air, the horizontal component of the force of water on keel (Fkh) becomes equal and acts opposite to the horizontal component of force of wind on sails (Fwh) despite the keel area being a small fraction of the sail area. Such equilibrium is the righting moment.

And, there is the drag force (Fd) that acts against the direction of motion of the boat. The vector addition of Fk and Fd is the force on hull and keel (Fh) that is numerically equal to Fw but acts opposite to it. The net forward thrust (Ft) has to overcome Fd if it has to drive the sailboat.

Taking the Bernoulli Principle into consideration, a net secondary force will also act on the sails. This will be in the same direction as that of Fw. In the interest of avoiding complications, we have not portrayed this force in our vector diagram.

Bernoulli’s Principle is the extension of the Principle of Conservation of Energy to fluid flow. It states, the total energy of an incompressible fluid remains constant for a streamlined flow.

Total energy consists of static pressure energy, kinetic energy, and potential energy. For flow at constant elevation the potential energy upstream and downstream is constant. Therefore, any increase in velocity comes with a proportional decline in pressure.

In the diagram above, wind moves faster on the outer side of the sail and slowly along the inner side. The static pressure on the inner side is therefore higher that on the outer. This creates a thrust from the inner side towards the outer side and pushes the boat further along Fw.

Tack Changes the Windward Side of the Boat Image Courtesy of Kangel at

Tack Changes the Windward Side of the Boat
Image Courtesy of Kangel at

Such movement of a sailboat (at an angle) against the wind is called tacking. Tack is the deliberate changing of the direction of a boat so that the wind blows into another side of the boat.

In the diagram above, the wind is initially blowing into the starboard (right) side of the boat. After tack, the wind blows into the port (left) side. Tack is necessary to sail upwind.

In the initial position when the wind is blowing into the starboard side, the vessel is in starboard tack. Later with the wind blowing into the port side, it is in port tack.

Incidentally, the spelling of tack differs from that of tact by just one alphabet. Wonder if this is a deliberate design of those who introduced the word ‘tack’ into the English vocabulary. You do need tact to fight forces stronger than yourself. And you need tack to sail against the wind.

After studying tack, let us now revisit the description of a proa for better understanding:

A proa is a sailboat with two parallel hulls usually of unequal lengths. Also known by various alter names such as perahu, prau, and prahu, it sails with one hull to the windward side and the other to the leeward. The craft therefore needs to shunt to reverse direction when tacking.

Proas shunt when changing tack. Shunt means, the bow becomes the stern and vice versa. The main hull is the vaka. It is longer than the smaller, windward hull called ama. The ama always remains windward so as to provide ballast. Akas or crossbeams connect the two hulls.

Jibe Changes the Windward Side of the Boat Image Courtesy of KVDP at

Jibe Changes the Windward Side of the Boat
Image Courtesy of KVDP at

Along with tack, jibe forms the two fundamental maneuvers of sailing ships. Whereas sailors execute the tack when the boat is sailing at an angle into the wind, they can jibe when the boat is sailing with the wind, at an angle of course.

Sailing ships move faster when the wind is blowing into them at an angle from behind (called broad reach) than when the wind blows straight from behind (called running). Because this higher speed more than makes up for the increased distance along the zig-zag path, sailors jibe their boats.

But because sailing a boat in the running mode is more convenient than sailing on broad reach (i.e. jibing the boat), sailors rarely jibe. Tack is more common than jibe because boats cannot sail directly into the wind.

Sailors can complete a jibe more rapidly than they can a tack. A jibing boat’s sails are therefore powered while a tacking boat’s sails are not when the bow crosses through or into the direction of the wind.

14 - Why Tack & Jibe Propel Sailboats Faster than Wind

Why Tack & Jibe Propel Sailboats Faster than Wind

For ships sailing into the wind, there is a no-go zone – the angle on either side of incoming wind into which the ship cannot sail as the sails cannot produce enough thrust. The width of the zone depends on the design, sails, and rig of the boat as well as on the sea state and wind strength.

Why can’t a boat moving straight downwind move faster than air? A jibing or a tacking boat can move faster than the speed of air. But, not a running boat. Here again, we have to request assistance from vector diagrams.

Suppose your sailboat is moving at 8 knots in the same direction as of a 12 knot wind. You will feel a relative wind of 4 knots. The mathematical representation of this, which applies to all sailing modes of the sailboat, is as follows:

vw = vb + vr

Where, vw, vb, and vr are respectively the wind velocity, boat velocity, and the relative (boat-wind) velocity. It is the relative velocity that drives the sails of your boat. And as the boat speed approaches the wind speed, the relative velocity drops to zero and there is nothing to propel the boat.

In this case, all the three velocities act along the same line in the same direction. This is not the case when you tack or jibe the boat. The faster a sailboat goes while tacking or jibing, the greater is the relative velocity to propel it faster.

Let us tackle tacking first. We can see that vw, vb, and vr are acting along different directions. And, that precisely is the crux of the issue. For a wind speed of 12 knots, a sailboat tacking at 40 degrees and at 16 knots will feel a driving, relative velocity of around 26 knots.

When jibing at 40 degrees at a wind speed of 12 knots and a boat speed of 16 knots, you will feel a relative velocity of just about 10 knots. Sounds easy? Not so simple in practice. It takes years of perseverance in angry oceans to master the art. Smooth seas never make skillful sailors.


As numerous experts have reiterated in the recent past, the need to minimize emissions and fuel costs will be the primary drivers for advancement of shipping technologies in the 21st century.

The world is bracing up to stand up to the terrible and all pervading menace unleashed by Global Warming and Climate Change. With hands on attitude, shipping is taking up this challenge head on.

But the CargoProa and TankerProa hold the potential to take center stage among all the technologies that aim to cut shipping emissions and fuel use. The catch is, they must succeed on the big stage, the global platform.

For more such glimpses into futuristic technologies in shipping, visit our blog.

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