Sailing in SL - Introduction+
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Sailing in SL - Introduction+
This page forms an introductory course into the principles of sailing and how they apply to sailing in SL. It does not concentrate on any particular yacht but does identify how the principles are applied to the yachts within SL of which the author has experience. This page is therefore complementary to the basic TAKO sailing course held frequently at Starboards Yacht Club and other introductory information held on the wiki.
Sailing Upwind
Ancient Egyptian sailing boats (the first recorded yachts) did not sail upwind. Their use for trade on the Nile was based on the fortunes of nature. River currents brought the boats down river and the wind blew them back.
The essential advance in yacht design between then and now is the invention of the keel. Modern yachts can sail upwind very successfully – but note that they still cannot sail ‘directly into’ the wind.
The following diagram shows the forces acting on a yacht when sailing into the wind. The wind flowing across the sail (acting as an aerofoil much like aeroplane wing) generates the ‘sail force’; the keel acting against the water generates the ‘keel force’.
The following diagram shows the force components along and across a yacht. As long as the ‘driving component’ is greater than the ‘drag’ the yacht will go forwards. The amount of sideways motion (leeway) depends on the difference between the ‘side force’ generated by the keel and the ‘side force’ generated by the sails.
There is also a further factor caused by the action of the wind on the sails and that is heel. The pressure of the wind on the sails causes a yacht to heel. The effect of heel is to decrease both the ‘driving component’ and the ‘side force’, with a resulting loss of boat speed and an increase in leeway, both undesirable if you are trying to sail quickly. In RL the leeway made by a yacht can be observed by looking at the wake and the angle it makes to a yachts course.
In RL there is a further aspect to consider - that of water line length. The maximum speed of a displacement hull is proportional to its water line length, and a yachts water line length generally increases as it heels. There is therefore an optimum angle of heel that balances loss of keel force against gains in water line length.
So, what does this mean for the person at the helm?:
Firstly, a yacht cannot sail closer to the wind than the angle where the ‘driving force’ of the sails equals the drag of the yacht. This applies to RL and all SL yachts.
Secondly, a yacht must be kept at its optimum level of heel that balances losses to the sideways force with the gains made by increasing the water line length. In SL this probably only applies to Mothgirl’s Flying Fizz and Owen Oyen's ACC.
Sailing Downwind
Sailing downwind appears easier, even the Egyptians could do it, and it has not changed much since. A papyrus boat sailing dead downwind in 8 knots of ‘true wind’ would not have broken the 8 knot speed barrier and neither can any modern yacht design.
This is because as boat speed increases, the wind speed that is felt on board (the ‘apparent wind’) decreases. Suppose that you could sail dead downwind at 8 knots then the ‘apparent wind’ will decrease to zero. If you could sail faster than the true wind then it would turn into a headwind and the yacht would be in the situation shown in the previous slide. Sailing faster than the true wind (when sailing dead downwind) is therefore impossible.
The first diagram shows the forces acting when a yacht is going downwind.
The second diagram illustrates that boat speed plus apparent wind is equal to true wind.
The only way of increasing boat speed when going dead downwind is to minimise the drag caused by the keel. In a yacht with a fixed keel this is the designers job, in a boat with a centreboard (like the Tako) then it can be raised.
Reaching
Reaching is the fastest point of sail. Sailing faster than the true wind when on a reach is possible.
The diagram shows the reason. As a yacht moves forward then the ‘apparent wind’ appears to increase and come more from the bow than the true wind. As a yacht accelerates this tendency increases and feeds upon itself, and is limited only by drag.
Modern multi-hulls and windsurfers are the waterborne extremes of this, however the fastest yachts are ice yachts where the drag is so slight that speeds in excess of 100 mph are possible in a moderate breeze.
The fact that ‘apparent wind’ moves towards the bow as a yacht accelerates has implications for sail trim in RL and for those yachts in SL that respond to ‘apparent wind’.
Reprise - Points of Sail
The diagram shows the terms used to describe the points of sail. Note that ‘close-hauled’ is as close to the true wind direction that it is ‘practicable’ to sail. In SL the yachts vary in their behaviour when close to ‘close hauled’. Some lose all drive while some slow down more gently.
Helm's Role
The performance of a yacht for a given true wind speed and at various true wind angles can be plotted on a diagram called a polar diagram. In RL these are initially supplied by the yacht designer.
A typical polar diagram is shown. For Moonbeam in 12 knots of breeze the maximum boat speed predicted by the polar diagram is 8 knots at 105 degrees true wind angle. From the previous topic a bit of vector math tells us that in those conditions someone on the yacht would observe the apparent wind speed as 13.8 knots from an apparent wind angle of 60 degrees.
