Amaya had a spy

Talbot

Sorry if I misunderstood your post, wasn't sure if you thought I'd made up the plot.

The thought of relying on a laptop real-time on a boat on a dirty night at sea fills me with dread. "A fatal exception has occured at OE12:05AD" springs merrily to mind, together with the useful information on the blue screen that I shouldn't have closed down my computer like that and that I've only myself to blame for the 5 minute delay whilst it checks I haven't damaged my hard disk.

I'm a great fan of the Yeoman plotter plus hourly position entries in the written log.
Wonder why!

Regards
Tom




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Where I think the lapdog does have a position is in the planning of the voyage and translating this to waypoints. knowing that your straight line course is actually going to curve sufficiently to hit a bad area must be of value. Furthermore winchart nexia can show the best time to leave for your journey based on tidal windows and expected passage speed. I understand that in the near future, the system will also include wind calculations allowing raggies to have even better knowledge of what they are doing. IMHO this is useful as it provides the sort of information that an experienced sailor already understands, thus should add to the safety of the newbies in our sport.

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I'd agree - but then I'm neccessarily biased!

The only negative is that having the system do the work can have the effect of causing users not to gain knowledge they would otherwise have picked up, so intuition suffers. On balance I think the outcome is good - particularly for people who understand what the software is doing on their behalf.

Even an experienced, knowledgeable navigator can easily fail to take into account an otherwise important element in a passage plan.

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Quite agree with this. The laptop is great for planning and post voyage analysis (that's how I downloaded my data). It's the use of them en-route which I was really questioning.

I haven't looked at winchart for a while, thanks for the info.

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To all who have followed this thread!

Thanks for your interest. The website I put up has had an amazing number of hits (13,000) so I'm not sure whether thats's MainlySteam in NZ visiting 13,000 times (only joking John!) or it's a lot of people having a quick peek. Whichever, it's obviously struck a chord. It's clear to me that there is some thirst for the technical issues which happen to be my bread and butter. I'm lucky to have encountered Qsiv's expertise on the forum also, his input has added an extra dimension.

It has inspired me to take things further and I'll be publishing a short series of articles looking at NMEA data in some detail, particularly what happens when you start to connect equipment together. There are some surprises. I will post the first article in the next week which will look at the static situation. Following this, I will be recording some data under sail using some pretty sophisticated equipment on board which will look at the useability and accuracy of the dynamic data.

I'm going to do this with both Raymarine ST60 and Simrad IS15 instrumentation and would like also to have access to a boat with Nasa gear - PM me if your Solent based and interested.

I'll keep this thread alive so when I publish the first article I'll post a link here. You'll need Acrobat Reader to view the articles.

Fair winds
Tom



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13,000 tries so far and I have still not found the Rugby results page on it. Nor, more importantly, the sure prediction of the Cup winner that I know must be in there somewhere!

Anyway, for the benefit of the others, we set up Kaniva last weekend to do some recording too - we had NMEA and computer already hooked up so was just a software exercise to record a pile of data points. So, for those who want to give it a go once they have NMEA data available (and I am sure Tom's articles will be very helpful there) but are short of software capable of recording their track and all the other relevant information such as wind, magnetic compass, depth, log, etc the following is one cheap way of doing it.

The free PC chart plotting program SeaClear II, which I know a number of forumites use already (and a Google search will take you straight to it), has an add on available called SeaTrace (also free and a link to it exists from SeaClear site) which will record at set intervals a fixed set of 15 data points off a NMEA line plus another 8 associated with the next waypoint (eg bearing, XTE), if there is one, into a text file. It will also do a screen presentation of the file. The only thing which is useful that it does not record is tide, however, that can be calculated using the GPS course/speed and the log course/speed - I will try and set the solution for the tide vectors up in the spreadsheet I load the data into. SeaClear will, of course, overlay the track sailed onto a chart.

The only slightly difficult part is one has to set up the Windows DDE link between SeaClear and SeaTrace but that is just a one line text entry in an .ini file and the Help tells exactly what to do (I mention it because it doesn't work otherwise). I have not tried but for recording the data I suspect that you do not even need any charts loaded into SeaClear (except perhaps for its world map that comes with it), but in any event SeaClear will read scanned charts and it is simple to scan the section of the chart that you want your route over and calibrate and load that as your chart.

Of course if you have other software already use it, the above is just one way and it works. Would be interested to know what else people may use.

John

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John

Just a word of caution about your tidal calc. In general it's correct - in detail (perhaps the sort of detail Tom is looking at), it can be significantly wrong.

COG & SOG are 100% accurate within the parameters of GPS.
Heading & Boatspeed are both likely to contain material errors, particularly boatspeed.

The computed tide vector will have errors due to the heading / boatspeed fixed errors - but much more importantly leeway can completely invalidate the calculation at times.

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Yes, thanks for that QSIV. Unfortunately the thing I am most interested in is getting some better knowledge of the tide, but I think implicating the log is the only option I have.

Cook Strait is quite a complicated piece of water with many local influences on tide from both the land and the sea bottom - for example there are strong upwellings, strong tidal effects around islands, promentaries, etc, and quite wide general variations from the daily tidal predictions. The tide atlas is indicative only and some important effects on the routes we normally sail are not shown (in fact the atlas would appear to be wrong).

To try to get some information I was going to endeavour to record data on flat run/ broad reaches through the Strait (except in the infrequent light variable conditions the wind always blows one way or the other through the Strait) and preferably with the general tide as that lessens the likelihood of short steep seas which may cause problems due to things like aeration of the log paddlewheel. From observing the on board computed tide in the past, the strong sets show up in the sense expected from other observations but I do not know how accurately as the log is implicated of course. By taking frequent instantaneous log readings over each logging period and then averaging those for each data logging entry (say measured every 30 seconds, with those averaged over 15 minutes and logged) I hope that some of the fleeting log errors (eg exposure coming off a steep sea, aeration) will have less effect on the overall result.

In the end I will have to see what I end up with, but I think that at worst I will gain a much better understanding of the tides in Cook Strait for future planning. What I will have is a frequent record of tide and wind, with some unknown error in both, but which can be compared to the plotted track's deviation from the planned course and perhaps some judgements made as to the error. Also, I should mention that my interest is mainly for cruise passages not race ones, so am more interested in better knowledge for planning, rather than having an accurate data stream available during the passage for frequently refining the strategy to make the best of the conditions encountered as a race boat would.

I would welcome any suggestions.

Regards

John

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