Tidal flow, rate and direction
We have all been to the beach and are aware of the tide flooding and ebbing. For this to happen there must be mass movement or stream of water from one place to another. Charts provide us with tidal stream information for certain positions. There are sixteen Tidal Diamonds on RYA Chart 3, three of them are marked in Fig 2.32 and are located in positions where the tidal stream rate and direction have been monitored through successive tidal cycles.
The magenta coloured Tidal Diamond letters on the chart correspond to the data table in Fig 2.33 which is located at the top of RYA Training Chart 3. This numerical data tells us the tidal rate and direction of water movement in relation to the tidal cycle with reference to Port Victoria, a main port on the northern peninsula. So at any given point in time we can ascertain where and how fast the tidal stream is moving at the position of the tidal diamonds.
From the information in Fig 2.34 We can see more clearly that at High Water (HW), the tidal stream is setting at:
Arrow A – 271° at tidal diamond A
Arrow C – 010° at tidal diamond C
Arrow D – 014° at tidal diamond D
These figures indicate the direction the tide stream is moving. You can see from the data on the tidal stream table that the direction of the tidal stream changes depending on what the state of tide is.
The numbers in the two columns to the right of the tidal direction (set) figure indicate to us the tidal ‘drift’ which is the speed or rate the water is moving (Fig 2.35). This is measured in Knots (nautical miles per hour). We are given two numbers, the Springs tides where there is a lot of water moving about and the Neap tide rate where there is comparatively less flow of water. Intuitively we know that the larger number corresponds to Springs tides, because we have more water moving about in the same time frame, it must therefore be moving faster. Conversely, during Neap tides there is comparatively less water moving during the same time period, so the rate of flow will be less.
Direction as a bearing
We measure direction by using a bearing system of 360° in a full circle. So 090° is east, 180° is south, 270° is west and 360° or 000° is North. We always use three digits to express a bearing even when it is like the bearing for east 090°. By using this system we can accurately express any direction. In Fig 2.36 we are using it to describe the set at tidal diamond position A – 270° and C – 010°