Blind Nav in Sailing Exam.

[toad_oftoadhall]

During a recent race I incorrectly guess-timated an initial CTS to 'the next mark'. As so often happens the real world didn't match the predictions. In broad daylight with pots/previous mark to see the 'real' tide against the correction required was obvious and made before the leg actually started, so cost us no time. However, it set me thinking about Blind Nav for people taking UK sailing qualifications. It strikes me as fairly common for a CTS to be a wee bit off. That costs little or no time if you can see but if you were "blind" could have you the wrong side of a mark or a large distance off target.

So how accurate do candidates have to be and over how great a distance?

All the factors involved are fickle so to be within (say) two boat lengths off the target after two miles would strike me as being as much about luck as skill.

There seems to be a paradox. If the conditions are right and candidates are allowed a reasonable margin of error for the imponderables the candidate has proved little. If the candidate has to hit b) bang on the nose then the test becomes a measurement of how "lucky" the candidate was.

Discuss.


PS: The above assumes the blind nav 'test' is to start from a) work out a CTS to b) and follow that CTS to pass close to b) without checking position between the two [1], but correcting only for those factors which would be known in thick fog like speed etc? (And therefore comparable to a club racer working out a CTS that will take him to the next mark, with the only difference being the the qual candidate won't be allowed to see to make a correction for external position clues.

 

[Sandy]

So how accurate do candidates have to be and over how great a distance?

As accurate as you need to be getting into a harbour. For as far as you are from that harbour.

From what I have see it is not precision navigation, you are not trying to dock at the ISS.

 

[DJE]

PS: The above assumes the blind nav 'test' is to start from a) work out a CTS to b) and follow that CTS to pass close to b) without checking position between the two [1], but correcting only for those factors which would be known in thick fog like speed etc? (And therefore comparable to a club racer working out a CTS that will take him to the next mark, with the only difference being the the qual candidate won't be allowed to see to make a correction for external position clues.

Whenever I have practiced it without GPS I have run a constant DR plot and checked it against the depth, choosing a route to follow or cross an obvious contour if possible. I would never expect a DR position to be within a couple of boat lengths. I have heard of a technique where you hold position bow into the tide at each buoy using the log and compass to measure the actual speed and direction of the set before working out the CTS to the next buoy. That should be better than using a tidal atlas, but never tried it myself.

 

[flaming]

That's not the normal tactic of blind nav though. Normally you deliberately aim off, hit a contour line and follow it to your goal - precisely because you know you won't be super accurate in a CTS.

I did one several years back where the challenge was to find one of the lateral markers at the entrance to Cowes, starting from North of the Brambles. It was a beat, so I just had them sail on starboard until we started picking up the shallowing water of the bank, then tack until it got deep and repeat. When we tacked onto starboard and the water didn't shoal I knew we'd cleared the bank - but I didn't have an especially accurate fix at that point. So I aimed cautiously quite far West of Cowes, found the contour off the green and followed it to the buoy. Instructor told me to come on deck when we were 20 feet away. Safe navigating, but nothing like what my track would have looked like had it been the next mark of a race course.

 

[marklucas]

As said accurate enough to get you to your destination without hitting anything - so make sure you're away from rocks but can follow a contour or know when you expect to hit the next one.

With practice you can get very accurate - I have gone from Chichester bar to to Ballast in Portsmouth Harbour before now - and called the buoy (as it was then) to within 50ft.

The real trick is to be prepared: have a hand bearing compass on the chart table, make sure the log is working and accurate, either be very good at speed / distance mental calculations or use almanac tables and try to imagine what the sea bed actually looks like e.g. if there is a finger of shallower water then it probably extends beyond the 10m contour. This can give you some very accurate fixes during the passage so that any errors do not become cumulative.

 

[toad_oftoadhall]

As accurate as you need to be getting into a harbour. For as far as you are from that harbour. From what I have see it is not precision navigation, you are not trying to dock at the ISS.

contour line

So not precision nav and you can use the echo sounder. Nobody has ever mentioned that to me - funny how these things get built up in the recounting. Thanks all.

 

[whipper_snapper]

As others say, as long as you know roughly where you are and the amount of tide, depth can very often provide pinpoint accuracy. On two occasions when I have been doing blind nav for fun, the crew on deck had to alter course without telling me to prevent us hitting the buoy I was going for.

