Depth of water and Tidal Level Tables
The charted depths are indicated on the charts by contour and spot depths but must realise that there will usually always be a certain Height of Tide above Chart Datum. We can use the Tidal Levels information on the chart to quickly asses the Depth of Water (Depth of Water = Charted Depth + Height of Tide)
It is calm weather and we want take the yacht between Inner Rocque and Middle Rocque Islands directly northeast of Old Dawson (Fig 5.13). There is a charted depth of 1.2m between the islands. Our vessel has a draft of 2.3m, which is the amount of water needed to keep the vessel afloat. The tidal level is at Mean Low Water Neaps (MLWN). Can we make it through the channel without grounding, if so what will the clearance be under the keel?
- Firstly we need to look at the chart and examine the passage between the island to confirm the least charted depth is 1.2m. This is the depth of water as measured from Chart Datum, the lowest astronomical tide level.
- There should always be at least 1.2m but usually more water at this position.
- We can now examine the Tidal Levels table (RYA Chart 4) and use the Old Dawson tidal information as it is the nearest and therefore the most relevant. We can read along the table for information and indicates that there is 2.4m tide at MLWN. This is the Height Of Tide above Chart Datum at MLWN
- With these two bits of information we can quickly see that Charted Depth + Height of Tide = the total Depth of Water, (1.2m + 2.4m = 3.6m). The depth of water between the islands is 3.6m at MLWN
- Our yacht has a draft of 2.3m and there is a water depth of 3.6m so we know we can make the passage between the Islands but would be wise to calculate the clearance between the bottom of the keel and the seabed at MLWN. Depth of Water – Draft of Vessel = Clearance (3.6m – 2.3m = 1.3m). We will have 1.3m clearance between the bottom of the hull and the seabed, at MLWN as we make the passage.
So we will make it through between the island at low water neaps MLWN. However it is a different situation at low water springs MLWS. If you go through the calculations above using the values for MLWS you will see that we will be grounding the yacht when making passage between the islands. There is obviously a Height of Tide between MLWN and MLWS that we will just be able to make the passage.
On a calm day we would want to leave at least 0.5m clearance under the keel and have an opportunity before and after MLWN that will able to make the passage without grounding. It is not immediately obvious from the Tidal Levels information found on the chart when we will be able to make the passage at intermediate states of the tide. Later in the module we will examine more accurately what times we can proceed through the channel so that we have a safe minimum depth. Height of Tide needed = Draft of Vessel + Clearance – Charted Depth ( 2.3m + 0.5m – 1.2m = 1.6m).