Loch Tarbert, Jura

[DaveS]

Last weekend I had the place to myself and anchored in Cuan Bay. I came to the conclusion that the 111 (approx) transit is not correctly laid down on the chart (or the Pilot). The charted line leads quite close to the rocks at one side of the channel, but following the actual transit keeps you in the middle of the channel - which would seem rather more sensible. The recorded track line on the plotter was c. 2 degrees different from the charted transit, and it agreed with the real world, i.e. it's not a chart datum issue. Has anyone else found this?

I thought of going ashore to see if I could verify the positions of the transit markers (HH GPS and/or horizontal sextant angles - an interesting exercise in its own right) but couldn't be bothered to inflate the dinghy... Next time, maybe.

[claymore]

I've not got my chart to hand but is that the wee place on the left before you go through into the middle section?
I motored along the transit line before you hang the final left and was miles of track according to my nav.software.
Long John was with us and was throwing duckfits but we didn't hit anything.

[DaveS]

Yes, the bay to the north after you reach the end of the 111 transit and before going through the narrows "on sight". Lawrence describes it as "the most popular" anchorage, but I think most people carry on through the narrows.

So it's not just me then?

I think I will mount an expedition to seek enlightenment...

[Silkie]

I've not been in but have been thinking of it for this weekend - hence re-visiting this thread.

Comparing the sketch in my (1981) CCC directions with the brand new Admiralty leisure folio, the back beacon of the 111 approx transit is shown in a significantly different position. The CCC put it about 3/4 cable back while it's fully 2 cables away according to the Admiralty.

Also, the charted transit barely clears the rock on the Admiralty chart but it never gets closer than 50m on the CCC sketch. As near as I can measure (it is only a sketch) the CCC transit is about 108 which would probably put you on the rock on the Admiralty chart. The CCC seems to correspond more closely with your experience Dave.

In which direction was the 2 degree difference you saw?

[DaveS]

Can't remember without looking at the chart - which is on the boat. I'll get back to you. I was using the Lawrance pilot rather than the CCC one and his drawing seemed the same as the chart. Interesting if the CCC version is different - and correct!

[Silkie]

Too feart to post the Admiralty version (for the benefit of those who don't have it) in case I get Nick into trouble again.

They should really thank us for investigating these areas of course - perhaps an Honorary Doctorate of Hydrography might be considered appropriate.

[DaveS]

Yes, comparing the two Pilots, I think the CCC version is definitely closer to reality than the BA chart / Lawrence Pilot, and that the actual transit is about 109 rather than 111. The one oddity is that, as I recall it, the cairns are quite a distance apart both vertically and horizontally which corresponds better to the BA version ! Definitely requires a site visit! A project for later this year...

[claymore]

When Longjohn and I went in there was a yacht parked on the rocks on the first outcrop. We seemed to go to the left of the transit but the projector line on my laptop had us lining up nicely. Didn't hit anyway. So are you brave enough to go through to the final section? We went up in the dinghy but it would be a braver man than me that would do it on sight.

[DaveS]

It depends what you mean by the "final section". I certainly plan to go through the Cumhain Mhor narrows - just past the disputed transit - but I'm not so sure about the tangle of transits to get into the inner, inner loch. I think I'd want company for that so that steering and transit watching could each be given 100% concentration...

[claymore]

Thats the bit we went into in the dinghy. There seems to be a lot of transits but none of them seemed to make much sense. I think it recommends going in on a rising tide which is fun as it clips through the narrow rocky channel at a knot or two - definately was going down hill!

[DaveS]

Right, after much thrashing about in the bracken I've now got the positions of all 6 beacons fixed by handheld GPS. Guess what? All 3 transits are wrong! I've sent the HO my findings together with a photo of my chart with the corrections added. For the reasons given by Silkie above I'm a bit reluctant to post it, but I'll be happy to copy my note and/or the photo (over 1Mb) to anyone interested enough to PM me.

In brief, the 111 transit is really 107 and the beacons are c. 120m further N, the 061 transit is really 059 and the beacons are c. 30m further E, and the 247 transit is really 243 and the beacons are c. 120m and 200m further E. All of which agrees pretty well with the CCC version.

[Silkie]

So, is it the case that the physical transits will keep you right but are shown incorrectly on the chart or have the fairies been moving the cairns? Or is this beyond the scope of your study?

[DaveS]

The physical transits are right, but the HO doesn't show them in the right place. As for moving the things, they'd need tae be gey hefty fairies: they're cast concrete pillars about 4 foot high - rather like OS trig points in fact, but a bit rougher finished.

The "247" transit front marker gives good support to my claim: as charted it is about 60m back from the shore: my position places it less than 10m from the shore and, in fact, it sits immediately above the beach.

Given that the CCC positions agree, it occurs that I may not be the first to carry out this exercise and that this might already have been reported to the HO. Anyone know?

[Silkie]

Nice bit of work Dave. Keep us updated about the response from the HO if you can find the time these days.

[jim.r]

Why the 'eck do you want to take a perfectly good yacht into a rocky old place? Ain't there enough nice places up there?