Course to steer.

gravygraham

gravygraham

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Just been reading the 'Calculating a course to steer' article in April's MBM. Having spent a few £k's on fitting plotter screens and an autopilot etc on Lady Lou when we bought her last year, I'm wondering if I'll ever use my rusty Day Skipper skills again. Having said that, I can see the reward to be had with making a 'manual' navigation passage.

The article mentions 'the boat's [compass] deviation card'. All have these to hand do you?
 
technically (me being a bit sad really) if you are sailing about in tidal waters its worth working out a course to steer as you will use less fuel. If you just follow the rolling road on the plotter you end up travelling in an arc and hence further. saying that if you are only going a short distance there's probably not a lot in it.
 
Surely if you plot and then follow a course (on a plotter) from A to B, you'll be sailing along the ground track, as indeed you would if you'd built a paper course with tidal vectors? Must admit, we haven't got to grips with the rolling road yet.
 
Using auto helm on little ship can work 2 ways

Course to steer..... The boat will always steer the same course and will be influenced by the tide, that means you will be pushed down and back up as the tide ebbs and flows.

Go to...... The boat will travel in a straight line to the point, if over a full tide the boat will face into the tidal flow keeping a straight line to the set point.

Well I understand what it means :D

Tom
 
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If you simply follow the course to waypoint you will follow the non tidal compensated track, as a result you will likely end up steering a large sweeping curve if the tide / wind offset has any legs, this will make your distance through the water longer.

You can use the cross track error to assist reducing this, but some people end up steering a zig-zag, simply to stick to an imaginary ground track like their lives depened on it, which means a longer distance.

I simply check the Bearing to waypoint / destination as you start your leg, then trim the auto helm heading to keep the Brg to Waypoint constant, so if the BRG to WPT increases, increase your heading, if it decreases lower your heading. I find once the leg has settled, this means a few light tweaks is all that is required.

If the cross track error does start increasing then I reset the cross track, as it is inefficient to simply keep to a track line that has now changed. For this reason I do not set my auto helm to 'Track' as I then find the autohelm will insist the boat sticks too rigidly to the initial track.

If the tide is offsetting one way or the other I may add a couple of degrees to compensate one way or the other, say if I am heading from more sheltered water to a mid tide stream.

This is fine for short legs up to say 10 miles on most PBs travelling at say 15 + knots as the tidal offset is taken into the account in the initial setting up tweaks, and the rate of offset itself won't vary that much during the leg.

On a longer leg, say cross channel, I would study the tide and balance this against the estimated time for the overall distance. Then apply a few degrees of offset to compensate for the tide over the longer leg period. Then an hourly correction of a few degrees should keep you close to rhumb line (ground track), and thus shortest distance.

Now the important bit (IMHO) - if offshore (i.e. more than say 3 miles from land) I maintain a paperchart update of my position every 30 minutes or so. If the GPS set goes off for any reason then converting to old fashioned DR & EP will be from a recent proper position. You can also use the established Heading -v- Course made good deviation (if you have taken notice of this !) to assist with the onward predicted track under EP, for at least a further hour.

I have been mid channel in a nasty electrical storm and all external comms simply disapeared. I do not rely ever just on my GPS, no matter how fancy it is.

Whether offshore or not the chart for where I am is always on the chart table, even if this is the Solent from Hamble to Cowes.
 
gravygraham said:
Just been reading the 'Calculating a course to steer' article in April's MBM.

The article mentions 'the boat's [compass] deviation card'. All have these to hand do you?
Click to expand...

If you are using your boat's magnetic compass, you really need to know how accurate it is - hence compass it should be swung to establish the deviation. Remember the 1 in 60 rule - one degree error in magnetic heading will put you 1 nautical mile off course in 60 miles.
 
Piers said:
If you are using your boat's magnetic compass, you really need to know how accurate it is - hence compass it should be swung to establish the deviation. Remember the 1 in 60 rule - one degree error in magnetic heading will put you 1 nautical mile off course in 60 miles.
Click to expand...

