Waypoints, cross track errors and tides!

[snowleopard]

Re: Enter the devil\'s advocate...

Why fight the tide? - well it's horses for courses. If your passage average speed is 4 knots and you're expecting 1.5 knots of tide it would be a bad idea to sail on XTE. On the other hand if you plan passages on 7.5 knots and the tide is under 1 knot, it isn't worth the hassle of hour-by-hour tidal plotting. Another factor is whether you enjoy the chartwork and your stomach likes you being down at the chart table.

Having been through the YM theory course, then sailed with professional merchant officers I was amazed how apparently casual their navigation was - the detail stuff was saved for when it was necessary.

 

[iangrant]

Woody,
Lots of good advice and responses already but her's my tuppence worth on the GPS.

If I understand you correctly, I'm sure you are OK with working up your course to steer using the tidal streams atlas to arrive at the right bit of the french coast using conventional means. I reckon sort of 205/210 to Cherbourg is about average from the Nab area.

Now then, a good start of course is to plumb in the waypoint and double check it (first time I did it I was nearly on the rocks outside Cherbourg looking for a light that was in the wrong place, ahem, should have plotted it on the chart first to verify the numbers plumbed in are a position on the water not land!) Use the luxury of the GPS to confirm your position as you cross the Channel (or just plot the lat and long from it onto the chart like everyone else these days).

There is no need to hit the goto button again (thus performing a reset of the displayed XTE) when you fall off the road. All Garmin GPS units (and I think MLR but not tried it) allow you to determine the width of the motorway! I think the default is half a mile.

Change that to five or 10 miles and you'll see how far you are drifted from the center line left then right, or t'other way round depending on tides! A quick calc of "sideways tide drift" distance over time should confirm the tidal stream.

Good luck, enjoy the sail (motor if you are off this weekend)


Ian

 

[duncan]

Joss - best you pm tome , it's his software.

 

[[2574]]

you seem to be an advocate of the Maxsea software - I am really struggling to get a clear view of the s/w available on the market and it's relative pro's and con's. The price seems to range from zero to £1,100 (for the Raymarine s/w). I think Maxsea is fairly fully featured? Also Maxsea seems to be dedicated to a particular chart type - I'm using Cmap with Raymarine hardware - will it interface okay with Raymarine (to include passing route and waypoint data?)

I'd be grateful for some views from users of the various different packages?

rob

 

[BlueSkyNick]

woody,

Just last night I did the sums for our crossing on Friday night. We are aiming to clear Bembridge at 2000 and arrive in St Vaast about 0800. Just to give you an idea of what to expect, we will drift up to 11.5 miles east on the crossing. The spring tides obviously have a significant impact.

I don't bother with the motorway on the GPS, don't even know how to us it. Its for powerboats IMHO. /forums/images/graemlins/smile.gif

 

[snowleopard]

[ QUOTE ]
I don't bother with the motorway on the GPS, don't even know how to us it. Its for powerboats IMHO. /forums/images/graemlins/smile.gif

[/ QUOTE ]

Can be handy for us as well. If you have a route through hazards you can draw a pair of parallel lines in safe water either side of the ground track, set the width of the virtual road in the GPS to match. You simply steer to stay within the 'road' and bingo - a safe course with no need to allow for tides or leeway.

 

[jimbouy]

I've seen this done and had a brief play on seapro. I don't think the Lite version does the tidal stuff. If I could jusify the cost I'd go for it.

www.euronav.co.uk

 

[jimi]

I do the course to steer (revising occasionally if speed different from planned) until about hour out of Cherbourg then put a waypoint in the entrance and adjust CTS until COG = the course to the waypoint. One could pick up transits if steering manually but the autopilot prefers a CTS.

 

[Woody]

I don't know about the package shown above but I use Neptune for passage planning. Very easy to use and can be linked to GPS to upload/download waypoints.

PS I have no connection with Neptune, just a happy customer.

Neptune

 

[tcm]

harumph, it is for those med powerboaters who don't know any better - the sort who when asked for ideas of what they would do if the wind suddenly strengthened replied "I'd probably tell the girls to come in from the foredeck and put the glassware away" !! - no, NOT anyone hereabouts but a real answer!

 

[duncan]

they would be the 'what I can see is what I might hit' navigators perchance?

 

[Guest]

Try this group .....

With members all over the world using near all packages generally available as well as software producers etc. included - you can get answers to near anything GPS / charting related :

http://groups.yahoo.com/group/gps-navigator/

 

[bob_tyler]

How about the simple way.

Estimate expected average speed through the water for a Southerly course.

Mark the chart hourly along a straight line from UK to France (eg Cherbourg).

Using a tidal atlas/PBO almanac calculate the no of miles E & W you will be swept.

Subtract one from the other.

Aim off the the amount of the answer in NM along the French coast

Use the GPS just to check that you are being swept E or W at the appropriate time and let yourself be swept.

PS. The above assumes sailing from either end of the IoW to Cherbourg

Gentle correction in the last 5 miles or when you can see the Cherbourg Harbour Entrance.

Mind the offlying rocks to port and you are home - no bother.

Have a nice, relaxed trip.