How accurate is your navigation

[Rob_Webb]

This afternoon I was put under the sweats with my coastguard training. We blasted out of the harbour and I was put under a dark blanket with only radar screen and plotter in sight, had to call instructions to helmsman. By using combination of GPS/ plotter and radar I managed to steer him between 2 leading line entry posts only 10m apart and into entrance to Gulf Harbour with breakwater 10m either side of boat. Combination of GPS/plotter for initial indication of course but radar for final physical reference.... except that within about 15m of target radar signal disappears... eek!

Was pretty happy with result, done with group of experienced CG chappies under calm training conditions hence zero risk. Great training.

Not done this kind of thing before as part of routine sailing leisure time but having done it I can see huge benefits. How often do you guys try this kind of thing?

 

[fireball]

Went out for night sail training in Solent - Cowes to Hamble .. easy run ... well, would've been if the Fawley tower hadn't disappeared into thick fog when we got to Bramble bank!!
We did have laptop on as chart plotter and I had to call up heading requests - keeping out of the channel! Found that the GPS didn't update quickly enough and had to swap the GPS input to the Garmin Etrex!
Pleased with the end result as none of us could see where we were going but managed to get into Hamble river with no problems - we knew it was really foggy when I called out what lights they should see and they couldn't see it till it was along side!!

 

[paddywackcocker]

Had the same sort of thing at the end of day sckipper practical just for fun, back in 92. Had to navigate into Falmouth with just, compass, depth, chart, log and wind direction.
Under sail, you had to call out change in course to the helmsman and work off the contours. It was good fun but hard work especially when the helm shouted no wind and you had to give a nother heading and then try get back to being one step ahead again.

 

[Stevie_T]

Yes its fun to do and a very useful skill to learn.
I presume though you did not forget use of depth guage as the most useful aid of all?

 

[webcraft]

Sorry Rob, but I don't see what is hard about blind navigation using a chartplotter!

Using only the log, compass and echo sounder is a bit more challenging though - you will get to practice this on any RYA practical course and will be examined on it if you sit your Coastal Skipper or Yachtmaster exam.

- Nick

 

[Birdseye]

[ QUOTE ]
Sorry Rob, but I don't see what is hard about blind navigation using a chartplotter!

Using only the log, compass and echo sounder is a bit more challenging though - you will get to practice this on any RYA practical course and will be examined on it if you sit your Coastal Skipper or Yachtmaster exam.

- Nick

[/ QUOTE ]

Not true - or certainly wasnt on my YM practical. When down below and laboriously working out an EP etc, I was asked by the examiner why I wasnt using the available GPS for my blind navigation. Mind you he was a really practical seaman, and took a dim view of me sticking up the cone to motor into harbour too.

 

[T_S]

Yep agree with you there, just recently completed my C.S pracitcal and had to navigate around sand banks on the east coast!

Enjoyed the sense of achievement!

 

[Rob_Webb]

Webcraft - perhaps I didn't give enough background/context.

Firstly, my point is that the chartplotter is not 100% accurate and when making a blind approach the only reference you can rely on is a physical one. And don't forget that not all hazards are visible on a plotter - the obvious one being other vessels! Hence my point about learning to use a combination of inputs inc. plotter and radar with tight accuracy.

Secondly, I didn't mention the speed element. Of course we made the actual final entrance at slow speed. But the initial approach into the area was at 40 kts. We departed the marina with me under the covers and then made a high-speed run in various directions to disorientate me. Then I was tasked to provide quick answers in terms of CTS, TTN, speed etc. Being a yachtie I'm used to making decisions at 5kts - but at 8 x speed decisions suddenly have to come faster - hence the sweating factor!

The purposes of the whole exercise was to simulate a rapid response callout when we might be navigating at high speed at night in poor vis towards a casualty.

 

[Ships_Cat]

I don't see what is hard about blind navigation using a chartplotter!

It is so hard that it is impossible to do, for example, in the instance that Rob mentions of navigating between two entry posts 10m apart. Using a chartplotter assumes that both the GPS fix and the chart are accurate to within much less than 5m - it won't happen.

If you find it easy (and accurate), perhaps you could let us all into the secret /forums/images/graemlins/smile.gif.

John

 

[DeeGee]

[ QUOTE ]
I don't see what is hard about blind navigation using a chartplotter!

It is so hard that it is impossible to do, for example, in the instance that Rob mentions of navigating between two entry posts 10m apart. Using a chartplotter assumes that both the GPS fix and the chart are accurate to within much less than 5m - it won't happen.

If you find it easy (and accurate), perhaps you could let us all into the secret /forums/images/graemlins/smile.gif.

John

[/ QUOTE ]Using a chartplotter assumes that both the GPS fix and the chart are accurate to within much less than 5m - it won't happen.... not wanting to get too deep into this argument, but surely, it DID happen. That is all he had, unless he left out something? My GPS on a good day says it is accurate to about 5-7m, and surely modern fixed shoreside stuff can theoretically be as good as DGPS, which is simply dead-on.

 

[BrendanS]

"That is all he had, unless he left out something?"

He didn't miss it out. He also used radar.

 

[Rob_Webb]

Brendan is correct I used radar as well. The process was to use the plotter to get an initial approximate location for the target (in this case 2 posts). Then once on a course towards them to use the radar overlay to find the true physical position. As we approached a difference opened up between the theoretical chartplotter position and the true (radar) position. At this point I ignored the chartplotter position and guided us thru on radar only. The only hearstopping moment came when approaching to within about 15m of the target (posts) and the image disappeared from the radar. The next time they showed up again they were immediately astern of us once we were about 20m clear. Meanwhile the chartplotter was showing them both a good few metres off centre and so if we'd used plotter only to try the maneouvre we would have missed going between the posts or worse still hit one!

 

[webcraft]

Re: 40 knots

Ah well . . .

That certainly makes a difference. At 40 knots any precision instrument navigation is difficult, so well done that man and ignore my previous remarks . . .

Would be fun to try it assuming GPS failure and using just log, compass and echo sounder sometime though - total heart failure at 40 knots I would imagine.

- Nick

 

[DeeGee]

Ooops, twas me that missed it out - maybe I was subconsciously nulling it out, as I find it no good within about quarter of a mile? I must admit to surprise that he managed to get within the 10m gap, but it should be theoretically possible,no?