Tuesday, July 07, 2009

Review: Sailing with Navionics iPhone Charts

Before heading off for my Sunsail bareboat charter holiday reviewed earlier, took the time to download some Navionics charts. It was an even better deal than before, costing around US $10 for the whole of the Meditarranian.

So what was it like to use it actually on a boat?

We had effectively three different navigation tools:
1) Chart, Pilot Book and eyes
2) On deck chart plotter
3) iPhone Navionics charts Version 2.1

The chart plotter and Navionics both showed similar chart data as can be seen by the screen above. We could have put in way points to Navionics but to be honest as have mentioned before have found it a clumbersome process, so didn't bother to test that functionality.

I did switch on the "Track" function and it does work as can be seen in the screen shot above. The track is shown as a red line and then the arrow shows the direction of travel based presumably on the last few fixes.

However that only works if you keep the iPhone switched on and if the power saving mode switches it off you end up with big jumps, as in the figure below:

The 255V socket on the boat didn't seem to function unless we were connected to shore power so wasn't able to charge onboard (this wasn't a problem as could just plug it in to recharge at any restaurants we stopped at).

The chart plotter had a very useful predicted line based upon current COG (course over ground) which Navionics does not, and in general it is very helpful. However there was some debate over accuracy at one port where our COG on the chart plotter intersected the land but simple eye-balling suggested we were just fine.

I'm not sure whether that was due to there being a lot of lee-way or due to inaccuracies in the GPS (which we should never forget can have significant errors in its data).

But there was a bigger question here - why use GPS, whether chart plotter or Navionics, at all?

Because in practice we were doing day sails island hopping and we could almost always just look up and see some form of easily identifiable landmark. The pilot books in particular had a wealth of information including historical points of interest and even the names of the taverna owners.

And there is a danger of always looking down at the instruments, not least we spend our working days gazing at a screen do we really want to spend our holidays doing the same?

Having said that there was point at which the Navionics iPhone application proved its worth. There was something on the paper charts which might have been a reef or it might have been a smudge due to a retsina spill, but which was it?

Out with the iPhone, couple of drags and zooms, and yup it's a smudge!

Phew - and so we sailed on with confidence.


Carol Anne said...

OK, so how do these devices compare with a sextant?

Alas, I'm going to miss the opportunity to sail with a boatload of astronomy geeks who would really be into celestial navigation. But Pat and I do have a couple of sextants -- including a really nifty high-quality replica of a Royal Navy lifeboat sextant from the 18th century, the same model that Captain Bligh used to get to civilization after the mutineers set him adrift.

JP said...

I might post on that: during the ARC we took a couple of fixes using one of the sextants we had on board and it was a slow process!

Parkie78 said...

...and realistically the best fix you can get with a sextant is 1 mile accuracy compared to around 10 metres with GPS!

JP said...

... which is enough when crossing an ocean, but not for (say) navigating up the Solent to Hamble!

But I guess that's what eyes are for