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Shiphandling Under Sail

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Shiphandling Under Sail Sail Theory Grade III Wind • True Wind is the actual wind force and direction. If you are sitting still this is what you feel • Relative Wind is the wind force you are creating with your motion. On a perfectly calm day, you will feel your relative wind as being exactly contrary to your motion. (If you head north at 10 kt, your relative wind will be a southerly 10 knot breeze) • Apparent Wind is the combination of both. This is the wind you are actually experiencing (Apparent means readily seen or open to view) Apparent Wind • Some points to remember: – If you are going upwind the apparent wind will be greater than the true wind – If you are going downwind the apparent wind will be less than the true wind – Wind direction will also be affected, becoming somewhere in between true and relative wind. – The amount of change in either speed or direction will depend on the relative strengths of the wind, particularly the strength of the true wind • 25 knots of true wind will not change very much for vessels of our speed, 7 knots will be readily affected. Calculating True Wind • You can use vector diagrams to calculate true wind. • One leg will be the vessel’s vector (relative wind), one leg will be the apparent wind as measured, the resultant is the true wind. • For a rough idea you can just look at the waves for wind direction and strength. Sail Theory • When sailing off the wind or downwind then sails just catch wind and get pulled along. • If you are sailing upwind then the sail is acting as an airfoil. This is essentially a wing, and if viewed from above you will see how a sail resembles a wing in cross section. Wing Sail Sail Theory • A neat thing about studying sail theory is that it is still largely a theory. There are good ideas on why and how sails work, but none have been proven to be entirely true. • For our purposes though, there are two principles that have been proven to work on a sail, regardless of what may or may not be going on at a more theoretical level. Sail Theory • Bernoulli's Theorem: States that in the case of a fluid in motion, the ratio of speed and pressure will, to a certain degree, remain constant. – This means that as the velocity increases, the pressure will decrease. Sail Theory • The Venturi Effect: Is an applied instance of Bernoulli's theorem, in which a fluid is run through a tube of decreasing diameter. – Using a tube of decreasing diameter, a Venturi tube shows the decrease in pressure that accompanies an increase in flow rate. Lower Pressure High Pressure Quick Flow Slow Flow Bernoulli's Theorem and the Venturi Effect, as Applied to Sails • Although this is an oversimplification of the phenomenon, the air travelling over a wing or a sail acts in a similar way to that fluid travelling through the tube. – Because of the curve of a sail and a wing, the air travelling over the forward/upper edge is compressed, increasing the flow rate and decreasing the pressure, while the air over the aft/lower edge is unchanged. Diagram of a Wing Fast Air Low Pressure Slow Air High Pressure Forces at work • The wind acts upon the sail • This force is transferred through the rigging to the hull (This is an important point. Next time you have a chance take a look at the attachment points for the running and standing rigging as well as the masts, all force is being transferred through these relatively small points of contact) • The force is then transferred into motion, and the keel works to translate as much as possible into forward motion. Leeway • Leeway is the sideways movement of the vessel. – The boat makes more leeway when going upwind than downwind – Leeway also increases with stronger winds and particularly with large waves – It is important to calculate for it when navigating, and you can get an idea of the amount of leeway by taking a bearing down your wake to see what the difference between course steered and CMG is. CE vs. CLR • CE is centre of effort. This is the theoretical centre of the forces acting up on the ship (sails each have their own CE, the sum of them all will create an overall CE) • CLR is the centre of lateral resistance. This is the theoretical centre of the underwater surface of the ships hull, it is the pivot point. – The CLR will move slightly depending on angle of heel, fore’n’aft trim and speed but is essentially stationary for our purposes. – The CLR is located slightly forward of midships CE vs. CLR • When setting and dousing sails you move the CE. Depending on where the CE is you can have a lot of effect on the behaviour of the vessel. – Knowledge of how to manipulate this will allow you to get the most out of the vessel when sailing and when maneuvring • Easing a sheet or sheeting it in will also affect CE, so minor adjustments can be made without changing sail. CE vs. CLR • The CE can also raise and lower (discussed later) or move athwartships. • When going downwind you will want to keep CE close to the centreline, or if it must be offset keep it as forward as you can. – Sometimes we reef the main for steering purposes, or even take it all together when running – A stuns’l can help balance this out Steering and Balance • Remember that to steer the ship you need water flow past the rudder. The faster you go the more responsive it will be. • While sailing you can also use the balance of the sail plan to assist in steering – If your sail plan makes the bow want to point into the wind you have Weather Helm – If your sail plan makes the bow want to point downwind you have Lee Helm • You usually want to have the forces almost in balance, otherwise you will need to use a lot of rudder which is inefficient as it creates unnecessary drag – Well balanced sail press will allow the boat to stay on course with almost no input Points of Sail Points of Sail: A: In Stays B: Close Hauled C: Close Reach D: Beam Reach E: Broad Reach F: Run Wind In Stays • This is when the wind is dead ahead. • Generally we avoid this point of sail, but when we tack we go through it. • It is also a good way to stop the ship down in a real hurry: Square the yards, sheet in the main and head up. Close Hauled • Apparent wind will be stronger than true wind. • All sails are acting as airfoils • On modern high-performance boats this is the fastest point of sail. – The design allows the boats to get to very high speeds, and they can often sail faster than the true wind speed. • For the brigantines this can be a very good point of sail as every sail is drawing well, but it is not our fastest. • Generally speaking we will keep CE aft of CLR, so as to create a slight amount of weather helm, which helps in pointing and making it to windward. Close Reach • Apparent wind is still stronger than true wind, but less so. • All sails are acting as airfoils • Sail trim is more or less the same as for close-hauled. • CE should be about balanced, or slightly aft Beam Reach • Apparent and true wind will be roughly equal • Sails are still acting as airfoils • CE and CLR should be roughly balanced Broad Reach • Apparent wind will be less than true wind. • Squares are being pushed now, fore’n’afters might be getting pushed, might still be acting as airfoils • This is the fastest point of sail for our boats. – The large sail area can be fully utilized, and as the brigantines are not modern lightweight craft they get pushed more efficiently than they are pulled. • Keep the CE forward, otherwise the boat will try to round up and broach. Running • Apparent wind will be less than true wind • All sails are being pushed • Not as fast as broad reaching as the forward sails are somewhat blanketed by after sails • Keep CE forward Sailing by the Lee • This is when you are going downwind and the wind is coming over the same side as your fore’n’afters are out on. – This is to be avoided for a few reasons: • Inefficient, you are blocking the forward sails • Major danger of an accidental Gybe Evolutions • An evolution is the term given to the actions taken whenever any major alteration is done to the sail press – This includes Tacking, Gybing, Wearing Ship and Bracing – It can also include setting/striking sail and heaving to Tacking • When beating to weather it is very important to keep good, tight sail trim and to make sure your helmsman is steering full-and-by (not just full!) • Hold each tack as long as possible, every time you tack you lose speed and are momentarily stopped. – If two boats sail the same distance, one taking 2 long tacks, one taking 4 short tacks, who will get there first? Tacking • Surging is an important aspect of tacking. – This is when you put the wheel over slowly enough that you use the momentum of the ship to ‘surge’ to windward before you pass through the eye of the wind and lose your headway. – If done properly you can gain a boat length or so directly to windward from each tack, this adds up to a lot of ground over time.
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