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As others have pointed out, laptops are virtually impossible to read on deck in daylight and are inadequately waterproof to be used there, or even below decks while offshore, in any event. They also use much more electricity than a dedicated chartplotter.
On my last boat I purchased a standard-horizon colour chartplotter and an identical unit in black and white as a back up. This was kept in the box and could be placed in the oven during an electrical storm. This has the advantage of giving you a direct 'plug and play' replacement for the head, the antenna and the cable should any of them fail.
On my current vessel I have a Garmin 2006C mounted outside at the helm and have just purchased a 3006C for the chart table. Both units can operate off the same charts/antenna, the second essentially as a repeater. Again, in the case of an electrical storm the one unit can be disconnected quickly and placed in the oven, and I now have a back-up cable and antenna usable on both units.
In addition, I have a separate fixed gps and a handheld gps and finally, a plastic sextant if I am very unlucky (and of course, I also keep paper charts as a backup).
Overkill? Maybe. But the chance of both chartplotters going down in these circumstances would be miniscule in comparison to the risk with a non-waterproof/resistant laptop with a single antenna/cable. Yes, the Bluecharts are expensive if you are planning on covering large areas (as were the cmap nt+ used with the Standard-Horizon). Yes, two chartplotters adds to your capital outlay (although in the present case, my boat already had the 2006C and the 3006C was compatible and still available). However, it strikes me that there are four areas where redundancy is critical to the offshore sailor: spare halyards/sheets/furling lines; spare sails; an accessible and usable emergency tiller; back-up navigation equipment/charts.
Sometimes more is more. If you can only afford one system, I would still recommend a dedicated chartplotter that is available in both colour and black and white models. I would mount it below decks where, despite being 'waterproof', it would be apt to develop less problems. I would, of course, still have a handheld gps, a sextant and paper charts as a backup. Later, if I had some additional cash, I would look for a black and white version of the unit as a back-up.
Brad
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