Question 1

A bridge has a charted clearance of 6m MHWS. Your vessel's air draft is 6.5m. The current predicted HW height is 5.2m and MHWS at this port is 5.8m. Can you safely pass under the bridge?

Accepted Answer

The predicted clearance is 6.6 m, but the resulting 0.1 m margin does not establish that passage is safe.. The question specifies that the bridge clearance is referenced to MHWS. At the predicted high water, clearance is 6.0 + (5.8 - 5.2) = 6.6 m. This exceeds the vessel's air draught by just 0.1 m. Such a small predicted margin does not establish safe passage: actual water level, vessel motion and measurement uncertainty must be considered, with an appropriate safety margin. MHWS means Mean High Water Springs, not the highest normal tide. Clearance reference levels vary, so check the chart's stated datum and relevant notes.

Question 2

You are approaching a drying harbour with a bar that dries 1.2m. Your vessel's draught is 1.8m. The predicted height of tide is 3.5m. What is the depth over the bar?

Accepted Answer

2.3m, calculated by subtracting the drying height from the predicted height of tide.. DRYING HARBOUR calculation: depth over bar = height of tide − drying height = 3.5 − 1.2 = 2.3m. Your draught is 1.8m. Clearance = 2.3 − 1.8 = 0.5m. This is marginal, add safety margin (ideally 0.5m minimum under keel). Best to enter with more rise of tide available. Always check whether tide is rising or falling when planning entry to a drying harbour.

Question 3

You need to cross a shoal charted at 0.8m. Your draught is 1.4m. Safety margin required: 0.3m. What minimum height of tide do you need?

Accepted Answer

0.9m. MINIMUM HEIGHT OF TIDE: depth needed = draught + safety margin = 1.4 + 0.3 = 1.7m. Depth over shoal = charted depth + height of tide. So: 0.8 + height of tide ≥ 1.7m. Height of tide ≥ 1.7 - 0.8 = 0.9m. You need at least 0.9m of tide to cross safely. Check tide tables to find when height of tide exceeds 0.9m and plan your crossing accordingly.

Tidal Heights, questions and explanations

A bridge has a charted clearance of 6m MHWS. Your vessel's air draft is 6.5m. The current predicted HW height is 5.2m and MHWS at this port is 5.8m. Can you safely pass under the bridge?

You are approaching a drying harbour with a bar that dries 1.2m. Your vessel's draught is 1.8m. The predicted height of tide is 3.5m. What is the depth over the bar?

You need to cross a shoal charted at 0.8m. Your draught is 1.4m. Safety margin required: 0.3m. What minimum height of tide do you need?

Practise this properly

Tidal Heights, questions and explanations

Questions

A bridge has a charted clearance of 6m MHWS. Your vessel's air draft is 6.5m. The current predicted HW height is 5.2m and MHWS at this port is 5.8m. Can you safely pass under the bridge?

You are approaching a drying harbour with a bar that dries 1.2m. Your vessel's draught is 1.8m. The predicted height of tide is 3.5m. What is the depth over the bar?

You need to cross a shoal charted at 0.8m. Your draught is 1.4m. Safety margin required: 0.3m. What minimum height of tide do you need?

Practise this properly