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DarkBlue · 13-01-2009, 10:00

Threads like this one and the answers given by expert sailors like you guys are one of the reasons why I have joined this great forum. I'm really learning a lot.
Thank you.

Lodesman · 13-01-2009, 10:10

Quote:

Originally Posted by rigormortis

Thanks boys.

I am still puzzled about this idea of steering to a waypoint.
If I were under engine power all the time it would make sense. But with sailing, the damn wind blows from where you want to go and it is a never-ending compromise of headings one steers when trying to make the objective.
Where do waypoints come in?

Open mind, but vacant.

Pete.

Pete,

I don't think it's entirely necessary to steer to a waypoint - obviously, as you've stated, wind will determine the direction you must steer and the track between the waypoints only indicates the ideal course at the start of your transit. Tacking back and forth to an imaginary line in the ocean doesn't make much sense, unless needed to avoid navigation hazards or to take advantage of wind or currents (eg. use the tradewinds). Ultimately, you only have a couple of key waypoints - where you are presently and your destination; this could also be an intermediate destination, such as the mouth of a channel. When you start out, your GPS or Cornell gives you an ideal heading and you try to get as near to that as wind, sea, safety, fun, enjoyment, etc allow - unless that's the same as the ideal, you'll diverge from your initial track, but as long as you're making progress towards your destination, it doesn't matter. Eventually conditions will change, or for the sake of periodic checking, you'll have to re-aim at the intended target (destination). At this point it doesn't make sense to go back to your original track, as the shortest distance to your destination, is a great circle from your present position. Recalculating gives you a new "ideal" track and you can then determine the sailing course that best approximates the ideal while considering the other factors (hazards, wind, current, desire to stay above or below certain latitudes, etc.) As to your question "why waypoints" - they are simply the electronic equivalent of a pencil line on a chart - no more and no less.

Kevin

Tropic Cat · 13-01-2009, 10:15

Quote:

Originally Posted by martinworswick

how does a gps work out its route between 2 waypoints, is it great circle or a fixed heading, i've only ever used a gps with distances short enough that it wouldn't matter.

GPSs calculate great circle distances and bearings

martinworswick · 13-01-2009, 11:40

Quote:

Originally Posted by Lodesman

I think most can do either, depending on the set-up; check your user's manual. If that doesn't help, then punch in two waypoints; use the exact same latitude, but put WP2 100 degrees West of WP1, then calculate the course. If it's 270 then you have a rhumb line; if it's 3-hundred and something, then you've got a Great Circle departure. Don't use zero for Lat.

s44 59,284 e168 48,690 to 44 59.284 e088 48.690 comes out with 239t

Lodesman · 13-01-2009, 12:41

Great circle - sorry my 300-something answer didn't apply to the Antipodes.

MarkJ · 13-01-2009, 13:25

Quote:

Originally Posted by martinworswick

between 2 waypoints, is it great circle or a fixed heading, i've only ever used a gps with distances short enough that it wouldn't matter.

Any of the newer ones do by default the Great Circle Route.
However its really just academic for us cruisers as we want to stay in the tropics and go west

Even our 3,000 mile leg Galapagos to Marquesas had virtually no Great Circle component. Our 8,000 mile Panama to Sydney, if done direct would have had to have been Great Circle and would have cut 500+ miles off. However very few cruisers will do that as they want to go stop on an island somewhere and see if the local women are worth a nail from the planking.

In high latitude sailing i.e. Trans Atlantic from New York or England then I think Great Circle would be very important. If you were doing those legs then you would have a chartplotter that would default to Great Circle.

One other thing, I don't think there is a 'button' to change it into or out of a Great Circle mode so the User Guide may not mention it.
Also remember that GPS/Plotters normally work in Magnetic. A long high latitude leg may require a change to Grid (True). That could be another thing to research

I am being plagued here by 15 degree Magnetic Variation: The weather wind directions are given in True. That means an Easterly breeze is from NORTH of east, not east! And not varying to the south of east but more NE to E. Get it?

Mark

jackdale · 13-01-2009, 13:47

Quote:

Originally Posted by MarkJ

Also remember that GPS/Plotters normally work in Magnetic. A long high latitude leg may require a change to Grid (True). That could be another thing to research

I am being plagued here by 15 degree Magnetic Variation: The weather wind directions are given in True. That means an Easterly breeze is from NORTH of east, not east! And not varying to the south of east but more NE to E. Get it?

Mark

Not sure about you comments re: changing to True. If you chartplotter is set to Magnetic, the variation will change accordingly. (Do not forget deviation - but I deviate too much.)

You can set your chartplotter for True or Magnetic. In the Magnetic setting the variation can be automatic or manual.

As for wind directions, the wind is coming from true east (090), but 075 magnetic. I assume you have 15 degree ~~West~~ East variation.

Jack

jackdale · 13-01-2009, 17:04

Quote:

Originally Posted by jackdale

As for wind directions, the wind is coming from true east (090), but 075 magnetic. I assume you have 15 degree West variation.
Jack

Let's make that an East variation. Too late to edit.

Jack

RBEmerson · 13-01-2009, 18:57

Nice test! Thanks for the tip!

MarkJ · 13-01-2009, 23:11

Quote:

Originally Posted by jackdale

Let's make that an East variation.

Jack

Excakkery!

Thanks Jack. yes you are right about the variation being done automatically. It looks pretty screwed up in the plotter on M with 15 deg variation. But looking at my pilot charts for the southern indian ocean (yep, not going there!) The M Variation is up to 55 degrees West. Cape town is about 30 degrees W. But the chart in the plotter when North is Up is gunna look crackers!

Maybe there they change to Head Up. But I canna do that myself, I have to have North Up. And when I hear the weather guy say "Easterly winds" thats where I want it from!

Mark

**Wotname** · 13-01-2009, 23:42

As some others have already posted, GPS bearings ARE great circle routes. Perhaps somewhere there is an odd GPS manufacturer that include an rhumb line option but I have never seen one since first using a GPS pre first Gulf war.

Additionally all the ones I am familar with allow for a user setting of either true or magnetic north. Normally the magnetic variation is automatic.

Opps... just saw Jacks post - note to self: read more carefully before posting!