GPS Repeaters

[webcraft]

Re: Compass display vs highway

Using XTE and continually correcting your heading to stay on track keeps distance travelled over the ground to a minimum.

Most navigators believe, like yourself, that drawing a traditional triangle taking tide and leeway into account for every hour on passage and adjusting your course accordingly will minimise time on passage, but that on some passages (eg cross-channel) it is possible to work out and allow opposite tidal effects on passage to cancel each other out. Either of these traditional approaches can be rendered very much less efective by inaccurate tidal information, inaccurate estimates of leeway or inaccurate predictions of boat speed based on inaccurate forecasting of sea or wind conditions.

I have seen it argued that using XTE to minimise distance travelled will have exactly the same effect as doing the full calculations even when everything goes according to plan. I am not enough of a mathematician / geometrician to work this one out - perhaps others would like to take this up. I certainly do not believe that you would beat me every time across the channel if I followed a direct track and you used all your navigational wiles - although you might a majority of the time.

However, I think you missed the essence of my post. The person who started this thread sounded as though they wanted to make the GPS their prime method of navigation. Bearing this in mind, what I said was that XTE was more important than heading to waypoint. By following your track you are certain of getting to your destination safely - the primary objective, surely. Blindly adjusting course to follow a moving bearing to a waypoint, however, may well end in disaster.

Of course anyone who is comfortable with GPS and trad nav can mix and match the information in a myriad of ways - using the drift off track or heading change to waypoint over time to calculate leeway, working out clearing lines of lat and long on the GPS etc - but if unsure, plot a safe track and stay on it - works every time. It's also an accurate way of crossing shipping lanes at the prescribed right angle.

- Nick



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[Oldhand]

Re: Compass display vs highway

Your welcome to stear your bow into the current to keep your XTE minimal, most of us find it faster to go with the tide as this obviously increases speed over the ground. Some of us make channel crossings in 9 hours or less and the currents don't cancel out but if we followed your principals the trip would probably take 12 hours.

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[webcraft]

Re: Compass display vs highway

Maths please

- N

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[peterb]

Re: Compass display vs highway

It's not distance over the ground that matters in terms of time taken, it's distance through the water. And that is minimised by holding a steady heading all the way across. If you try to hold zero cross-track error (XTE) then you will be continually heading into the instantaneous tide, and hence travelling further than if you just aim off sufficiently to counter the total tide for the trip.

The other common way of doing it is to point the boat in such a direction that the course over the ground (COG) is the same as the bearing to the waypoint (BRG). This gives very similar results to those of the XTE method, but usually gives much larger errors since in the early stages of the trip BRG changes little even with quite large XTE.

But more worrying is your final statement. Rule 10 is now quite specific in saying that the heading of a crossing vessel should be at right angles to the general direction of travel of vessels using the traffic lane. Heading, not track. If you are using GPS to stay on or near the rhumb line to the waypoint, then your heading may be very different to that line, and you will be in conflict with Rule 10. When you're in the traffic lane, then you should forget GPS, point your vessel at rightangles to the lane, and recover your line once you are across.

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[webcraft]

Re: Compass display vs highway

'A vessel shall so far as practicable avoid crossing traffic lanes, but if obliged to do so shall cross on a heading as nearly as practicable at right angles to the general direction of traffic flow.'

So - technically you are correct - bearing in mind the 'as far as practicable' bit. However, most people will want to spend as little time in the traffic lanes as possible - and the easiest way to assure this may be to follow a direct track across at 90 deg

I am still not convinced that aiming continuously into the tide and staying on track is necessarily always slower than making an overall calculation and steering a heading calculated this way. People just say it is without apparently doing any maths to justify it. It is certainly safer if there are fixed hazards to avoid.

And why this post has been hijacked to only deal with the specific issue of crossing the channel (at some undefined point) is beyond me. Most of my navigation involves avoiding rocks, finding narrow gaps and making sure I am at tidal gates at the right time, with cross-tide calculations being a relatively minor part of the whole thing. The point I was (originally) making was that the compass heading screen on the GPS serves very little practical purpose whichever way one is navigating, and can lead to disaster if there is one of the aforementioned rocks not too far to one side of the proposed track. Rather more worrying I would think than having one's bow a few degrees squint while dodging shipping . . . . presumably you wouldn't deviate from this rigid 90 deg apparent attitude to avoid a collision??

