Amaya had a spy

[qsiv]

Re: Qsivs Analysis

My word - thats a very, very impressive set of documentation you've put together.

Sterling effort!

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[MainlySteam]

Re: Qsivs Analysis

Many thanks from me to Tom and QSIV for your efforts in presenting the information and methodology.

Regards

John

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[BlueSkyNick]

Re: Qsivs Analysis

HEAR HEAR !!

Really interesting stuff, even for a relative novice such as myself.

Now looking forward to taking this further on Moondance .... but next year (sob, sob)

Well done chaps, and thank you.


<hr width=100% size=1>I started out with nothing and still have most of it left.

 

[bedouin]

Re: Qsivs Analysis

I am confused by the suggested track. That seems to suggest sailing much closer to the rhumb line - doesn't that imply a longer water track and so a slower passage?

Probably not important - but I think the wind shown is not accurate - I believe it was pretty much due E (at 25kts); the plot shows ESE

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[tome]

Re: Qsivs Analysis

Hi Bedouin

I believe you are correct about the wind and this may have been due to a lack of calibration in my heading sensor.

Since there seems to be a lot of interest in the technical side of this I've added some more details about how to gather the data. I'll add a diagram of the connections also, but am having some PC probs.

As for the track, I now believe that the polars were far too optimistic and I'd be interested to see a revision based on a crossing time of more like 9h45m if Qsiv has time to do this. This may show a very different picture.

I reckon we should have come up by 10 degrees at around 1230 to come closer to the Nab and stay up-tide of Chichester.

Cheers
Tom

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[tome]

Re: Qsivs Analysis

Ian

One shortcoming of the Nav6 is the very small field of view - you have to be right in front to read it. You can set the font size much bigger (visually impaired old git mode).

It does take a bit of setting up and if you ply me with ale I might be tempted to visit your rather cramped nav station! BTW, where did the boat come from? Could it be that it's set for a different area? If it's connected to GPS it can be set to find the nearest Navtex station.

Regards
Tom

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[iangrant]

Re: Qsivs Analysis

Tom
Duncan had a play with it and said - that's OK now - err nought - the set isn't connected to the GPS but is set for Niton 518. it's one of those things that have slipped down the lists of things to do, but beer will be forthcoming in the furure if you fancy squeezing into the nav station.

Ian

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[tome]

Nav 6

No problem Ian - give me a shout sometime

Tom

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[bedouin]

Re: Qsivs Analysis

Ah that explains it - having now looked at the polars I would agree that they are probably optimistic given the sea state.

I presume from looking at the plot that it is advocating a very different start time - to coincide with West-going tide from Cherbourg. In my experience that is usually a bad idea since I think there is more N/S component in the tides on the French side of the channel than tends to be shown in the Tide Atlases


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[tome]

Re: Qsivs Analysis

Also, if you look at our track the W going tide appears to start about an hour earlier than predicted (NP250).

I've added a to the <A target="_blank" HREF=http://home.btconnect.com/Amaya/Amaya_Web1.htm>site

Regards
Tom

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[qsiv]

Re: Qsivs Analysis

Bear in mind the tidal vectors dont come straight out of a tidal atlas, rather they come from Proudman's tidal engine, which accuarately renders tidal vectors (Rate, direction, height - corrected for atmospheric pressure) in 1.2 km grid elements.

I believe the output to be accurate - it is as far as I am aware widely used by SAR centres to model casualty drift.

I have amended the polars and re-run the model - the net effect is surprisingly small. The passage started a bit later, and held more to a rhumb line. The key element in the strategy was to avoid the 'sweep' downtide and downwind which would have markedly reduced the apparent wind speed. If you look at the log extract you will see that as the passage draws to its conclusion (about 14:45) that reefs are being shaken out. Looking at the nav data the True Wind Speed is holding fairly steady. What is happening is that the apparent wind is dropping due to the tide sweeping the boat downwind.

Just out of interest I will send to Tome the 'forecast' the system can generate for a point just N of Cherbourg, for the day concerned. It woul be intresting to get your view on ow the tidal directions compre with what you saw empirically.

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[qsiv]

Re: Qsivs Analysis

I'll re run with 'slower' polars this afternoon!

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[tome]

Re: Qsivs Analysis

<A target="_blank" HREF=http://home.btconnect.com/Amaya/Amaya_Web1.htm>Updated site</A> now includes GRIB files!

I'll post revised passage when Qsiv gets a chance to re-iterate based on slower polars

Tom

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[bedouin]

Re: Qsivs Analysis

If the course is holding closer to the rhumb line - does that mean the prediction is advocating changing heading on a regular basis?

If you are investigating it would be interesting to see what happens if you bring the wind round to due East - which I believe to be closer to the true state of affairs

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[qsiv]

Re: Qsivs Analysis

I've actually used the data from Tom's electronics = the average wind was 117.5, and I called that 120 for the sake of simplicity. Similarly the average windspeed was 24 knots, and I rounded this to 25 for simplicity.

If you PM me with a real email, I'll send you a JPG with an 090 wind as basis.

Just for a hoot, I ran the polars for the new raceboat over that trip - that would have been quite a day as the return trip would have taken abot 3:30 - leave after 08:00 back before midday, but it would not have been for the faint hearted.

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[tome]

Re: Qsivs Analysis

Cor blimey, not for the faint hearted indeed!

I've posted the modified plan which is beginning to look more like our track. I'd like to put up the 090 wind plan also. Can you force a 5.30 departure?

Regards
Tom

<hr width=100% size=1><A target="_blank" HREF=http://home.btconnect.com/Amaya/Amaya_Web1.htm>Cherbourg Data site</A>

 

[qsiv]

Re: Qsivs Analysis

To reply to the other question!!

yes, the CTS does vary with time from 359 at the start to 020 at the Nab (but not in a uniform way...) The 'leglets' on the x channel leg are about 2 1/4 miles (dont ask me, I only wrote the software), so thats about every 15 to 20 minutes.

If one were running the sooftware for real, the system would recalculate an optimum rout from the current position to the next waypoint periodically (perhaps 1 minute in race mode, perhaps 15 or 30 minutes in cruising mode), and you would use that to give a new course to steer. One of the other analyses the system does is to try and assess whether the difference between the current wind you are experiencing is an 'offset' against the forecast, or whether the conditions are following the forecast, but at a different speed - i.e. a front has arrived early. As such the interim recalcs can provide quite markedly different strategies. In race mode I analyse the wind patterns and try to extract the frequency and range of periodic windshifts so that you work the shifts, not against them.

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[qsiv]

Re: Qsivs Analysis

It's on its way. You'll notice a difference between the 090 wind on the plan, and the actual wind you logged on the replay track...

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[tome]

Re: Qsivs Analysis

New track on the site

Webmaster!

<hr width=100% size=1><A target="_blank" HREF=http://home.btconnect.com/Amaya/Amaya_Web1.htm>Cherbourg Data site</A>

 

[Talbot]

As a perrenial sufferer of bad software, I sympathise. What I was trying to say (and not phrasing very well), was that your track looked very similar to the tidal curve predicted for a straight line course on that particular software. I dont actually use it myself, I have a plotter adjacent to the helm, which IMHO is the best option. waterproof and low power. lapdogs are fine for those in large stinkies with lots of power. Plotters are particularly good for short handed sailing especially if it is bumping around a bit!

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