S5C3: Sailing Ships

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a) Ship shape

We have already covered the main means of transport on solid ground with the trains and pipelines in Chapter 1 and cars and trucks in Chapter 2, it is now time to turn our gaze towards the seas and oceans to understand transport over these liquid states of matter. We’ll start with the wind-powered sailing ships and in Chapter 4 we will cover the engine-powered versions used in freight, the cargo-ships, as well as the underwater vehicle appropriately named submarines.

So, how does one move over water without engines? There are two parts to the answer, the first is to use air pressure, and specifically differentials in air pressure, the main reason behind the phenomena we called wind, which is the diffusion of air from a zone of higher pressure to one with lower pressure. In addition to air pressure, itself the result of differential heating, wind is also generated by the Earth’s rotation and the fictious force called the Coriolis effect it engenders – something we have already mentioned in S3 Section 3.c on the key climate variables. We will look into how wind is made use of to propel a sailing ship in the next section and in this one we will discuss the second part of the answer, the shape and structure.

Indeed, having a propelling force available at or close to the surface is one thing but the ship needs to remain there to avail it. In other words, it needs to stay afloat and not sink. This sounds trivial and yet it is far from being easy to achieve over long periods and in rough weather with large waves. Overall buoyancy is quite easy to figure out: air is much less dense than water so one only needs to build a barrier between liquid and gas and architect the dry, gaseous part so that a) it keeps water from crossing the barrier, b) it can accommodate cargo, passengers and the wind harvesting equipment, and c) it can do all of this without being too slow, i.e. with reduced friction.

Friction in water is mainly a function of the speed at which the craft is moving, the surface area moving through the liquid, and the angle at which the interfacing occurs when considered from the perspective of the direction of motion. Thus, to reduce friction, we would need a watertight hull with favourable hydrodynamics, or at least as much as can be afforded by the purpose of the ship, and if possible we also want to leverage the propelling force of the wind to somehow lift as much of the hull out of the water. This can be seen more spectacularly in windsurfing and in racing sailing ships where there is little volume immerged in water. Racing is not the main market however, so typical hulls would be more pragmatic and may sport hard edges or smooth curves and flat bottoms or v-bottoms. For added stability, to provide lateral reactive force and help right a ship, a longitudinal appendage called keel can be laid and fixed under the hull. Longitudinal means following the centreline, from the bow at the front to the stern at the back. Port is the left side when facing the bow and is signalled by a red light while starboard is the right side and signalled by a green light. Access to a ship typically takes place port side, the origin of the term.

The hull also needs to be tall enough to prevent waves from capsizing the boat too easily or water entering each time one crashes against it. Nonetheless, this needs to be optimized because a hull costs money to build and is heavy, thus affecting performance. Once this superstructure is ready, several floors can be laid out, depending on the size of the ship, to accommodate food reserves, potentially cannons for warfare back in the days, sleeping and living quarters, and a top deck where most of the sailing ship’s operations take place and the mast or masts to which the sails are rigged can be fixed.

b) Dancing with the wind

Before we look into the intricacies of having the wind in our sails, we should ask ourselves how it is that wind could provide motive power. What is wind? As mentioned earlier, it originates from a differential in atmospheric pressure and from the spinning of the Earth upon itself. This doesn’t provide an immediate answer to the question then, so we need to clarify that this process of pressure equalization entails the movement of gas molecules from one place towards another. This is the key: wind is the movement of matter; it is kinetic energy. And this energy can naturally be converted into other forms of energy, as we saw repeatedly over the previous series.

If you have already observed a sailing ship move over water, certain scenarios are intuitive enough, others not so much. It is quite straightforward to understand the motive force when heading downwind, essentially the wind is pushing the sails and, since the sails are tied to the mast, the entire ship moves forward at a speed which cannot exceed that of the wind. It is obvious there is more to sailing because otherwise one would be entirely beholden to the exact direction of the winds and some voyages in direction opposite to the prevailing winds would be physically impossible. Without going to the extreme of going against the wind, how can a partially sideways wind be used to move forward – and not just forward but potentially faster than the true wind speed, that measured in relation with the surface of the water, as opposed to the apparent wind speed which is the one measured from the reference frame of the boat and is therefore a function of the true wind and boat velocity.

