Module 3 -
Navigation
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In this module we are going to learn how to get from A to B and to keep a
track of where we are when there is nothing to fix with. Nothing to see but
sea!
Topics covered in this module are - Dead Reckoning.
- Estimated Position.
- Course to Steer.
- The Running fix.
- GPS fixing techniques
Including leeway drift and errors.
There are a few terms and standard notation to revise first:
On some courses, particularly when the wind is on the beam or forward of
it, our water track is governed by two forces:
This method is used to work out our position when we cannot see any marks, or we cannot positively identify the marks we can see. Here is an 8 step guide for plotting a Dead Reckoned position: - From a known position (Fix), note your log reading and time. (4 M at 0900UT)
- Then steer a course. e.g. 100° Magnetic.
- After some time, perhaps an hour on a passage, take a further note of the log. (8M at 1000UT).
- Take the first log reading from the second and you have the distance run in the hour since you took your fix.(4M)
- Next reduce your course steered to a True course... eg, 100º Compass. No deviation. 5° west variation becomes 095° T.
- Plot this bearing from your fix.
- Open your dividers to measure 4 miles on the latitude scale.
- Place the dividers on the fix and mark off 4 miles along the course. This is your DR position.
Dead reckoned positions are useful when you need a quick idea of where you are or when there is no tidal stream. After the last module though, you know the tide might be going somewhere other than where you’re going and faster! So we need a better method……
This method takes account of both tide and leeway. Here’s a four step guide to working out your Estimated Position. - Plot a DR, but include leeway.
- Find out the tide experienced in your area between your fix and the time of the EP.
- From your plotted DR position, plot the tide
- The end of the tide is your EP
Let’s work through an example.
Step 1. We are crossing the channel from France and luckily sight EC’A’ Buoy (50º 01’.0 N 005º 10’.6 W). The time is 0300 BST and the log reads 20
miles. We are heading for Fowey. Our course is At the buoy draw a circle and put the time and log reading next to it. We also have a If that is a bit difficult to understand, try to remember: -
With the dividers measure 8 miles (minutes of lat) opposite your position and mark that off along your water track including leeway, to produce a DR. So far, we have completed Step 1 of the Estimated Position
Step 2. By using either the Tidal Diamond on the chart or the Tidal Stream Atlas, we find that the tide over the LAST hour in our area has been 230ºT at 1.2 Knots.
Step 3. From your DR position, plot a line at 230º T for 1.2 miles.
Step 4. At that point draw a triangle and write down the time and log reading. That is your new position and your vessel has travelled between the circle and triangle, not along the two lines! The line which joins the circle to the triangle is called the Ground
track. If you measure the distance between the two, you will see your
vessel’s speed made good or
Speed over the ground. Notice that we are not making a good ground track for Fowey. We can either carry on doing one hour EP's, plotting the tide and Ep every hour like this.
Or if you are on a longer trip or just haven't had time to do an hourly EP, you can just add up your distance run, plot it then plot all the different tidal rates and streams on the end. As you can see, the result will be the same. If you had to alter course for some reason eg tacking to windward, then you simply plot your water tracks including the leeway, then plot the tide on the end.
Occasionally you will need to use this technique (EP) to look forward in time
to find out where you are going to end up. Your log will tell you how far you
have gone, say in the last half an hour. So you assume that speed and can
Estimate where you will end up in half an hours time by plotting your suspected
water track and then the tide on the end. This method is a guide only as your
speed may change over the period.
Using Estimated position has lots of error. - Your log will not always give you a true distance covered.
*See Module 1, Logs* - Your course steered may well not be accurate, due to helmsman error or
various compass errors.
*See Module 1, Magnetics* - Leeway is hard to judge accurately, even with experience.
Because the boat does not go where we point it we need to be very careful. If there were a danger along the ground track then we would have hit it! Remember EP is finding out where we have been! not where we are going. When the wind is against us we have no choice about our course. We steer as close to the wind as possible. Usually about 45º off. We have to EP.
This is the ships log. A legal record of the yachts activities. You must keep
a log. There are two reasons for keeping it - It's the law
- It is impossible to navigate effectively
**without**one.
In the log, you keep a note of all
This is where we take control of our destiny and work out a course to make best use of wind and tide, rather than simply pointing the boat and suffering them! To learn how to shape a Course to steer, we will use the same situation as in EP above. Here is a 5 step guide. - Plot your desired Ground Track.
- Find out the tidal rate and direction in your area for the
**next hour**. - Work out your True Course to Steer, allowing for tide.
- Counteract your leeway.
- Convert to the Magnetic Course to Steer.
Step 1. We have a fix at the EC’A’ buoy. It is 0300. The log reads 28 miles.
Wind is northeast and we have found that the tide will set at 230ºT and its
rate will be 1.2 knots over the
Course to steer is planning into the future.
Using the simple formula. "Dinner Time Session" (I was taught this one by a Royal Marine Who was terrible at math’s, but loved going to the pub at dinner time!)
To explain this, let’s see when we will leave the "area to be avoided" around our buoy? We know from measuring the ground Track, that our speed is 7.8 knots. I make the magenta limit line to be 0.7M from the buoy. Put the numbers into the formula: - Therefore, to find the time taken to leave the area, we use the formula as follows;
To convert the time from hours to minutes, just multiply by 60 = 5.38 minutes.
Time equals Distance over Speed. (Multiply by 60 to convert hours to minutes) Speed equals Distance over Time. (Change minutes to hours first - divide by 60) Distance equals Time multiplied by Speed. (Change minutes to hours first - divide by 60)
Even if you are only using your course to steer for a few minutes, you still draw the tidal vector for an hour. All those lines and arrows are a vector diagram. If your chart covers such a small area that you cannot plot the boat speed and tide for an hour then Mini vectors If crossing the channel or your destination is more than one hour away, you could decide to compile a multiple tidal vector to stay on the fastest ground track over the whole passage. There are a few problems with this method. First you must be sure that the courses steered do not take your vessel over or too near to dangers. Second, for this method to work, your speed needs to be relatively constant throughout. (difficult on a sailing vessel!) Here's how.
Take a bearing of the lighthouse and note the time and log reading and course steered. Wait until the bearing has changed by at least 40 degrees, then retake the bearing and note time and log again. Plot fix like this. On the chart, plot the first bearing to the
lighthouse. Then from anywhere on that line draw out your course
steered. Mark off the distance run along this line. (logged distance
between the two bearings) On the end of your distance run, plot the tide for the
time between the two bearings. Plot the second bearing.
Ok now a few words about cheating! We all have GPS these days and here are a few methods to make good use of the machine
Lets assume that we are tacking into Torbay in a westerly wind, in the dark.
there are only two of you aboard so not much time for navigating!
We are now on our way to Holcombe Bay. its still dark and we are two up, but
at least the winds behind us for a change! Unfortunately the leading lights are
obscured by bad vis. A cross track error ladder can show us quickly and easily
where we are. Again with out plotting Lat lon. You cannot see the scale on this picture, but I hope you get the idea. the GPS says XTE 0.3M and the distance to go (to the waypoint) is 1.5M so that's where we are!
That’s all for this one. I hope you know the meaning of a line with one, two, or three arrows on it. You know that in navigation, the yacht’s position is not known exactly, there are lots of errors, but by keeping a good record and careful work you can reduce those errors sufficiently to make your EP usable. If by some miracle the wind allows you to go where you want to, you can lay off a course to make the best use of the tide and you will know how long it will take to get there. You are never going to forget that boats don’t go where you point them, they are always going somewhere else, and that somewhere else is governed somewhat by your course steered, but also by leeway and tidal stream. |