Heavy Weather Dynamics: Upwind Sailing, Windage and Resistance

Last Updated on 05 August 2016 by Eric Bretscher

With tropical cruising becoming increasingly popular and accessible in the last 2-3 decades, trends have developed in outfitting yachts for ocean cruising. Some of these trends that originated from sailing in moderate settled weather areas have resulted in significantly increased risks when a vessel is caught in severe conditions.

Many of today’s cruising yachts, whether of modern design or not, have become top-heavy through numerous “standard” additions ranging from stern arches, davits, dodgers, shade cloths, solar arrays, wind generators, dinghies on deck, antennas, radar scanners up in the rig, furling headsails and sometimes many more items such as jerrycans, kayaks, surfboards and outboard motors stored on deck when in fact they should really be stowed down-below – if on board at all.

While some awareness usually exists about the impact of weight on and above the deck in terms of reducing the stability range, the fact is often discounted until it becomes perceptibly dangerous – which is a long way down the track and far too late. However, the first consequence of outfitting as described above is windage and its effect is more insidious.

Outfitting for ocean cruising while assuming the best case scenario is playing a game of Russian roulette

Since many tropical cruisers predominantly opt for downwind routes, high windage mostly translates into additional drive and no major issues are experienced. Since heavy weather at sea is far from a common occurrence and simply doesn’t occur in Trade Wind areas outside the possible cyclone seasons, one could arguably cruise the tropics near indefinitely with minimal wisdom and without ever encountering truly severe conditions. In reality, most yachts engaged in Trade Wind cruising do stray from the easy latitudes to some extent at some point: rounding Cape of Good Hope, descending to New Zealand or sailing to or from their home port. Areas of strong ocean currents such as the Gulf Stream, the current descending the East Coast of Australia towards Bass Strait, the current of Brazil or the Aghulas Current off South Africa can all result in dangerous sea conditions at times if stronger winds are experienced. This is where incidents usually start being recorded.

Upwind Aerodynamics: The Effect of Windage

Aerodynamic drag forces acting on a yacht are oriented with the wind direction and increase with the square of the wind speed. This means that, in a 40-knot blow, the drag will be four times higher than in 20 knots of breeze; it will be 9 times higher in a 60-knot gust. This has detrimental consequences on sailing ability that can seriously compromise safety in bad weather.

One must remember that both driving forces and drag increase as the wind speed gets higher. Reefing the sails lowers the centre of effort of the rig, allowing higher sail forces and more propulsive power for the same heel angle. On the other hand, the bare upper section of the mast now only contributes to drag. Still, a well configured yacht should have no trouble sailing upwind in storm conditions.

Ensuring that propulsive forces keep up with the increase in drag as the wind picks up is very important, including and especially on cruising yachts. If overall windage is increased through “additions” above deck as described earlier, the balance of forces depicted can become compromised:

 

At that point, the vessel is unable to advance forward due to the absence of a net propulsive force. Without speed, it can’t maintain its angle in the sea and the bow falls off, until a new point of equilibrium is reached:

The difference now is that the vessel is travelling at an angle much more parallel to the sea and is unable to do better.

Since all propulsion originates from aerodynamic lift forces, the net effect of adding aerodynamic drag to a sailing yacht is deteriorating the lift/drag ratio. As highlighted above, the yacht naturally compensates by falling off the wind because this increases drive and reduces the forward resistance component. It falls off the wind until a new equilibrium point is found. As a result, added drag always reduces the upwind ability of a yacht.

Incidentally, hydrodynamic hull drag has the same effect on pointing ability in the sense that it adds to the resistance component and also needs to be overcome by propulsive forces. Large wetted surface, poorly designed appendages, protruding hull fittings and transducers all lead to poor tacking angles. Increasing displacement also tends to opens up the tacking angle, because hull resistance is proportional to displacement.

Safety Implications

If conditions become severe enough to cause the sea to break, inability to point to weather can result in a very dangerous near beam hit. Since windage also results in additional heeling moment and is often coupled with weight aloft, the available stability margin is reduced just as the likelihood of broadside impact is greatly increased. What started as a dodger, a stern arch and other structures aimed at improving convenience has now turned into a significant hazard. Heavy, old-fashioned long-keeled boats get into this situation quite quickly, often long before the wind truly reaches gale proportions, hence the old rants about heavy weather tactics and the various ways of lying in the sea. Those are not only inept, but very hazardous on board modern vessels with considerably more exposed hull area and much less inertia.