When wind heads into the sail, the side exposed to the wind is subjected to more pressure than the opposite side of the sail; these are called respectively the windward and leeward sides. This pressure can be decomposed into two vectors representing drag, in the same direction as the apparent wind, and lift, perpendicular to drag and the vertical plane of the sail. These are illustrated in Figure 2A below and you will note the lift (L) pushes the boat sideways and slightly forward whereas the drag (D) also pushes it sideways and backward with the total aerodynamic force (F_T) being the combination of those two vectors.

Figure 2A: Wind force on sail

Credit: HopsonRoad (CC BY-SA 4.0)

We are halfway there and the second set of forces is the result of Newton’s third law of motion we saw in S1 Section 6.d, which states that “for every action, there is an equal and opposite reaction.” And so, beneath the surface, the hull and in particular the keel, are subjected to an opposite hydrodynamic force (F_I) which can be decomposed like the aerodynamic force F_T into a hydrodynamic resistance (R_I) and a hydrodynamic lift (P_I) with drag increasing as the speed of the craft increases and remaining constant when the resistive force is equal to the forward driving force (F_R on the diagram). Until this point is reached, the forward component of the force exerted into the sail is what causes the boat to accelerate and propels it forward in relation with the water. This vector is both a function of the apparent wind velocity (V_A) and the angle of attack (α), which is the angle formed by the imaginary vertical plane of the sail called chord line and V_A. All these forces are illustrated in Figure 2B below and the relevant relationships can be plotted on a polar diagram to derive the optimal angle of attack.

Figure 2B: Forces on sail and boat

Credit: Bcebul (CC BY-SA 4.0)

This explains why, in racing, the geometry of the hull and keel and having real-time information about wind velocity is crucial to work out the optimal route. Indeed, from the above it is clear one doesn’t want to navigate bow first into the wind and some zigzagging may be required. This is called tacking and changing tack – on a side note, it is unbelievable the number of expressions in the English language that come from sailing ships and their manoeuvring.

The ease with which one can change tack partially has to do with the disposition and shape of the sails. This configuration is optimized according to the task the ship is meant to carry out and the geography in which it operates, including the strength of winds. Hence, designing a ship involves drawing a sail plan with one or several masts, sails that can be square or triangular-shaped, and different types of rigging. This is an entire topic unto itself so I am including a link to the Wikipedia entry at the end of this chapter and will only mention a few points as follows:

Square rigs refers to squarish sails carried on spars; the horizontal beams attached to the vertical mast. They are orientated at a right angle to the ship’s centreline and thus its intended direction of travel. This set-up works well going in the direction of the wind and it is easier to keep the sails rigid.
Fore-and-aft rigs have sails laid in the median plane of the keel, not perpendicularly. These work well for tacking.
Rigs are systems designed to control the orientation, tension and deployment (or not) of the sails. They consist of ropes or cables linked to the sails via pulleys as well as to the masts to provide support against sideways forces (shrouds) and fore-and-aft forces (stays).

c) Operating a ship

Maritime navigation is extremely manpower intensive, or at least it was in the days where trade was carried out by sailing ships. The size of the boats and related large number of sails meant a lot of ropework depending on wind conditions and direction with tacking requiring a lot of coordination and the speed at which a crew could reef sails, meaning reduce the surface area exposed to the wind, could make the difference between saving a ship and wrecking it. The saying “if you’re thinking of putting a reef in, you should have already done it”, conveys the importance of acting in unison, that’s discipline and training, of knowing the ropes, that’s experience, and remaining aware of one’s environment, that’s keeping watch.

