Clearance heights using Tidal Levels Table
Clearance heights are indicated on the chart and give the clearance height in metres from Highest Astronomical Tide to beneath a bridge, pylon etc. We would expect there to be at least this clearance height, if not more at all states of tide. We can use the tidal level table on the chart to quickly assess the situation. Instead of the draft of the vessel under the waterline we are more concerned with the air-draft or how much structure we have on the vessel above the waterline.
We are on a small sailing keelboat with a mast measuring 5m above the waterline. We would like to take the vessel under the Hinder Island bridge leaving at least 1m clearance between the top of the mast and bottom of bridge (Fig 5.15). We can call the height of the mast and the safety clearance we need, the yachts air-draft. Its getting near High Water Neaps MHWN, will we make it under the bridge?
First we examine the chart to see that the clearance symbol as indicated by one bar over and one bar under the clearance says there is 4.5m. This is the measurement between the bottom of the bridge and the Highest Astronomical Tide level.
We know that we have a 6m air-draft. We obviously do not have enough clearance to get under the bridge at the highest Astronomical Tide level!
We need to work out how much the tide will fall from a HAT level to the MHWN level. Looking at the Tidal Levels Table (RYA Chart 4) for Endal Bay we can see that the Fall of Tide = Highest Astronomical Tide – Mean High Water Neaps = 5.1m – 3.5m = 1.6m. So in other words the MHWN tide level is 1.6m below the Highest Astronomical tide level.
So we can no work out if we can get under the bridge. Total bridge clearance = charted bridge clearance + fall of tide = 4.5 + 1.6m = 6.1m
We are going to make it under the bridge safely at the MHWN tide level and do not have to wait for the tide to drop further. Remember our air-draft for the boat, 6m, has a safety factor of 1m already built in, so in actual fact we have a total clearance of 1.1m. As you will notice, it would have been a totally different situation at high water springs and may have been quicker to sail around the outside of Hinder Island rather then wait for the tide to drop to get under the bridge.