If the wind is blowing onto a lee shore, once leeway is taken into account, the course on the ground may not even be better than 90° from the wind: the vessel becomes unable to gain to weather and may get driven ashore.

When running downwind is ruled out either by the proximity of land or the behaviour of the vessel in following seas, a yacht with excessive windage and/or hull resistance quickly runs out of safe options by also being unable to point into the weather. It is a very unfortunate situation, because this is avoidable through sensible choices and sailing at moderate speed to weather under reduced canvas is often the simplest and safest way of handling a blow with heavy seas that could otherwise cause a knock-down with damage and potential injuries.

Practical Considerations

In the figures above, all aerodynamic drag components were lumped together into one single drag force. This included the windage of the entire vessel as well as rig and sail drag. Adopting an efficient rig is the beginning. From this point of view, modern Bermudian sloop rigs still offer the best results in terms of practicality, simplicity, strength and upwind aerodynamic efficiency. Traditional, low aspect ratio rigs can offer more power off the wind than a Bermudian sloop rig, but they are very unfavourable upwind. In itself, it should be a good reason for discarding them because they don’t allow pointing properly in high winds and this leads to a dangerous situation once the sea can break. It has nothing to do with racing. Ketch, schooner and yawl rigs all lead to much greater aerodynamic drag due the second mast and set of shrouds. In most instances, they make no sense today unless the vessel is so large that splitting the sail area becomes somewhat desirable for handling, cost or sail manufacturing reasons.

Regardless of rig choice, aerodynamic efficiency is the key starting point in remaining able to gain to weather in the breeze

One should therefore refrain from destroying the efficiency of the rig through a proliferation of undesirable attachments. Radar scanners should be installed on a post at the stern, rather than up the mast. Achieving range through scanner elevation is not a relevant consideration anyway. Protruding mast steps and ladder rungs installed in the lower shrouds are all bad ideas. Multiple headsail furlers are disastrous upwind, especially on small to mid-size vessels. The list is rather long.

Beyond this, consider the projected area of the yacht when viewing it from 45° from the bow and heeled 30°. Most of the profiling done seems based on a perfect head wind on an upright boat. This is not the way the wind blows at sea in a gale. Solar panels installed horizontally high up on arches don’t look so good when viewed from where the wind really comes from. Lower them down so they partly or completely disappear from view when the boat heels over. The local wind velocity is also much reduced at a lower elevation due to the vertical gradient and drag increases with the square of this velocity.

Ideally, from an aerodynamic point of view, one would want a perfectly clean-looking vessel, like a racing yacht. Practically, there is a need for generating some electricity and usually a desire for a degree of convenience that opposes the most efficient configuration. What can be tolerated is a question that needs answered case-by-case. Yachts with low resistance hulls and efficient keels don’t require as much driving force to keep progressing upwind and can often tolerate more windage than designs that already struggle to point properly in a minimal configuration. Windage can also be reduced significantly by removing offending features prior to undertaking any passage where heavy weather cannot be excluded, rather than “risking it”. This can include wind generators, dodgers and many more items that all contribute to increasing projected area. For some existing vessels normally confined to the Tropics, it may be the most practical option.

This is also a lot of gear that won’t be able to carry away and cause damage if solid green water sweeps the deck.

Upwind Performance as Wind Increases

Some of the strongest headwinds experienced at sea and away from land on the sloop Yarra were in the area of the gulf of Peñas in Chili. The wind was such that the deep-reefed main had to be handed in, leaving a single storm jib forward. In order to take the mainsail in, I had to progress sitting in the windward side deck, my back to the wind all the way. Standing up was out of the question. Once I reached the mast, I mostly leaned against it while working to resist the pressure from the wind. Under storm jib alone, the sloop was climbing upwind at 4.5-5 knots heeled hard, well-balanced, and the turret of the steering vane was exactly at 45°, i.e. we were sailing at that angle from the wind, true wind and apparent wind being very near the same in those conditions.

This ability to claw off any shore and punch into virtually any wind conditions encountered was one enormous safety factor for ocean cruising in this relatively small vessel, a safety factor many larger yachts were sorely lacking.

There are two aspects that are useful to dissociate and discuss separately when it comes to upwind performance: the tacking angle and the angle made good on the ground, in the absence of current.