There is no standard watchkeeping schedule per se but some are more prevalent than others and these include the splitting of 24h into 6 watches of 4h each, starting at 20h00 until midnight, that’s an 8-12 watch or the fourth watch, and ending with the 16h00 to 20h00 nicknamed “dog watch”. This makes it easy to split across either two or three teams and if some degree of rotation is called for among the crew to make things fair, then oftentimes the dog watch would be split in two periods of two hours to make the total number of watches equal to seven over the course of 24h. Six hours shifts also work well, divided across two teams or across three with each team completing 4 watches over three days in the latter case. On merchant ships, the six periods of four hours dominate with different officers responsible for each watch on top of their other responsibilities.

Besides setting sails and keeping watch, the ordinary hands (as opposed to the captain and other officers) would need to be proficient in ropework, which includes rope splicing (joining them together to act as one) and tying knots as well as repairing and maintaining the sails and pulleys.

Housekeeping and more broadly the ability to keep the place in perfect order, “ship shape” as we say, was more than valued, it was imposed on the crew for obvious safety reasons considering the swaying of the boat and the necessity of keeping the top deck neatly arranged despite the dozens of rigging present so that the right rope could be found where it is meant to be and there would be no obstacles on the way.

As for the captain, he would be in charge on the ship, the supreme authority and arbiter of justice when called for. Part of his job would be to decide on the course to be followed by the ship and he would be assisted in this role by a navigation specialist. Nowadays, one would use GPS (a technology we explored in S4 Section 9.c) but before that, when not in sight of recognizable landmarks such as coastlines, dead-reckoning, a technique we mentioned in S4 Section 9.b on the gyroscope and accelerometer, and celestial navigation were the only practical methods. The use of astronomy was discussed briefly in S3 Section 9.a in the context of the historical relevance of astronomy and the measurement of angles between visible objects, including stars and the horizon, was made possible by a sextant – a process known as sighting. With these values in hand and the tables containing the predicted position of celestial bodies enclosed in a nautical almanac, it is possible to mathematically derive the latitude and longitude coordinates, being one’s position on a map.

In our modern days with refrigeration and engine power, it is quite easy to feed a crew over several weeks. It was not so in the age of discoveries when crews could be at sea for weeks on end without food or water resupply. This meant cured meat and starches would be on the menu every day, including the famous and oh-so-appetizing ship’s biscuit also known as “hard tack” consisting of cereals and flour. The lack of fresh fruits and vegetables translated into an insufficient intake of vitamins, a nutritional concept unknown of at the time, and this deficiency would ravage crews with the scurvy.

Adding insult to injury, sailors could genuinely end up dying of thirst, even when surrounded by water, because the human body system cannot process so much salt concentration. Hence, storing and rationing fresh water was of utmost importance and the main way to top off reserves was the collection of rainwater. The use of distillers or evaporators for sea water desalinisation was too ineffective back in those days.

d) Construction and maintenance of a ship

It is not only the crew onboard a ship that should survive a sea voyage, but the boat also has to. Well, what could go wrong when wood is immersed in water for years and the hull, metallic or wooden, should not let water in? Quite a lot as you may expect, hence the need to keep a watchful eye and carry out regular maintenance.

Woodworms might be listed as enemy number one before the advent of iron and steel hulls. This wood-eating larva is laid by beetles boring into moist wood, which would cause structural weakening and potentially water leakage calling for chemical treatment or replacement of the timber planks. Second on the list of biological adversity would be algae and barnacles finding an underwater home on the hull surface and creating a drag when moving through the water at speed if they were not scrubbed off. Hulls would thus be protected by layering noxious products such as tar on them. Periodically, a ship would be beached or dry-docked to proceed with urgent repair or scheduled maintenance work.

A dry dock would be a sort of homecoming for a ship, starting with its keel, is constructed in isolation from its aqueous environment. Shipbuilding takes place in shipyards and is extremely manpower intensive, it is therefore considered a strategic industry in terms of providing regional employment and tends to be overly competitive for that reason – a dynamic quite similar to the auto manufacturing industry.