- N

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[peterb]

Re: Compass display vs highway

"the easiest way to assure this may be to follow a direct track across at 90 deg"

Well, no, it isn't usually. Under certain special circumstances it might be (such as being close hauled and being unable to hold the correct heading), but assuming that the water speed remains constant then a right angle heading gives the shortest time in the traffic lane.

Difficult to give the maths without a diagram, but basically if the tide is parallel to the lane then the tidal diagram is going to be a right-angled triangle. If your heading is at right-angles to the lane, then the two right-angled vectors will be your velocity relative to the water and the velocity of the tide, while the hypotenuse will be the ground track; your ground speed will be greater than your water speed. If your ground track is at right-angles to the lane then the two right-angle vectors will be the ground track and the tide, while the hypotenuse will be your velocity relative to the water; your ground speed will be less than your water speed.

If your water speed is a lot faster than the tidal rate then there won't be much difference, but if, say, you are doing 5 knots with a tidal rate of 3 knots then the time you spend in the lane will be increased by 25% if your track is at right-angles compared with keeping your heading at right-angles. Oh, and your bow will be pointing nearly 40 degrees off the correct course, a bit more than "a few degrees squint".

On the question of the compass display, though, I agree with you. I've always wondered what it was there for, but suspect that it might be more useful for walkers. If I use GPS for my coastal navigation, then usually I use the 'rolling road' display. I preplan my route as a series of straight lines on a paper chart, with a waypoint at each turning point. I work backwards from each turning point, putting ticks on the line at each mile (or sometimes half-mile). Then by using the distance to the next waypoint together with the cross-track error I can get an almost instantaneous position on the chart, and I can easily dodge an isolated hazard.


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[webcraft]

Re: Compass display vs highway

Yes, your ground speed will be greater than your water speed.

But . . . your ground track will be longer, so the time spent in the traffic lane will be as long if not longer.

I do not see the importance of speed through the water . . . perhaps we should be talking about VMG to make more sense of this argument.

- Nick



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[peterb]

Re: Compass display vs highway

Heading at rightangles:

Distance to be travelled through the water to cross the lane = width of lane (you'll go further over the ground, but your ground speed exactly compensates).

Track at rightangles:

Distance to be travelled through the water to cross the lane = square root of (square of tidal drift plus square of lane width) (by Pythagoras's theorem), which must be greater than the lane width.

The time taken to cross the lanes will be the distance travelled through the water divided by the boat's water speed. Assuming the boat's water speed is the same for both cases, the heading at right-angles must give the quicker crossing.

The result becomes very obvious if the tidal rate is equal to or greater than the boat's water speed. The crossing time on a right-angled heading will be exactly the same as if there were no tidal stream, but the boat will be unable to cross at all if a right-angled track is attempted.

 

[webcraft]

Re: Compass display vs highway

That sounds pretty convincing, Peter - maybe a right angle heading does equal least time spent in the lane. Of course, you may not be where you wanted to be when you get to the other side . . . but that's another matter, more in line with what was being discussed before the red herring of traffic lanes came along.

(Whoops - maybe it was me started that . . .

- Nick



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[philip_stevens]

That is more or less what I will now do for next season.

I have a motor sailer, a Konsort Duo, with a chart plotter inside on the chart table/steering console. Two weeks ago, there was an old AP Mk8 GPS advertised in the For Sale for £50 -that I bought. It works a treat, and will be fitted outside in the cockpit to be used when under sail. It is better than a repeater, as it is a full GPS with any display required, and, should the main GPS plotter fail, I have a back-up.

For anyone who wants a cheap back-up GPS, there is another Mk8 advertised for sale on the Westerly Owners site. It does not have an aerial, but I would think Comar on the Isle of Wight could sort something out.

<hr width=100% size=1>regards,
Philip