 

[Barnacle Bill]

During a recent race I incorrectly guess-timated an initial CTS to 'the next mark'. As so often happens the real world didn't match the predictions. In broad daylight with pots/previous mark to see the 'real' tide against the correction required was obvious and made before the leg actually started, so cost us no time. However, it set me thinking about Blind Nav for people taking UK sailing qualifications. It strikes me as fairly common for a CTS to be a wee bit off. That costs little or no time if you can see but if you were "blind" could have you the wrong side of a mark or a large distance off target.

So how accurate do candidates have to be and over how great a distance?

All the factors involved are fickle so to be within (say) two boat lengths off the target after two miles would strike me as being as much about luck as skill.

There seems to be a paradox. If the conditions are right and candidates are allowed a reasonable margin of error for the imponderables the candidate has proved little. If the candidate has to hit b) bang on the nose then the test becomes a measurement of how "lucky" the candidate was.

Discuss.


PS: The above assumes the blind nav 'test' is to start from a) work out a CTS to b) and follow that CTS to pass close to b) without checking position between the two [1], but correcting only for those factors which would be known in thick fog like speed etc? (And therefore comparable to a club racer working out a CTS that will take him to the next mark, with the only difference being the the qual candidate won't be allowed to see to make a correction for external position clues.

You've put your finger on why it's rather an artificial 'test'.

As good practice, I would encourage people to do exactly what they would actually do if they needed to get somewhere in a fog. Yes, use GPS: it's as likely to be working as echo sounder is. And use the radar if you have one. Excellent training, that you might actually use some day!

 

[alant]

During a recent race I incorrectly guess-timated an initial CTS to 'the next mark'. As so often happens the real world didn't match the predictions. In broad daylight with pots/previous mark to see the 'real' tide against the correction required was obvious and made before the leg actually started, so cost us no time. However, it set me thinking about Blind Nav for people taking UK sailing qualifications. It strikes me as fairly common for a CTS to be a wee bit off. That costs little or no time if you can see but if you were "blind" could have you the wrong side of a mark or a large distance off target.

So how accurate do candidates have to be and over how great a distance?

All the factors involved are fickle so to be within (say) two boat lengths off the target after two miles would strike me as being as much about luck as skill.

There seems to be a paradox. If the conditions are right and candidates are allowed a reasonable margin of error for the imponderables the candidate has proved little. If the candidate has to hit b) bang on the nose then the test becomes a measurement of how "lucky" the candidate was.

Discuss.


PS: The above assumes the blind nav 'test' is to start from a) work out a CTS to b) and follow that CTS to pass close to b) without checking position between the two [1], but correcting only for those factors which would be known in thick fog like speed etc? (And therefore comparable to a club racer working out a CTS that will take him to the next mark, with the only difference being the the qual candidate won't be allowed to see to make a correction for external position clues.

When doing my Prep many years ago, my instructor started us at one of the green marks at the outer entrance of the Hamble River & we used the 5m contour to go blind all the way up the river, crossing over above Mercury & picking up the next one, proceeding to our intended mark, which was a post near Moody's. All of us got within about 10m of the post.
No 'tide' as such to worry about, but it all seemed like 'white mans magic' to get that accuracy.

When teaching CTS in the Solent, getting within 100m, is acceptable & achievable, even with inexperienced DS candidates.

PS
Many, many years ago, when originally being shown how to follow a contour, I was given the helm & asked to go from Gurnard to Newtown Creek. We were under engine & I concentrated hard on steering +/- 3m along the contour. Our boat wriggled its way toward Newtown, eventually reaching the entrance, near the little red marker buoy. The instructor & crew were well impressed with my capability in succeeding. It was only afterwards, whilst we were at anchor, I realised that the depth guage I had been using, was in fact the one reading windspeed. I told no-one.

 

[alant]

You've put your finger on why it's rather an artificial 'test'.

As good practice, I would encourage people to do exactly what they would actually do if they needed to get somewhere in a fog. Yes, use GPS: it's as likely to be working as echo sounder is. And use the radar if you have one. Excellent training, that you might actually use some day!

One 'trick' I was shown, if fog/mist seemed imminent, was to find the nearest lateral mark & check the sea depth around it with your sounder. Rather than reliance on a calculated figure, this should give a instant & more accurate depth reading of the contour it is on, allowing you to proceed along it.

 

[toad_oftoadhall]

you might actually use some day!

From the average age here, the overwhelming majority of us will have experience of feeling our way about in fog before GPS came along, myself included.

Somewhat fewer of us will have done 'Blind Nav' as a "test" and that's what my question was about.

