This would be a four second, 25m long wave. So, if you have enough wind force to travel at 8kn in calm water, that will enable you to surf steadily on a wave travelling at up to 12kn. Firstly, if a yacht is moving at say, 8kn in calm water, then you put the boat in waves with the same propulsive force from the wind, you can pick up and surf-ride a wave that is about 50 per cent faster than your calm water boat speed. The tests revealed two important findings. They happened to be interested in high speed powerboats, but subsequent tests, Kan, 1987 on a range of hull shapes, led to similar generalised conclusions. There is a very old and oft forgotten piece of research (du Cane and Goodrich, 1962), where they measured what happens to boats travelling down waves at model scale in a test basin. Let us park this bit of knowledge about rudder effectiveness for the moment and move onto surging and surfing. This is what gives the helm that soggy helpless feel just before the waveĬonversely, the orbital velocities in the trough are 4kn in the opposite direction, giving you a healthy 11kn flow speed over the rudder, if you can maintain boat speed. A 10 second wave of 7m height, has an orbital velocity of about 4kn so, when the yacht rudder is on the wave crest, the flow over the rudder is just 3kn (7kn minus 4kn). If a boat is sitting with its stern on the crest of a long wave doing, say, 7 knots, then the flow speed over the rudder will not be 7kn, it will be 7kn minus the orbital velocity of the water particles at the wave crest. The greater the speed of the water flowing over the rudder, the more steering force it can generate. And now the naval architectureĬonsider the effect of those orbital velocities on the effectiveness of the rudder. ![]() Our example wave, of period 10 seconds and height 7m, will have a wave particle (orbital) velocity of about 4kn. This circular movement of the water particles is given the term ‘orbital velocity’. This can be proven mathematically and verified by putting a cork in the ocean and tracking its movement. They travel with the wave at the crest, in the opposite direction at the trough and vertically at the mid-slope in between. The particles of water do not travel with the wave, they go round in a circle: see Figure 2. Let us return to the issue of the speed of the water particles making up the wave. If its height is more than about 20m it will break. It would probably have a height of around 7m but it could be as high as 14m. The oceanographic equations show that it would have a length of 150 metres and travel at 30 knots. To put this in perspective, a typical deep ocean wave might have a period of ten seconds. It is important to note that the particles of water in a wave do not move at this speed, it is the speed of the shape of the wave we are talking about here more on that in a moment. Speed is the speed at which a wave crest moves across the surface. Steepness can be many things, but an easy definition is height divided by length Length is the distance between successive crests Height is the vertical distance from peak to trough Period is the time between successive crests passing ![]() ![]() ![]() Fortunately we can reduce this to just height and speed because length, period and speed are directly linked, as we shall see in a moment. Waves can be described by height, steepness, length, speed and period. I am not going to examine other storm tactics such as heaving to, lying a-hull, or active steering. We will also only consider the situation where the yacht is travelling in exactly the same direction as the waves broad reaching brings in extra complications. I also assume we are in deep ocean, not coastal waters, as the physics of coastal waves is similar but the final numbers come out different. I shall assume we are dealing with a yacht sailing in a regular pattern of waves all from the same direction, none of them breaking over the boat. That is much too complicated to investigate mathematically, so I follow standard engineering practice and strip it down to something simple enough to understand, but not so simple that it is meaningless. The ocean is a mish-mash of waves coming in all shapes and sizes from many directions. Debate still rages over whether you should deploy drag devicesto slow a boat down, or whether you should run as fast as you can in order to maintain steerage.
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