Interesting comments

[Guest Rocket]

You misunderstand my language - I did not plot onto a chart I stared at the screen and saw myself happily motoring over the hills - GPS inaccuracies! I use a GPS plotter as an aide and therefore feel very comfortable ignoring them when I know they are wrong. My point is your primary senses should be used - stick your head up and look. You wouldn't drive down the road relying on your Tomtom to steer off, why would you think driving on the water is any different...

[Guest]

Yes I understand however the chartplotter has two things going on , one is the gps and the other is the chart callibration and outline. I think gps is very accurate but chartplotters are sometimes let down by the accuracy of the chart. To make my point....

I drove with my new gps handheld down the road a few miles and came back. The track on the chart showed parallel lines the whole distance I travelled. these lines were exactly parallel for the whole distance. From that test I reckon the gps , when moving , is extremely accurate.

I've found the same with the charts though, being on land when sailing so just trying to get this clear. I agree wholeheartedly, can't beat watching where you are going.

[wheels 543]

Yes GPS can be extremely accurate. Providing you have at the least 4 birds being received, you should be able to be close to at least 3m or so. The more satellites received, the more accurate the position will be and it is highly possible to have an accuracey of 1.5m. To be closer than that, you need a landbased station as well. This is how they are now doing surveying with GPS, with accuracies down to a centimeter or so.

It is the Chart and it's calabration or "offset" that will let a plotter/GPS down in accuracey.

The Satellites are now regularly checked with land based Laser beams. The beam is fired at the bird and bounces back, which takes about 17.7uS and from that they can range find the bird and make corrections in the timing. So the entire system has really become much more accurate. Well....they kind of had too.

The new European Gallilao naivgation system is underway. Infact just last week, two of the brand spanking new birds arrived at French Guiana and are being readied for launch next month hopefully. If weather and all other things go well, they hope to launch the first rocket on the 20th Oct. This system is going to be quite exciting because it is networked with a worldwide array of ground stations, which will constantly calculate and correct the inaccuracies as I understand it.

[Guest]

We will still have the same charts though so same problem. Guess we will just have to keep on looking out. I think its accuracy is better than indicated on the screen. Mine goes down to about 4 mtres but the resulting tracks in the above experiement don't deviate much at all. Seems to jump about when stopped.

[wheels 543]

The jumping about is the slight inaccuracey giving different positions.

[Guest]

The jumping about is the slight inaccuracey giving different positions.

 

G'day 'Wheels' I've noticed that too . My 'team leader' complains about that 'slight inaccuracey' also. Any solutions that come to hand?

 

Change subject - cause I'm 'challenged' - mentally + over 70 & male + ex-pat Canuck + live in OZ - geeze - how's taht for handicaps & that's before the starting line & 15 min prep signal.

There's a bloke in M4US that's looking for someone to assist in mfgr, promotion & sales of a 'brake' of some kind for sailing yachts. Any of you pro's in NZ interested in talking to him.

 

As you can see I don't know how to use this sight - to old to see screen properly but I can still sail - I think. Don't know how to post 'new' subjects' & maybe that's a good thing, eh!! Ciao, james

[rigger 47]

for what it is worth at sunrise and sunset GPS systems can suffer errors - have worked DP systems and it was noticable how often we would get alarms from the system warning of errors in the GPS data at those times. We were running 2 DGPS systems and a Fan Beam system. and off course the over riding system was Mark 1 eyeball monitoring the whole lot and looking out the window to enure the distance between ship and platform was ok.

We had a max off position distance of 5metres and heading allowance of 3degree max

[wheels 543]

Any solutions that come to hand?

Keep the boat moving

for what it is worth at sunrise and sunset GPS systems can suffer errors

hmmm, interesting.

[Guest]

wonder why gps varies in the mornings or evenings.

[wheels 543]

Ionisation of the atmosphere. A similar thing happens with Radio as well.

[Guest Rocket]

So final conclusion?

 

Use your eyes and treat your GPS as an aid to navigation.....

[Guest]

Gps is great for vmg. Velocity Made Good is King. Lets me know whether to bear off a bit or which tack is faster. Now I wonder why one tack is usually faster than the other. I've read that it may be due to the hull being uneven or the mast not in the centre. One thing I read had wind shear causing this but I don't really understand wind shear. I thought may be due to currents helping a little on one tack and hindering a bit on the other. Even a half knot of current would then add to one knot of difference.

[PaulR 3]

bbay:

 

1). GPS Have you set a "WAYPOINT"point in a route or positioned the cursor and created a "GOTO" Most GPS's require this before they can calculate VMG.

 

However VMG strictly speaking is into the eye of the wind.

 

Your GPS is displaying VMW (or sometimes VMC) being Velocity Made Good towards WAYPOINT (or sometimes desired COURSE).

 

Personally I prefer VMW for most yachting around the gulf / coastal races.

 

So a few degress to several degress difference between VMG into the WIND and VMM to WAYPOINT is to be expected as there is only one place on the sailing course when VMG and VMM are identical, being when you are exactly down wind from your waypoint at that instance of time whilst the wind is constant at that same instance of time, but as well all know, the one true fact about the wind is that the wind is constantly changing.

 

So GSP is VMW.

 

To get VMG you need your wind sensor / instument linked to hull speed as a minimum. Extra black boxes of a compass and "server" to process more data can be added for greater accuracy; and even then more options become available if your GPS is linked in as well; sending and receiving the sailing (NMEA) data between different instruments and processors (servers).

 

Hope that helps one's understanding in simple terms.

 

2). Tides & Currents. For some reading, google "Lee Bow". Arguements for both sides of the debate are to be found.