 

[capnsensible]

From the average age here, the overwhelming majority of us will have experience of feeling our way about in fog before GPS came along, myself included.

Somewhat fewer of us will have done 'Blind Nav' as a "test" and that's what my question was about.

Im lucky enough to get to practice contour following nearly every week.

2 tips, keep up to time with the tide curve if you are in serious fog.

Rather than trying to use a set depth eg 7 metres, try a slow weave between , say, 6 and 8. Shallow angle in, bit sharper out.

Have used this plenty in real fog, once from Cape Trafalgar to Cadiz. SHM of Cadiz harbour was the first thing we saw in 10 hours....

Practice makes it straightforward and makes a 'test' simple.

Oh yeah, its often better to end up in the unusual position of down tide from your destination, keeps sog low whilst you approach the end.

Hope this helps.

 

[Ehecatl]

This is part of the Coastal Skipper test, well it was for mine.

 

[Tony Cross]

The blind nav test on my exam was done just as we left the marked channel into Poole Harbour, we were heading for Swanage. I got us to Swanage though not with a CTS but by following the 5m depth contour as has been mentioned. I must admit that I chose the 5m almost at random, and in the wrap-up after the exam my examiner said he was pleased to see that I got out of the main ship channel quickly, an important thing to do in fog he reminded me. Naturally I didn't tell him that in my panic I'd forgotten all about us being in the ship channel....

I hope he's not reading this!

 

[marklucas]

One 'trick' I was shown, if fog/mist seemed imminent, was to find the nearest lateral mark & check the sea depth around it with your sounder. Rather than reliance on a calculated figure, this should give a instant & more accurate depth reading of the contour it is on, allowing you to proceed along it.

This is just general good practice.

Whenever I charter from Hamble, I always sail / motor directly between port hand piles south of Hamble Point - they're on the 0m contour - depth gauge calibrated!

 

[flaming]

This is just general good practice.

Whenever I charter from Hamble, I always sail / motor directly between port hand piles south of Hamble Point - they're on the 0m contour - depth gauge calibrated!

I assume you never leave at low water then!

 

[toad_oftoadhall]

Whenever I charter from Hamble, I always sail / motor directly between port hand piles south of Hamble Point - they're on the 0m contour - depth gauge calibrated!

It shelves steeply there and the contour isn't shown as dead straight between the two poles (and isn't in practice).

I'm sure you'd see the depth climb/fall through the zero CD point but isolating the bit that's bang on 0 would be impossible in my view.

So ok as a very rough check - no more.

...and that assumes the chart is gospel.

 

[Daedelus]

The CTS is the start and then the depth is the cross check - after you have allowed for current Height of Tide.

In an exam I once was asked to get us to the Hamble from the Bramble bank and decided I'd aim for Southampton Water and when it deepened to show I was in it turn and follow the contour up Southampton water.

This would have been alright but the increasingly plaintive cries from below of Are you sure you're steering 300? Were met with constant reassurances from the helm until finally the examiner took a peek at the compass and said No, you're not you muffin, you're steering 330.
Oh sorry, says helm, I haven't got my reading glasses and can't actually see the compass.

This lead to the total collapse of examiner and the rest of the crew until finally someone recovered enough to say that we were now steering 300 by autopilot.

There are useful ups and downs in depth along the straight line towards the Hamble so you can sort out pretty closely where you are. SWMBO told the examiner he was being rotten to me when he called down to tell me the IoW ferry had taken out the lit E Cardinal so I couldn't use that to enter. He just grinned and told her he wanted to see what I could really do.

We eventually turned into the Hamble on blind nav (they had reassured me they wouldn't let me hit one of the piles) and the examiner said ok, where do you think we are? When I said, close to No3 pile he said come and have a look. I could have hit it with a well aimed digestive biscuit and he professed himself satisfied.

One thing I did find invaluable was the kitchen timer with the time and distance tables so that I knew to the second how long between depth changes and could relate this to distance travelled.

 

[snowleopard]

This is part of the Coastal Skipper test, well it was for mine.

Rather to my surprise I wasn't asked to do it for my YM (1993).

A good tip I heard for estimating tidal set was to get the helmsman to go alongside a buoy and stem the tide, i.e. keep station on the buoy. You can then read the speed and direction of the actual current from your instruments down below.

 

[Simondjuk]

So not precision nav and you can use the echo sounder. Nobody has ever mentioned that to me - funny how these things get built up in the recounting. Thanks all.

Sometimes the echo sounder 'breaks' and a lead line must be used, which livens things up a little.