Navigation

From CruisersWiki

Revision as of 21:02, 14 June 2012

Navigation

Basics

The basics of good navigation?

Navigation Tools

• GPS
• Sextant
• Radar
• AIS
• Dividers have dual functions. They are used to measure distances on a chart and they are used for determining latitude and longitude. Two varieties can be found. The less expensive version requires two hands to set. The traditional one-handed divider is more expensive.
• Plotters are used to determine bearings and courses on the chart. Most people are used to seeing parallel rules, but common plotters are Portland Plotters, Douglas protractors, course arm plotters.
• The hand bearing compass is an essential piece of equipment. Nautical hand bearing compasses are used to take bearings accurate to 1°. They cost more than a simple hand-held compass, but are worth the expenditure. They can also be used at night.
• Binoculars can be used to identify aids to navigation at a distance. Nautical binoculars are usually “7X50”. They magnify to a power of 7 times. Any binoculars more powerful than this are unstable on a moving boat. They have a 50-millimetre objective, or front, lens to admit sufficient light for use at night. Some binoculars have a built-in compass.
• Knowing the depth of water in which you may find yourself is crucial. Traditionally this was determined through the use of a weighted lead line which was “swung.” Knots along the line indicated the depths in fathoms. Electronic depth sounders use a technology similar to sonar. Depth sounders can be set to that they account for the depth of the keel. Ensure that you are aware of any offset programmed into a depth sounder.
• Knotmeters or logs as they are more correctly known measure the speed of the vessel through the water. The speed and direction of any current must be taken into account to determine the true speed across the bottom.
• A watch or clock is another essential navigational tool.
• Time, speed, distance calculations and other mathematical tasks are simplified using a digital calculator.
• A time , speed, distance computer, a misnomer, is a circular slide rule that can be used in place of a calculator. No batteries are needed.

Paper Charts

The Title Block

When first using a chart, several items of information on the chart require close scrutiny. Read the chart title block carefully for:

Scale

The scale of a chart indicates the degree of detail available. Small-scale charts are generally in the range of 1:40 000 to 1:80 000. On a small-scale chart you will notice that some aids to navigation are not shown or are shown by a small dot. There is a chart that covers the whole of the North Pacific. It shows a very large area, but very little detail. Large-scale charts are required for entering harbours and anchorages. They will show much more detail. All rocks and aids to navigation will be included. Remember small-scale charts have little detail while large-scale charts have lots of detail.

Type of Projection

The projection used will affect how distance is measured on the chart. On a polyconic chart, look for a distance scale. On a Mercator projection use the latitude scale.

Depths and Elevations

In considering depths two factors must be considered: chart datum and unit of measure. Chart datum is the point from which depths are measured. Canada and the United States use different reference points, the Canadian being shallower that the American. If using an American chart, use American tide tables; Canadian charts require Canadian tide tables. The units of measure can vary. Older charts may be in fathoms and feet; newer charts will be metric with depths in metres and tenths of metres. A typical sailboat has a draft of 6 feet. That is either one fathom or 2 metres. Be careful. Elevations and clearances are generally measured from the highest watermark. This adds a measure of safety. Again, Canadian and American charts use different reference points. As an aside, the areas of the chart shown in green (foreshore) are between chart datum and the high watermark.

Horizontal Datum

If you are using a GPS ensure that you have set the horizontal datum to coincide with that of your chart. The most common is WGS 84, which is equivalent to NAD 83.

Special Notes and Cautions

These call attention to the appropriate Vessel Traffic Services, restricted areas, and other similar information.

Date of issue and corrections

The date of issue is printed on the chart. A stamp indicates the date up to which the chart has been corrected using Notices to Mariners.

Electronic Charts

C-Map

Available as either a plug-in memory module for stand-alone Chart Plotters or as a DVD for installation on a notebook computer. C-Map uses the CM-93 format for charts. The included chart plotter software supplied by Jeppesen on the DVD - called ECS 4.01 - is well out of date but free alternatives like OpenCPN are readily found on the internet and can use the same map format.

When the C-Map software is used from DVD a GPS module is also required for the computer. These can be serial (old and usually not available on laptop/notebook PCs), USB (mechanically dodgy and unreliable), Bluetooth (can drop out at random) or Compact Flash formats. The last of these is preferred although you may need to buy an adapter to make it work in the PCMCIA slot if no CF slot is present.

Considerable savings can be made by using the DVD instead of a chart plotter, and the notebook computer has many other uses such as TV, video, music and internet. If you can find one of these DVDs it's a bargain at any price under $150.00 while chart plotters with equal performance go for $800 or so (June 2012) PLUS the cost of the chart modules.

The OpenCPN website has a large list of free electronic nautical charts on this page.

The Logbook

Can be integrated into chart plotter software (ECS), a stand-alone program or even the traditional physical book. This is a personal choice and really depends what you plan to do with it. Remember that most countries want a history of your last few ports of call before entry, so at least that much information should always be kept.

References

Publications, etc.

Forum Discussions

List links to discussion threads on partnering forums. (see link for requirements)

• budget setup navigation gps chartplotter notebook
• navigation courses
• celestial navigation questions
• navigation compasses binoculars deviation

External Links

• American Practical Navigator (Bowditch) Probably the definitive navigation text book, pdf download
• British Admiralty: The Mariners Handbook pdf download
• British Admiralty: Digital chart catalogue
• TideWizard Software for worldwide tide height predictions

Personal Notes

Personal experiences?

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