Fig 6 Theta & Froude*40 vs Depth of water In fig 6 above, Speed of the vessel is kept constant at 5 m/s. Vessel is cruising in to shallow waters from deeper region. So, depth of water goes on reducing and so Froude number will shoot up. Theta takes a nose dive at a water depth of 5m. At this instance, Froude number is 0.7. 5. Mathematics of the waves Consider a case, where a vessel is travelling in a harbour channel of draught 15m. Speed of the vessel is 12knots (6.18m/s). Tide is one metre above chart datum. From equation (2),
From equation (3),
From equation, (1)
For intermediate waters, from linear wave theory,
This can be solved by calculator. Period of the wave is obtained as 3.23s. Now Change the speed of the vessel to 20 knots (10.294m/s) F is obtained as 0.822, θ= 31.104o and C = 8.814m/s Period of the wave works out to 5.885s 6. Height of waves Height of vessel generated waves depends on various factors. Vessel speed is the most prominent factor. There are other factors like type of vessel, water depth and distance from the sailing line to the location of incidence. A sample of a wave record is given in figure 7 below. It is interesting to note that wave height increases to a maximum quickly and dies down slowly.
Fig 7 Example of vessel generated wave data (Courtesy: Coastal Engineering Manual) Courage is going from failure to failure without losing enthusiasm
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