Goat Herders and passes
Rhiann Marie - Round the World
Stewart Graham
Mon 24 May 2010 19:09
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May 24 0733 Local 1733 UTC
15:14.05S 146:36.62W
We left off the blog last night with us leaving
Toau. That was several days ago now and from Toau we went to Apataki. We had
planned to do only three atolls, but if we get safely in and out of Rangiroa
this week we will have visited 5 atolls and to be honest I could spend another
year here and not do the Tuamotos justice.
Before telling you about Apataki, first I have to
go and have a good lookout on deck as the depths are falling......... OK
everything fine, we are less a third of a mile off the east of Arutua and the
depth is now only 801 metres! You may ask why I am at the keyboard when in such
close proximity to a coral atoll and of course that's a good question. However,
I am sitting at the nav station aboard Rhiann Marie, which gives excellent
visibility, I have the digital depth sounder on which measures depths almost to
1000 metres depending on conditions (it is now 635 metres ....... ) I have the
radar overlayed on the chart to check the GPS/Chart accuracy, God is in heaven
the sun is in the sky and I have to catch up with my web log!
Passes - what do you know about passes? Those goat
herders and football players among you will know plenty about passes -
but not the kind you find in the Tuamotos.
I have already told you about the coral atoll and
the theory of the sunken mountainous island in the middle (that bit I am not
convinced about - but let's play along with Darwin for the moment). So to recap,
you have a coral ring which is the remains of an island that it once
surrounded and has now gone South to middle earth. The coral ring is
at sea level and over the years "motu" have built up on the reef to
form a ring of islands. These are separated by "hoa" which are closed to
seaward by the coral, over which, the sea spills and flows through the many
hoa like rivers, filling up the enclosed lagoon. Then there are passes -
one, two, perhaps three, perhaps none. These are open to the sea from the lagoon
and navigable, some wide and deep, some narrow and shallow and some - well you
get the picture don't you?
Hold it there - I'll just nip outside again for an
eyball check - we are a bit close now .......... yep - I can touch the coconut
palms with my left hand and over the starboard side it is 535 metes - but I'll
adjust course just to be safe. OK.
Passes. Now some people have theorised that these
passes were once the river beds of the now missing mountainous islands
........hmmm. Too far! Whatever they were, we know what they currently are.
Extremely fast flowing tidal channels that can be treacherous but are for the
most part predictable with the tides. They can flow at up to ten knots but
normally three to six knots. This is bread and butter stuff for sailing the
coast of Scotland so they have not fazed me at all, but I must not be complacent
and I have two more to go.... I sort of wish I hadn't said that
now!
All those interested in flowers and pretty fish and
all things bright and beautiful should skip the next couple of
paragraphs. When the tide outside is rising the pass
will flow into the lagoon and naturally, hours three and four will generally be
the fastest flowing (remember the rule of twelfth's?). However, the Ocean is
very large and the pass very narrow and the lagoon capacious, so all the high
level outside takes a bit more than a tide to flow into the lagoon so slack
water in the pass is normally about an hour or so after high water, or of
course conversely, low water. The actual time of slack in the pass will depend
on the ratio of the lagoon volume to the potential pass flow rate and number of
passes in general. So take high water and low water times and add an hour, to an
hour and a half, and glide through at slack water. Or do the sums as we do but
make your plans regardless, turn up at the pass and charge through. "Tut tut" I
hear all the traditionalists say. But hey, today at 0600 I came through a
pass at 13.5 knots - now that's a blast and worth getting up for. Maybe a
touch silly but where's the fun in ......... you know. So today we got up at
first light and left Apataki. We prefer that than doing the pass in the dark.
The best time we can arrive at Rangiroa is when the tide is mid way between high
and low so we should have maximum out flow. I'll let you know how I get on. Off
course if it goes pear shaped you won't hear from me for a while till I get my
story straight.
Passes. So that's the basics. Now consider an
atoll, which has extended reef and is very large and has a pass on the east or
soth east side. The prevailing wind is from the East but of course blows from
all directions. Now take low pressure and a strong blow. You can easily
figure out that the coral reef, even the higher bits of it, will overtop much,
much more than normal. Add spring tides, and you start to get the picture. The
lagoon will fill up hugely more than normal. The tide will fall and you will
have a raging torrent flowing out of your east facing pass into the east wind of
course. But more than that. The tide in some circumstances can turn and
there will be so much water in the lagoon that it has not yet emptied so there
can be out flow even as the tide is rising and in some cases the flow can even
completely reverse! Facinating don't you think?
So while we have listned to many boats obsessing
about what the tide is doing (all 0.5 metre rise and fall) and arranging their
sailing schedules to coincide with their nine digit mathematical calculation of
when slack shoud be (we even met one boat who had built a spread sheet to try
and predit the time of slack! ), others slowed down sailing to spend all night
ghosting along at 2 knots, others made (in my view ) uneccessary overnight
sails to coincide with the slacks, we have got up early and stayed up
late and turned up at the pass and dealt with it. Life is for living. On the
same subject we have met a huge number of boats who are obsessed with the
weather and the weather forecasts. "The wind was 5 - 10 knots more than the grib
file said". Off course it was, look at the mountainous islands and the local
conditions. I am not exaggerating when I say, I met one boat who was taking
seven weather forecasts. For God's sake guys - look out the window
occassionally and if you are that worried stay at home.
Don't get me wrong, we are not in any way
complacent at all about the weather and in no way are we complacent about
our navigation. Our number one priority is to have a safe ship. Non -
negotiable! But neither are we sailing in daily fear of the
environment we are in. Respect - yes, fear - no.
Now that those who left out the last couple of
paragraphs of the blog to watch another back episode of the Blue Planet are back
- oops sorry to tell you we have run out of time and I will have to tell you
about our Black Pearl farming exploits in Apataki ....... and my best catch
so far in the next blog.
Trish's birthday
tomorrow.
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