Arguably the most impressive and grandiose shipyard the world has ever known, adjusted for relative historical levels of economic and technological development, was the Venetian Arsenal, where arms and ammunitions were also manufactured. The city-state navy and merchants had their ships mass-produced and maintained there, employing more than 10,000 people and reaching an output of nearly one ship a day. A stunning number by any standards and a key foundation in projecting Venice’s mercantile and military powers across the Mediterranean. Without the Arsenal, Venice would not have become the wealthy city it remained for centuries. I include a link to the relevant Wikipedia entry in the last section if you wish to read more about this complex.

Early shipbuilding techniques would first build the hull and then lay the reinforcement from within whereas later techniques would see the keel and remaining of the frame be built first and then the shell layered on the outside. In the shell-first scenario, there were three widely used manners to keep the wooden planks together: sewing or tying them together, partial overlapping with nails traversing them (this would be clinker and reverse-clinker construction), or a system of wooden pegs working as joints.

e) Modern technologies

In section b) we have seen the main force slowing down a ship is the hydrodynamic resistance, denoted as R_I in figure 2B. However, it was also clear a sail is a foil that happens to move through the air, a liquid with low viscosity. Like any foil, depending on the angle of attack, it is able to generate more lift than drag. Using the same principle, one can fit a ship with hydrofoils that, with the proper angle of attack and above a certain speed threshold, will be able to lift the boat vertically and, with the hull partially or even fully out of the water, this greatly reduces the drag faced by the ship – delivering more speed and energy efficiency in the process.

And indeed, it works in practice; there is no engine or magic involved. Because this requires a minimum speed to be achieved, the technology is most often seen on high-speed ferries or racing sail boats – quite a sight to behold as the hull seems to levitate above the surface. For a long time now, the speed records have been held by such boats and the best speed average ever measured over a nautical mile is just above 102km/h (55.3 knots).

It is also possible to create vertical lift from below as well as from above using airborne foils bearing the generic name of kites. Typically they are shaped like paragliders but may also have rigid elements and can be used by kite surfers as well as to reduce the drag of a boat’s hull in the water and even pull it forward. Hence, autonomous versions can be tethered to ships, including engine-powered ships without space for a mast and sails on their top deck. This is still mostly in trial stages for large cargo ships and it is worth noting that, at the contemplated altitudes of several hundred metres above the surface of the sea, the winds are significantly stronger.

Wind assistance, even for freight and traditionally engine-powered passenger transport is gaining widespread interest in order to cut down on fossil fuel consumption and reduce greenhouse gas emissions. The use of carbon fibre and plastic allows for the manufacturing of tall, rigid and lightweight frames that can also be stacked and deployed or retracted telescopically depending on wind conditions, all at the push of a button or even in a fully automated fashion. Knowing the ropes might become a thing of the past outside of leisure yachts.

f) Trivia – Sail materials

We just mentioned plastic and carbon fibre but these were not always a thing, so what materials are sails traditionally made of? Sailcloth comes in different fashion (!) and the canvas was historically made of natural fibres such as cotton, linen or hemp, or even wool in the case of Viking longships and pandan leaves in Austronesia. The choice of fibre is important, yet it isn’t the only criteria in the performance of a sail and the weaving quality is another important part of the equation.

With the rise of petrochemical products came fabrics such as nylon or polyester. More recently, it is aramids (aromatic polyamide) that has seen the most traction in the racing industry, in particular Kevlar on account of its very high strength-to-weight ratio and elastic modulus.

Modulus is the ability to resist stretching and can be a sought-after property, as can be the resistance to UV light (this can impact the fabric strength), the ability to retain stretch resistance over time (the technical term is “creep”), and the quantity of force that can be experienced without tearing (breaking strength). Depending on the use case, light weight might be paramount and in others it might be modulus. The ongoing development of advanced materials exhibiting extreme properties, such as those seen with graphene, could herald more improvements as we set sails towards the future.