 

I still favour the fact that the lee bow tide pushes the boat into the wind just that little bit, which means you point higher, and have a slightly increased apparent wind speed as well.

 

Others can argue differently.

 

Personally what happens in reality is best. I have been 2nd in the Round Alderney Island race TWICE. Here the tide that passes through the Channel Islands and the Cherbourg Peninsilar is the strongest in the English Channel. Just a few miles south at St Malo, Dinard and Mon St Michelle, the tidal rise and fall is 45ft / 15m or more, and comes in "faster than the speed of a gallopping horse".

 

One time I was helming a Swan 41 with the Swiss Admiral's Cup crew. Thus the yacht was well equipped and no crew bunnies on board.

The wind was very very light and most boats sailed up to the northern top of Alderney and anchored. Most is not quite correct. All except us anchored.

 

We pinched our nose right into the rocks and looked really good until we hit the eastflowing 10+ knot tide. We went sideways so fast, the apparent wind totally changed and we had to tack the headsail to get any steerage at all. We ended up gybing, doing a 3 to 5 mile circle and eventually got back to the point, accompanied by the cheers from those anchored.

 

Second attempt we just sneaked around and were away . . . only to be pipped at the line by a 65ft yacht. Slowly the others weighed their anchors and finished.

 

So the real lesson was the tide TOTALLY altered the apparent wind by about 180 degress.

 

Yes, it was an extreme Tide on the Bow case, but the memories of a lesson well learned . . . . I think the party went on for a long time . . .

[Guest]

PaulR, thanks, quite a read and I learned a lot. I've just been using the vmg to a waypoint. I see my furuno ls 4100 fishfinder has vmg included in its nav data but needs hull speed and wind info so correlates with what you say. I'll be looking at fitting wind info shortly. Can't decide whether to get the wind or the ais next. Just using telltales and angle of heel for wind strength.

The current effect that I'm talking is more subtle but I think important... say on a port tack hard on the wind you are getting 6 knots..tack onto the starboard side and speed drops to say 5.5. I would guess there's a current running towards your port side somewhat and helping along a bit but hindering by the same amount on the starboard tack.

[PaulR 3]

Can also be wind on tide causing a different wave pattern.

 

On one UK Admiral's Cupper team boat, we would change headsails on EVERY tack, just because of the wave / slop action, especially inside the Solent, working up the Lymington shore. So we would swap from a Hood to a Banks headsail because of the different construction shapes: panels being simple strips from luff to leech as opposed to radial shaped panels from Tack, Clew mainly, and less radials from the head.

 

The joys of twin genoa halyards, twin grove headstays and only one sheet on each sail.

 

We had 2 each of Light Genoa, No 1 genoa, No 1 heavy genoas, No 2 & No 3 genoas.

 

We only had single sails in the jibs #4, 5 & 6 which were were all Hood IIRC.

 

These days the 3DL type sails may be different.

[PaulR 3]

Can't decide whether to get the wind or the ais next. Just using telltales and angle of heel for wind strength.

 

Wind instruments work well at night. In NZ we have relatively very little shipping on our coastal passages, such that often one does not see any ships at all. Quite different to the congested English Channel, Panama and the Med where it is common the see a couple of strings of boats going both ways in the traffic separation lanes.

 

Where are you sailing to That is the big decider, but certainly wind instruments would be used a lot more often and at night, well before you do any coastal or ocean passages.

 

IMHO WIND gear will give you more useful information sooner.

 

However once the wind goes above 60 knots, a lot of wind gear is off the scale then and days can go by before the wind speed lowers and is readable again, especially if one is positioned relative to a low weather system moving along with you at the same speed, say 20 to 30 knots, on your course. At this stage, you will become used to life and can tell when the wind increases further as the surfing goes on for longer and longer, and the bows fall further off the tops of the waves.

 

Then you know when the wind is higher still and you remove all sails.

Currently my fastest speed has been 14 knots under bare poles off Argentina.

[PaulR 3]

The current effect that I'm talking is more subtle but I think important... say on a port tack hard on the wind you are getting 6 knots..tack onto the starboard side and speed drops to say 5.5. I would guess there's a current running towards your port side somewhat and helping along a bit but hindering by the same amount on the starboard tack.

 

Yes could well be but also, have you calibrated your log to know true water speed

 

Did you do this under motor (We generally do.)

 

Have you the log on the centre line of the hull, or off set a little

 

Is the log too near the keel

 

Is the sailing weight in your boat evenly distributed or are you already listing a few degress, possibly to port The old IOR Inclination test could show this up.

 

Are your hull lengths when heeled the same Beautiful looking yachts have been found to be different, sometimes by 6 inches 150mm. Stick a Tape measure around your hull when next hauled out, at about a 20 to 25 degree heeling.

 

Let us know

 

Have you tried sailing along a measured mile

The wind is seldom favourable IMHO so ofte one has to use other beacons = maths.

NB use fixed beacon transits, knot movable buoys.

[Guest]

Arrrrh, getting buried under variables, too many of them, I give in.......arrgh

[PaulR 3]

Just consider one variable per day. FOCUS FOCUS FOCUS.

 

Write each variable on a sheet of paper and add pointers queries etc.

 

SWOT analysis or CRITICAL PATH planning may help.

 

Don't try to do too much all on the same day.

 

 

A good siesta and a good night's sleep will help focus on each variable in sequence.

[Guest]

Hey PaulR, thanks mate . I reckon a sailboat is about as complicated as the space shuttle. And thats trying to keep it simple. Understanding the usefulness of vmg is a big step up.