Winds of Change WARNING. CONTAINS TRACES OF MATHS. On a day with no wind at all. A day with a gentle breeze

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1 WARNING. CONTAINS TRACES OF MATHS. Winds of Change Wind is the schizophrenic of the weather family. Sender of icy blasts but feeder of fires, dryer of washing yet bringer of rains. The word northerly in other contexts means toward the north, but in the case of wind, it means toward the south. In keeping with its ambivalent nature it can be both friend and foe to the kayaker. Another peculiarity of wind is that its speed and direction appear to change as a boat moves. This is not as strange as it might at first seem. On a day with no wind at all, anything moving, such as a kayaker paddling, will experience a breeze. This is called apparent wind and it appears to flow against direction of movement at the same speed as the boat moves through the still air. On a day with no wind at all Imagine a day with a gentle breeze and a kayak paddling at a true speed (i.e. relative to the sea bed) exactly the same as the speed at which wind is passing over the surface of the water. Heading directly downwind, there will be no air flow on the paddler as both boat and air are moving in unison the apparent wind speed is zero. If the boat then turns and heads directly into the breeze at the same speed, a wind gauge would show an apparent wind double the true wind speed. This results from the true speed of the kayak plus the true speed of the wind which is now moving past it in the opposite direction. Easy so far. Wind affects boat performance in many ways and to various extents. I seem to spend a lot of time paddling into or out of the wind, and noticed that some points did not seem to add up. To investigate I conducted trials which involved paddling at a steady effort for a fixed time. I used a GPS to record speed and distance covered, both up- and downwind, which revealed a puzzle. A day with a gentle breeze In gentle and moderate breezes before stability becomes an issue or surfing is possible, you d think that any extra time spent heading into the wind would be cancelled out by the time saved coming back down wind. The results of the tests however suggested that on a run of this type, the time taken over the two legs is greater than it would be in calm weather how can this be? Further tests were done to find out how wind affects a drifting kayak. First I measured speeds just drifting and pointing directly downwind, using a GPS as a speedo. However I 1

2 needed to know not just the drift speed, but the true speed of the wind that resulted in a particular drift speed. I used a GPS and an anemometer (wind speed gauge). This was easy when drifting down wind, however I was not recording the true wind speed, but the apparent speed. To remedy this I tried paddling just hard enough into the wind to hold myself still (by the GPS) and taking wind readings, but I had one hand too few. Then I realised that if I took both measurements while drifting downwind and added the drift speed to measured (apparent) wind speed, I could calculate the true wind speed. An anemometer used on land shows the true wind speed For example, drifting downwind at 3 km/h with a measured or apparent wind speed of 9 km/h means that the true wind speed is = 12 km/h. I soon found that in various true wind speeds up to 20 km/h, the drift speed in an unladen sea kayak was consistently about one quarter, or 25% of the true wind speed. It seemed likely then that when paddling into the wind, the speed achieved would be the normal calm weather rate of progress minus the drift speed. Applying this theory to a kayak paddled with an effort that would result in 8 km/h in calm weather: Into a headwind of 4 km/h, drift will be 1 km/h and progress would be 8-1 = 7 km/h. Into a headwind of 8 km/h, drift will be 2 km/h and progress would be 8-2 = 6 km/h. And so on until 32/km/h when drift will be 8 km/h and no progress will be made. Coming down wind, it seems obvious that actual speed would be equal to the normal rate of progress plus the drift speed. Consider a 16 km journey comprising a straight-line trip of 8 km directly into and out of the wind being undertaken at a level of effort that would always maintain 8 km/h in calm conditions. Time is distance speed, so on a dead calm day the paddler will take: 16km 2km/h = 2 hours 2

3 Now imagine the same paddler doing the same trip with the same boat, paddling with the same effort on a day with a steady 8 km/h breeze, and therefore a 2 km/h drift speed. The wind obligingly stays at the same speed and direction for both legs of the trip. You would imagine that under these circumstances, the extra time taken on the first leg would be cancelled out by that gained on the way home and the overall time taken would be the same. Wind picking up. Diane and Susan in Evans Bay Trials suggested that this is not the case and that the fickle wind somehow stacks the odds against us. To examine this and to test the progress-in-wind hypothesis, we need some assumptions: The wind blows at a steady 8 km/h and fixed direction for the whole period. The kayak and paddler drift at 2 km/h when no paddling at all is done. There are no significant waves that affect boat handling or which offer surfing rides. The out leg is 8 km directly into the wind and the home leg is 8 km downwind. The paddler always uses an effort that would achieve 8 km/h in calm conditions. There is no current or tidal stream. Returning from checking conditions outside Whangaparpara harbour on Great Barrier Island. 3

