Tidal Levels

[Polaris]

So I've got my Tides exam coming up shortly, and I just can't get my head around the calculations for passing under bridges or passing over drying heights. Primary and Secondary ports for European and Pacific are a doddle along with the theory, but just can't get much further than the basics for these questions.

Tried to get my lecturer to explain it time and time again but his methods are not helping.

If anyone can help with pointing me in the direction of a good website it would be much appreciated.


An example question would be.

On entering a port a vessel needs to pass under power cables with a charted height of 11.5m The chartered depth at this position is 6.6m The vessel is sailing with an even keel draught of 7.5m and the mainmast is 18m above the keel. When passing underneath the cables the echo sounder shows a reading of 4.5m (MHWS = 9.0m.)

What is the level of tide at the time of passing under the cables? What is the air clearance of the vessel?

[gadget123]

Usualy helps to draw a picture of the whole problem with the measurements written on. Then you should be able to see which numbers are missing and add up/ subtract to get the numbers you need.

[Clanky]

Exactly what gadget says, draw a line at chart datum and work from there, think of everything only in terms of vertical height.

For the question above, I would suggest you do it in the following order.

- draw a chart datum line
- draw a seabed line
- draw the ships bottom from the echo sounder depth
- draw the ship's waterline
- draw a line at the top of the mast
- calculate the height of the waterline above chart datum
- draw a line at MHWS
- draw the power cable
- calculate the clearance from top of the mast to the power cable

A similar method should work for most questions.

The important thing to get your head round is what conditions of the tide chart datum depths are quoted at and what condition of the tide charted heights are quoted at.

[Polaris]

Thanks for the advice, think I did all rght. I seemed to grasp the concept literally the night before but the exam questions are never as difficult as you think they'll be.