Note: firstly - the rapid drop in boat speed as a yacht gets too close to the wind; secondly – that the maximum speed is shown against a reach; thirdly – that boat speed again drops off towards ‘dead downwind’.
The consequence of the latter is that ‘generally’ the fastest way of sailing to a point ‘dead downwind’ of a yacht is in series of zigzags but gybing between each zig and zag. Comparison should be made with going directly into the wind in which a tack is made between each zig and zag.
Some more vector math is needed (or ask the designer) to determine where the gain in boat speed more than compensates for any extra distance traveled in order to determine the optimum upwind and downwind angles.
The role of the helm is therefore to plan and execute a route for the race course that takes into account: Wind Speed; Wind Direction; Polars; Hazards (islands, shallows, etc.); and (importantly) other yachts.
Sail Trimmer's Role
Each boat within SL has its own set of controls. But the principles are similar for each.
The aim of the sail trimmer is to set the sheeting angle to the optimum value for the direction that the boat is sailing.
For this the trimmer needs to make use of both the wind angle and sheeting angle information that is given on a hud or as floating text as appropriate.
The role of the sail trimmer is therefore to keep the mainsail sheet angle at half the wind angle. For the Flying Fizz the wind angle used should be the ‘apparent wind angle’. (note that ‘true wind angle’ is also displayed and be aware of the heel angle to avoid a capsize).
Where a spinnaker is fitted and flying then the sail trimmer also needs to adjust its sheeting angle. For the Tako it should be set to the value of wind angle less 90. The gennaker on the Flying Fizz is adjusted together with the jib and mainsail and is deployed automatically.
In RL the wind never stays still and it is co-ordination between the sail trim and helm that makes a yacht go quickly. In SL the windsetter and a yachts response to it simulate the RL variability to wind. For most yachts only small adjustments are generally necessary to the sail trim once it is set for a race leg. The Flying Fizz is slightly different – variance in yacht speed make a significant difference to the ‘apparent wind’. Sail trim (or course) must be adjusted to suit.
Upwind Strategy (Introduction)
In RL and SL the wind direction varies over time. Generally courses are set so that the base wind direction is blowing directly from the first turning mark (windward mark) to the Start Line. If so then all that is necessary is to sail the yacht so that it makes maximum VMG (Velocity Made Good) and pick appropriate tack points.
The windsetter may be set up so that there is an oscillation in the direction from which the SL wind blows. The default for NYC is plus or minus 15 degrees.
Consideration of from where the wind is currently blowing and which direction it is going can help the decision as to which tack to take first.
In general take the tack that makes the most gain towards the windward mark and sail towards an anticipated wind shift (and still sail the yacht for maximum VMG)
The diagram illustrates the advantage that can be made – both yachts have sailed the same distance.
Downwind Strategy (Introduction)
Downwind strategy is similar to that of Upwind.
Sail on the gybe that makes the most gain towards the leeward mark but sail away from any anticipated wind shift.
The diagram again illustrates the advantage that can be made.
Reaching Strategy (Introduction)
The shortest distance between two points is a straight line and it might seem that pointing a boat at the next mark and setting sails is therefore the fastest solution. It can be - but be aware of the conditions where it isn't.
A look at the Polars (again) will help you make the decision. An indication that a straight line may not be fastest is given when there is a significant speed jump at particular wind angles (e.g. the Tako when the Spinnaker can be deployed or the Fizz when the gennaker deploys, both at 90 degrees true wind angle <- is this true for Fizz?).
There may be advantages in sailing a lower course (than needed to lay a mark) at a higher speed and then sailing higher than would be the straight line case (albeit slower) for a shorter distance.
As an example consider an approximation to the leg from the Purple to Red buoys on the old NYC F4 course.
For the purposes of the approximation I will assume that the buoys are 640m apart N-S and 128m W-E. The shortest distance between them is the direct line of 653m. We'll also assume that the wind is blowing from the East.
Now assume that on that direct line (True Wind Angle) a yacht can sail at 7.5 m/s, or it can sail due South under spinnaker at 11 m/s and on a course of 146 degrees (True Wind Angle 56) at 6 m/s. In the former case the yacht will take 87 secs to complete the leg (653/7.5). In the latter case the yacht will take 79 secs to complete the leg (448/11 + 231/6). Those numbers are arbitrary (though approximate the Tako) but do not represent the optimal solution for the leg described.
As another example consider Moonbeam again. The polars suggest 8 knots on the reach and 5.8 knots on the fetch. Even at those speeds there is less than 5 seconds (this time in favour of the direct route) difference.