You don't say if you have a deviation card Piers?

Come on then folks - how many of you have had your compasses swung? I'm guessing very few indeed. I haven't.
 
+1 for the deviation card

My take on things is that knowing first principles allows you to make quick fag packet calculations in your head to make sure things are going ok.

Purely relying on electronic equipment without paper knowledge can lead you into trouble.

For example: whilst you might not take 3 point fixes every 20 minutes (at 30 knots you'll need to be bloody quick in order to avoid mistakes) you can use the principles to check your position against marks. Do those 2 buoys line up? Does that mark line up with that tower? Does my heading seem about right knowing which way the tide is going and indeed do I think the tide is heading in the right direction to make sure I haven't read the tide tables incorrectly?

It's the same as when you come into port, you know what your predicted tide height should be so you look at the shore line, a pile or dolphin to visually compare the expected state to how much seaweed and crud is exposed. Some places even have a tide gauge to tell you the exact measurement.

Knowing basic paper navigating makes using electronics much easier.

Henry :)
 
Well, I must say I'm impressed Piers and Henry.

Out of curiosity, who does one call up for a compass swing. I've never seen adverts or anything pertaining to compass services.
 
gravygraham said:
Well, I must say I'm impressed Piers and Henry.

Out of curiosity, who does one call up for a compass swing. I've never seen adverts or anything pertaining to compass services.
Click to expand...

Google "compass adjuster". Robinsons is a well-known name for the Hamble, for example.

I tend to do the nav manually to sense-check the electronics. I often don't bother to write it down, though.
 
Robinson Compass Adjusters next to Cinnamon Bay Indian Restaurant in Hamble.

However it is not that difficult to establish deviation for yourself. Plenty of mentions in lots of boating navigation books, or simply google Compass Deviation, I am sure something will come up. Once you know your deviation it is simply a case of applying it in the same way as variation - East Least (or minus), West Best (or plus).

Whereas the 1 in 60 rule is correct, most helmsman will steer +- 5 degrees, and a wise navigator will quietly note the bias of an individual helmsman and compensate by instructing a higher or lower course accordingly, as this saves having to nag a less experienced helmsman. Also this means if your deviation is less than 5 degrees for any course practically it can be ignored, but if you have areas of the circle where greater deviation occurs then it should be applied as a correction.

More than 8-10 degrees should be investigated to find the cause.
 
gravygraham said:
You don't say if you have a deviation card Piers?

Come on then folks - how many of you have had your compasses swung? I'm guessing very few indeed. I haven't.
Click to expand...

I too am quickly forgetting the majority of my day skipper course but it must be knocking around the grey cells somewhere in the long term memory ready to surface in times of emergency, I hope!

Deviation card. yep, got one of those, it came with the boat when purchased last year
 
Keeping up with the chart work whilst using the plotter for the main nav makes good use of the time on passage, and allows excellent realtime practice, as the plotter becomes the 'instructor' to check your work as you go, and besides this gives practice in a real environment, not a classroom or with practice charts.

If you lose the lot, could you navigate back to a safe and recognisable landfall with just a compass and guess of the boat's speed ?

Why not try working up your DR & EP with reference just to your compass & tide tables / diamond, and whilst practising like this you can assess the water flow past to determine your speed with the log working. If you assume one day this will happen, if it ever does you shall be ready for it.
 
superheat6k said:
If you simply follow the course to waypoint you will follow the non tidal compensated track, as a result you will likely end up steering a large sweeping curve if the tide / wind offset has any legs, this will make your distance through the water longer.

You can use the cross track error to assist reducing this, but some people end up steering a zig-zag, simply to stick to an imaginary ground track like their lives depened on it, which means a longer distance.

I simply check the Bearing to waypoint / destination as you start your leg, then trim the auto helm heading to keep the Brg to Waypoint constant, so if the BRG to WPT increases, increase your heading, if it decreases lower your heading. I find once the leg has settled, this means a few light tweaks is all that is required.