 

Tacking Angle

Generally speaking, the tacking angle of a yacht in relation to the water (i.e. as read between compass headings, not in terms of ground course) is worst in very light winds and should first improve as the wind increases because the effect of the apparent wind is less prominent in the breeze than in light winds and the boat is less headed by its own speed-induced breeze. More importantly, it should then remain reasonably comparable all the way to the point where no canvas can be carried any more, allowance being made for sea state.

If the tacking angle readily deteriorates as soon as the breeze picks up, the matter should be taken seriously. It is a sign that driving forces are not keeping up with drag as the wind increases and sea state worsens and the boat will likely become plainly dangerous in bad weather. The condition of the sails should be checked. Beyond this, windage should be reduced as much as possible. One should also look at hydrodynamic drag, such as large fixed propellers if relevant, but also anything else triggering turbulence on the hull surface, especially forward as this turbulence will then grow along the remaining length of the hull.

The tacking angle controls the angle between the oncoming sea and the axis of the vessel. It is important in terms of ability to reduce risks in breaking seas and as a result it matters a great deal for offshore vessels. Anything over 100-110° quickly becomes extremely questionable.

Headsail furlers are extremely detrimental to the tacking angle and cause catastrophic degradation in high winds if a partly rolled sail is used, because they ruin the aerodynamics of the sail from the leading edge.

Ground Course or Angle Made Good

The ground course angle always deteriorates when the wind increases because larger sail forces need to be matched by a larger keel side force, which can only be achieved through a greater leeway angle once maximum speed has been reached. However, this difference only amounts to a few degrees with a decent keel configuration.

Some yachts are simply too inefficient, exhibit too much overall resistance and don’t develop the aerodynamic lift force required to be able to climb to weather in a strong breeze. While this also comes with a poor tacking angle, a lack of draft or a poor keel configuration can result in high leeway angles while the boat still appears to be pointing reasonably.

Appendage side forces are produced through a combination of angle of attack of the flow on the section and flow velocity. This means that one can trade leeway for speed and vice-versa, and while lift forces vary proportionally to leeway, they vary with the square of boat speed. The practical implication is that it may be better to allow the bow to fall off a little and sail faster than trying to point higher in circumstances where it is difficult to progress, as long as the angle of the boat in the sea is safe enough as mentioned in the previous paragraph.

Practically

Sailing upwind in bad weather is a strategy that has been used by some ocean sailors since yachts have been able to achieve it. The early French circumnavigator Marcel Bardiaux, who was never much inclined towards losing time and ground at sea during his 1950-1958 single-handed voyage on board the cutter Les Quatre Vents, had adopted it quite unilaterally against accepted views at the time. Forereaching is a low-key concept that many very experienced sailors resort to today, which is essentially pointing into it upwind, but at low speed, something modern fin-keelers are usually very able to deliver under a single deep-reefed mainsail. It is more comfortable and easier on the gear than sailing at full speed upwind – which can be incredibly violent – as long as the conditions allow it.

A yacht pointing into a breaking sea still needs enough momentum to cross the breaking crests without being thrown back, which would most likely have very unhealthy consequences for those on board. However, it would take a far bigger sea to achieve such a feat than what is needed to roll and completely disable a vessel lying in the troughs without speed.

The longer and the faster the sea, the more powerful the breaking crests, the more forward momentum one is likely to need to deal with them.

While my temperament often steered me towards simply punching into it, I occasionally resorted to forereaching with the sloop Yarra. One such instance was in the Southern Ocean while sailing from the bottom of New Zealand to Tasmania. Concerned about a westerly blow developing into much greater proportions, I had spent a night making limited progress under deep-reefed main just to see what was coming. In other circumstances, forereaching would have been out of the question due to the amount of expended energy present in the sea, but punching was a valid option.

When the boat balances adequately, sailing a fin-keeler under a single small headsail can be very effective for punching in heavy weather, but usually too fast for forereaching. Hoisting a storm jib and dropping the deep-reefed main can be a very effective way of “changing gear” and gather momentum in very strong winds if the sea conditions call for it.

The sloop Nordkyn, with its large mainsail and deep bulb keel, can achieve a broad range of speeds and angles upwind without headsail while maintaining perfect course control, from sailing very slow and very high to punching at speed. Sailing both very high and fast requires the use of a headsail.

As usual, it is a matter of observation, assessment and decision at the time, at sea. It is also useful to assess what the boat can do whenever an opportunity arises. Storm force winds blowing off the land sometimes offer a useful testing ground for evaluating upwind performance and such conditions are quite commonly encountered when seeking an anchorage in high latitude areas.