From: giddy@astrix.trl.OZ.AU (David Giddy)
Newsgroups: rec.scuba
Subject: More on drift diving
Date: 8 Apr 92 04:32:30 GMT
Reply-To: d.giddy@trl.oz.au
Organization: Telecom Research Laboratories


Anthony DeBoer wrote a fascinating article on drift diving in the Niagra
river, so I thought I might contribute to the subject by describing a
different type of drift diving we do here in Melbourne (Australia not Florida!).

I have only limited experience, so I can only describe my one drift dive (so 
far). I got my C card only last December.

Melbourne is situated at the top of a large bay known as Port Philip Bay.
It's shape is roughly as follows:


				Melbourne
                                 ----\
                               /       \
                              /          \ 
                             /            \           1 - Portsea
                            /              \          2 - Queenscliff
                          /                 \             Bommies dive site
                        /                   |
             Geelong  /                    /
                      --------/ 2         /
                             \     ---1---         
                     ---------      \-------\               
                                             ---------- 

To give you an idea of the scale, it is about 90 km (56 miles) from Melbourne to
Portsea (1 on the map).

The bay is reasonably shallow, and has a mostly sandy bottom. The entrance to
the bay is only about 500 m across and so the tidal flow through here is 
very strong (it is known locally as "the Rip"). A little further into the bay,
the flow spreads out and slows down.

The dive I recently went on is known as the Queenscliff Bommies dive around
location 2 on the map. Here, the tide flows at around 5 - 7 knots and we
dived it on the flood tide. The depth is around 18 m (60 ft). This type of
drift dive is done as follows:

The boat finds the site and cuts engines. A long rope with a large buoy is
trailed out the back of the boat. All the divers then hop into the water and
space themselves along the rope at the surface with about 18 m of rope between
the buoy and the first diver. The dive master is usually at the end of the
rope. When everyone is ready, the DM signals down and you descend as a group to
about 1 m off the bottom. The aim is to keep the rope stretched in a line
(horizontal and perpendicular to the current) and fin slowly with the current.
At the end of the dive, as you get low on air, you can work your way along the
rope and up to the surface next to the buoy. The boat follows the buoy and
picks you all up when the dive is complete. It is a very relaxing form of
diving as you expend virtually no energy.                                         
The day I went on my first drift at this site was fantastic. There were only
three of us on the rope (the buddy system is not adhered to quite as strictly
as everyone is always next to someone else on the rope). We were carried along 
at about 5 knots past sponge gardens, lots of fish and giant bommies (outcrops
of rock rising out of the sea bed). The bommies were up to 15 feet tall and
you have to rise up over them and then drop down the other side. The Viz
was about 15 m this day, so seeing them coming was no problem. As Anthony
pointed out, there is a natural upwelling of water in front of the bommie
which helps carry you over it. When you are on a rope, it is not really
possible to stop and look at things as you will get left behind, it is more
like a leisurely cruise through an enchanted garden. The highlight of the
dive was a ray with about a 6 foot wing span that cruised past us.

The local rules of drift diving say that if you become separated from the rope 
and cannot get back to it, you should abort the dive. 

During my advanced dive course (PADI), I did the drift dive option and our
instructor told us of a phenomenon one has to watch out for. Port Philip
bay is quite shallow, but the sea outside drops off quite rapidly to
300+ feet. He told us once when he was doing a drift near the heads on an
ebb tide and got carried further than expected. He said that they suddenly found
themselves travelling at 12+ knots out through the heads and the bottom
dropped away like a cliff. The effect of the water flowing out was like an
underwater waterfall and they were all carried down rapidly to about 70 m !!
Luckily, all the divers were experieneced, didn't panic and so safely came 
to the surface. He said he will never do it again!

Anyway, that should give you some idea of the type of drift dive we do around
here. Likewise, anyone who wants to try this when they are next in Melbourne,
is welcome to drop me a line.

David.

P.S: If I am slow to respond to followups, it is due to our slow newsfeed. It
is currently about six days behind! (9600 baud). Email works fine.
______________________________________________________________________________
David Giddy,			                 |    Voice: +61 3 253 6388
Telecommunications Research Laboratories of AOTC,|      Fax: +61 3 253 6144
P.O. Box 249, Clayton, Victoria 3168, AUSTRALIA  |      Net: d.giddy@trl.oz.au
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