If the cross track error does start increasing then I reset the cross track, as it is inefficient to simply keep to a track line that has now changed. For this reason I do not set my auto helm to 'Track' as I then find the autohelm will insist the boat sticks too rigidly to the initial track.

If the tide is offsetting one way or the other I may add a couple of degrees to compensate one way or the other, say if I am heading from more sheltered water to a mid tide stream.

This is fine for short legs up to say 10 miles on most PBs travelling at say 15 + knots as the tidal offset is taken into the account in the initial setting up tweaks, and the rate of offset itself won't vary that much during the leg.

On a longer leg, say cross channel, I would study the tide and balance this against the estimated time for the overall distance. Then apply a few degrees of offset to compensate for the tide over the longer leg period. Then an hourly correction of a few degrees should keep you close to rhumb line (ground track), and thus shortest distance.

Now the important bit (IMHO) - if offshore (i.e. more than say 3 miles from land) I maintain a paperchart update of my position every 30 minutes or so. If the GPS set goes off for any reason then converting to old fashioned DR & EP will be from a recent proper position. You can also use the established Heading -v- Course made good deviation (if you have taken notice of this !) to assist with the onward predicted track under EP, for at least a further hour.

I have been mid channel in a nasty electrical storm and all external comms simply disapeared. I do not rely ever just on my GPS, no matter how fancy it is.

Whether offshore or not the chart for where I am is always on the chart table, even if this is the Solent from Hamble to Cowes.
Click to expand...

Unfortunately, the shortest distance over the ground is not the fastest route if the tidal current is not constant - which is usually the case. An example of how to work out the best course is given here : http://www.sailingalmanac.com/Almanac/Navigation/crossingchannel.html
 
guernseyman said:
Unfortunately, the shortest distance over the ground is not the fastest route if the tidal current is not constant - which is usually the case. An example of how to work out the best course is given here : http://www.sailingalmanac.com/Almanac/Navigation/crossingchannel.html
Click to expand...

When we used to have a fast cruiser we'd just let the autopilot navigate the ground course. When we upgraded to the Fleming and started crusing at 8 kts, we began looking at how to use the tide as an aid to saving passage time and fuel rather than something to contend with. In other words, could we steer a constant heading allowing the tide to take us off course but bringing us back as the tide turned so we'd arrive at destination. At first we judged it 'by eye' at the planning stage by looking at the tidal rates hour by hour and using the 1 in 60 rule to offset the heading. This worked quite well, but we wanted something more accurate.

Rather than use the nav learned on courses to do this, we found Tide Plan www.compasscard.co.uk (no connection, etc) which for crossing the Channel has worked really well. And this brings us neatly back to the deviation card and the need to be able to steer an acurate heading.

So, you need your magnetic compass swung. You also need to know your autopilot's flux gate compass is calibrated (swung) as well, and that's something you can easily do yourself by following the instructions it came with.

Happy constant heading cruising!
 
henryf said:
My take on things is that knowing first principles allows you to make quick fag packet calculations in your head to make sure things are going ok.
. . .
Knowing basic paper navigating makes using electronics much easier.
Click to expand...

+1. Tidal/Wind efficient CTS is more of an issue for slow sailing vessels than fast motor craft with no leeway. From a fuel point of view generally one wants the shortest distance through the water rather than shortest COG (ie a straight line through a moving body of water which arcs over the ground assuming you'll clear hazards with your choosen COG). Knowing the basics enables one to mentally do this fairly well using DR.
 
LittleShip said:
Using auto helm on little ship can work 2 ways

Course to steer..... The boat will always steer the same course and will be influenced by the tide, that means you will be pushed down and back up as the tide ebbs and flows.

Go to...... The boat will travel in a straight line to the point, if over a full tide the boat will face into the tidal flow keeping a straight line to the set point.

Well I understand what it means :D

Tom
Click to expand...

CTS, compensates for the effect of tide!
 
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