Paper Tidal Curves

Different authorities present their data in different ways. Although the curves will be similar, the method of extracting the actual heights for times other than high or low water will differ and may not be obvious to someone used to a different method. In the open sea and along coasts which do not alter the natural shape of the tidal curve (a sine wave) very much, a good rule of thumb that needs nothing except the heights of high and low water is the 'rule of twelfths', though you may find a...

Disadvantages of AIS

Only commercial vessels bigger than 300 GRT are required to fit it - you can see only vessels which have it fitted. It can transmit erroneous information. AIS Class B Transceiver If we fit a class B AIS transceiver (transmitter receiver) we must be aware of its limitations There is absolutely no guarantee that any commercial vessel will see our class B transmissions, either because of lack of output power, because our set cannot find a free slot or because of deficiencies of their AIS display....

Differences at Secondary Port

The method is almost identical to that used by UKHO. Remember to correct for local time after you've done all the other calculations. HEURES AU PORT PRI NCI PAL CE REFERENCE HAUTEURS AU PORT PRINCI PAL DE REFERENCE VE is equivalent to spring bdes ME Is equivalent to neap tides SHOM differences from standard ports

Electronic Tidal Curves

Without any doubt, these are the easiest and quickest ways to obtain tidal height information. Because each provider may use data from different authorities, there may be apparent discrepancies between different products. The differences may seem large at first glance, with times of high water sometimes differing by as much as half an hour, but when you look at the heights involved, these show that they are less than 0.05 metre (less than 2 inches) apart at the same time, so in reality there is...

Narrow Channel Marked by Buoys or Beacons at Frequent Intervals

This is the easiest form of pilotage just keep in the channel. At night, it's desirable to have a list of directions and distances between each buoy. In a curved channel, the channel will curve between individual buoys or beacons, so if you follow a straight line between buoys, you may run into shallows, which curve between them. You need to allow for this curvature. In a very narrow channel, the buoys, or especially any beacons, may be in water too shallow for your boat at low water, so don't...

Single Position Line Derived from Radar

Radar can allow us to measure both distance and bearing from a geographical position. Bearing is the least accurate and should not be relied on unless you have nothing better. Range (distance) is much more accurate. A distance from your boat to a feature as measured by radar will provide a curved position line, so you will need a pair of compasses to draw this on your chart.

Radar

Radar Beam Width

I he majority of boat owners buy radar for collision avoidance, for which proper training is required. However, radar is a very powerful navigation and pilotage tool as well. The radar scanner rotates at approximately 24 revolutions per minute, and while it does this, it transmits pulses of microwave energy. The time interval between each pulse is long enough to allow a pulse to travel out to the maximum range of the radar and back to the scanner. This means that the radar is listening for...

Course to Steer

UU nless you are operating in tideless and windless waters, you will need to calculate a course to steer to get from A to B. If you just steer the direct track between the two points, the wind and tide will take you in a different direction. At the first sight, it may seem a little complicated, but practice a few times and it should become second nature. One essential reminder is that unlike estimated positions (EPs), the tide comes first, not last In order to strive for clarity, the first...

Calculating the Depth of Water

The principal reason for using tidal height data is to check if the water is deep enough for your boat or that there's sufficient clearance to pass under a bridge or cable. You can use 'electronic' tidal curves if you have them - that's the easiest way - or you'll need to do it all on paper. It's a good idea to consult the chart for the general depths and to see if there are any rocks or shallow bits, but you won't be using chart datum or soundings. What you need to know is how much the tide...

Errors in Position Lines

A big advantage of visual and radar position lines is that you are not relying on the accuracy of the cartography to avoid hitting the land. The land is where you or your radar sees it, not where the GPS or the mapmaker tells you where they think it is. The position given by your position lines is correct in relation to the land, though it may not tally with the latitudes and longitudes shown on your chart - This is important if your charts have large errors, as some do. If your chartplotter...

Tidal Heights and Tidal Streams

Atmospheric Pressure Corrections Tide Tables UKHO Tidal Predictions SHOM Tidal Predictions Tidal Streams 11 is possible to obtain heights of tide for any day using 'online' sources. However, the navigator is often forced into a position where there's a need to calculate the height of tide at a particular time manually. There are a number of different methods of doing this, some more accurate than others. Where the tidal curve is of sine waveform, the rule of twelfths, as discussed in Chapter 5,...