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Chart Plotters and Wrecksite

Discussion in 'Computers and Technology' started by georgedavo, May 26, 2020.

  1. georgedavo

    georgedavo Member

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    Hi All,

    Over the lockdown I decided to start the process of planning my BSAC Advanced Diver Expedition.

    I was just wondering what people use as chart plotters and whether anyone uses the charts on wrecksite and how it compares to other chart plotters.

    I’m relatively new to chartwork so any input to the most practical methods of voyage planning would be much appreciated.

    Cheers!
     
  2. jb2cool

    jb2cool Moderator
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    I’ve used wrecksite before but never needed to do the chart work as I use commercial charters. Wibble May have some ideas as he’s of the nautical persuasion
     
  3. Wibble

    Wibble Fish don't talk
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    The same as for land mapping, nautical charts have changed immensely over the past 15 years. Paper charts have pretty much gone by the wayside, being replaced with the ubiquitous iPad and chart plotters. The greatest benefit is not having that massive tedious chore of manually updating charts, simply download an update et voila!

    Charts fall into two camps, raster and vector charts. Raster charts are basically scans of a paper chart — think PDF. As you zoom in, you just magnify what is there and no new information appears. Vector charts are like you have with google maps, more info appears as you zoom in

    The charts on Wrecksite are the standard Admiralty raster charts, augmented with the wreck positions.

    Using charts means learning the symbols, particularly the navigational marks and depth contours. Other key points are the tide and current markers, referring to tables on printed charts.

    Your focus depends on what you’re using the chart for. Passage planning, navigation or finding things. What do you want to do with charts?
     
    jps likes this.
  4. georgedavo

    georgedavo Member

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    Thanks both.

    I’m mainly looking at passage planning from a slip to wrecks and back, along with tides and currents for predicting slack etc.

    I’m currently using a trial version of navionics.
     
  5. Wibble

    Wibble Fish don't talk
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    Passage planning needs other information, not always available on the charts.

    Navionics (unfortunately been taken over by Garmin who are greedy data-slurping bastards) are great charts, using the standard symbols in vector format. The online version of the Navionics charts is a subset of the ones that come with an iPad, missing the tidal stream information. On the iPad you have to pay an annual Admiralty licence fee for the tidal info outside of a one week window.

    Passage planning depends largely upon the boat you're driving/sailing. If it's a RIB, all you're interested in is rough patches of water (e.g. tidal races around Portland) and restrictions as you've the speed to pretty much ignore the tides. Diving's after the slack periods, normally -- but not always -- corresponding to high or low water. If it's a slower vessel or with a deeper draft, e.g. sailing boat, you're far more affected by the tide which can add many hours to a passage duration and sailing direction; there's whole courses on this.

    The info you really want is missing from Navionics. This is a full tidal atlas which generally comes in book form, but is normally an appendix to an Admiralty chart. You can find it inline by searching ("tidal streams dover") but I've struggled to find this in a neat iPad format which doesn't rely on t'intarwebs (feck all use when you're at sea). These tidal streams maps have special nomenclature indicating neaps and springs flow rates in knots; you need to interpolate these values when between springs/neaps. You use the larger atlas for overall planning, but certain locations such as Portland require localised info for planning. The trick for using these is to scan for the optimum tidal direction at certain times -- known as a tidal gate -- and aim to be at that point at that time. Sometimes you're using the overall times to calculate the shortest duration, meaning the optimum departure time (e.g. westwards from dover is 4h after high water).

    Passage planning is a black art. Don't make the mistake of thinking tides are simple sine waves, they're not. The Solent has some of the most complex tides in the world, more resembling a molar than a sine wave. I used to have an old Windows program which was an excellent passage planning tool, calculating the optimum time for departure for minimum passage times. Alas this was never ported to iPad and has long been deprecated. Never found a useful one since, probably as most regions of the planet don't have the complex tides we have around the UK.

    The next problem that's slightly covered, but pretty poorly by Navionics, is the port info. For this you normally buy a "Pilot Book" -- such as Tom Cunliffe's Channel Pilot -- and resort to paper. For some crazy reason (that'll be copyright), these aren't available on Kindle or PDF. The vital info here is a local port chart, calling frequencies for Port Control radio, info about marinas (where to berth, numbers to call) and local instructions (where to get a beer and curry). Often they contain localised tidal atlases and passage planning info (for example Dover's a bastard to get into).

    I guess the last specialist info you need is wreck info. Wrecksite's the mutts nutts for this. Again, you have to pay the Admiralty licence for access to their (raster) charts. Once found on Wrecksite, you can transfer this over to Navionics and plan from there. Remember that exact locations of wrecks can be surprisingly inaccurate.

    And then once you do set off, don't forget to monitor your instruments -- depth, wind -- and make sure that you use the local navigation conditions to validate your position. Buoys move all the time. You look a right dickhead when you're aground on the Bramble Bank 'cos you believed the tidal info and didn't factor in inaccuracies of charts, tidal offsets, high/low pressure. As heard in the Solent once "... he's gone aground on the Bramble Bank and still has his sails up. Maybe he's in a ploughing competition...". Then a bloody mayday from the prat who should simply spend the few hours on the boat's side silently pondering what improvements need to be made to his sailing skills. (East Coast sailors hate the Solent numpties)
     
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  6. Wibble

    Wibble Fish don't talk
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    Aught to have said that there's a big benefit of physical books and charts; they don't need battery nor intarwebs access. Not much cop on a RIB though!
     
  7. georgedavo

    georgedavo Member

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    Thanks for the detailed answer, much appreciated.

    I've got a Nautical Almanac which has the tidal streams and tidal curves and a little bit of information on the ports. Since its RIB diving generally its gonna be pretty simple slipways rather than full-blown marinas.
     

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