4 On the outward leg the paddler goes at 8 km/h from paddling effort - 2 km/h drift speed = 6.0 km/h Time = distance speed, so the outward leg takes 8 km 6 km/h = 1.33 hours or 1 h 20 m On the homeward leg the paddler travels at 8 km/h from paddling effort + 2 km/h drift speed = 10.0 km/h The homeward leg takes 8 km 10 km/h = 0.8 h or 48 m The total time for the trip is 1 hr 20 minutes out + 48 minutes home = 2 hours and 8 minutes This is 8 minutes longer than the 2 hour the journey would take on a calm day, which deals a severe blow to the time lost = time gained theory. One way to think of it is that the headwind slows the boat, and acts for a longer time, thus has an increased effect. Conversely, a tailwind speeds the boat up which reduces the time it has to act, so it is less influential. The net result is that, all other things remaining the same, in a straight upwind/downwind trip, the journey will always take more time than in calm conditions. If we work the same calculation based on a 25% wind effect factor for the journey in a 30 km/hr wind, we find that the downwind trip takes 31 minutes but the upwind journey takes 16 hours. An experienced paddler will know that, 8 km (5 miles) does not take 16 hours upwind, even against a 30 km (16 knot) wind. The 25 % factor may be accurate for drift rates, but it does not work if added/deducted to or from normal speed. In high wind, use a suitable craft. Multisport rudders don t even reach the water in 30 km/hr winds. Photo: Charles Jarvie Trying different values for the wind effect factor on the same basis, in a 30 km/hour wind, allowing a 20% wind effect factor, the trip would take 34 minutes out and 4 hours back. At 15% wind effect factor it would take 39 minutes out and 2 hours 18 back. 4

5 Time in hours for the trip These lower wind effect factors seem more reasonable, and the actual figure that applies will depend on the individual as each differs in paddle power and kayak speed will vary with the boat s lines and characteristics. While a boat may drift at 25% of wind speed, we can rule it out as a realistic wind effect factor % seems more reasonable. Times and wind effect factor can be charted Times taken for a trip 8 km upwind and 8 km downwind at different windspeeds for three wind effect factors Wind effect factor ± 15% Windspeed in km/hr Wind effect factor ± 17.5% Wind effect factor ± 20% Times for an out and back trip at different wind speeds and wind effect factors Why does the chart shoot up so high at the end? The link between speed and power is a squared relationship. If you want to go 2 times as fast, you need to provide 2 2 (i.e. 4) times as much power. By the same token if the wind blows you back twice as fast, you need to paddle 4 times as hard just to maintain the same speed. For geeks, the relationship is: Velocity = Power (Nm/s) In SI units: m s -1 = kg m 2 s -3 Force (N) kg m s -2 In this case, force is the resistive forces acting on the boat and paddler. If you double the velocity, the resistive forces double and therefore the power needed must be four times as much.if you quadruple the speed and the force, the power must be 16 times as much. Let s assume that trials proved that a wind effect factor of 20% was about right for someone in their normal kayak. Sea trials reveal that paddling into the wind is in reality faster than you d expect i.e. the boat will travel faster than predicted by the -20% of wind effect factor. Most people will know the 5

6 I m going to get left behind feeling that affects group paddlers at some point and which manifests as the I m not going to get there syndrome for expeditioneers. One explanation is that that against a headwind, we basically paddle harder. Had my initial tests been done using heart rate to monitor effort I am sure that the upwind effort would have been higher. Into and out of a 40 km/hr wind. It's possible to make reasonable headway into a wind of 40km/hr (21 knots) in an unladen kayak On the other hand, coming down wind generally does not make progress as fast as you might expect i.e. in general you may not gain 20% of speed. I suspect that this is due to the fact that as waves increase, more effort is required to keep the boat upright and on course, and consequently less energy is devoted to straight line speed. This holds good up to the point where surfing becomes possible and speed increases significantly. So, in considering the whole journey into and out of the wind, adjusting by a 15% to 20% wind effect factor seems reasonable. Individuals may be able to fine tune a more accurate figure for themselves. If making estimates of time needed for a journey using this method, remember to adjust your normal calm weather speed by 15-20% of the wind speed, not 15-20% of your calm weather speed. In the real world, we don t always head directly into or out of the wind. We travel across it at all sorts of different angles, and the apparent wind calculations become entirely more complex. For example, imagine a day with wind speed and direction of 10 km/h from 270 (due west). If we then paddle north at 6 km/h and could measure wind speed and direction, we would find that the apparent wind has shifted to 239 and increased to about 11.7 km/h. Strange 6

7 stuff. The relationship is a bit elusive and involves much more frightening sums. To avoid some serious calculations, use one of the apparent wind calculators on the internet. The calculations shown at the top of page 4 work for similar circumstances such as a bike trip into or out of wind, but working out the equivalent to drift factor is tricky, as it is impossible to use the same effort down wind. It also works for kayaking or canoeing up and down a river or in a tidal stream by substituting the average current for the drift speed. I have an idea that a similar calculation would work for situations such as biking up and down hills, but I am not cyclically inclined to test the theory. Sandy Winterton Most photos by the author. Others by Susan Cade June