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Decompression Algorithms: ZHL-16C, VPM-B, DCAP, DCIEM, VVAL-18M... Does It Really Matter?

At the risk of annoying those who do have a preference for a specific decompression algorithm, the simple answer for most divers is NO the algorithm is not critical. There is no expert consensus that any one of the current crop of decompression algorithms is better than another. All of these algorithms used in dive computers and desktop table generation software, when set to their default conservancy values, will get you out of the water with an acceptable margin of safety. What we can say for sure is they are all imperfect representations of actual decompression in humans.

Numerous variants of ZHL-16C are very widely implemented in both sport and technical dive computers. For technical diving, versions of ZHL-16C that include user configurable Gradient Factor modifications are very popular because the GF values can be 'tuned' to provide different types of profiles for specific types of diving. VPM-B dive profiles typically have deeper initial stops, along with reduced time at shallow depths resulting in a 'smoother' profile although recent research calls into question the benefits of 'deep stops' especially for lengthy VPM-B profiles. DCAP was developed for use by early extended range divers (today we call them 'technical divers'.) VVAL-18M is the basis for the modern US Navy Tables. DCIEM has been extensively tested by the Canadian military to ensure its applicability to cold water working divers. RGBM (basis for NAUI tables with its roots in VPM) and DSAT (basis for PADI tables) are most often seen in no stop required sport diving applications. Recently, the RGBM model has been called in to question by a legal action, but it's not clear if the issue is with the algorithm itself or a specific dive computers' implementation, as most experts consider the RGBM model to be very conservative especially on repetitive dives.

The practices of decompression are not exact, in many ways as much about skill as science. Much of what we do in decompression diving is based on empirical observation and experience, rather than having a basis in theoretical science. Dr. R W (Bill) Hamilton, the late co-developer of DCAP and whose research in decompression is widely acknowledged as having a key role in opening up recreational extreme exposure diving in the early 90s, was fond of the saying 'what works, works'. The most important safety factor is not the decompression algorithm you select, rather your skill as a diver and that you closely follow the recommendations of that algorithm and safe diving practices in general. Also, best practice when diving as a team is that all divers should use the same algorithm in order to remain together as a team during ascent phase of the dive plan.

Corsair Rock

Bommie Dive Bommie Dive | Boat access Boat access

Advanced Open Water Rated Crayfish Dive Site Outside Port Phillip Reef Dive Site Slack Water

Depth: 3 m (9.84 ft) to 30 m (98 ft)

Level: Advanced Open Water and beyond.

The name Corsair Rock has sent shudders down the spine of many a seaman navigating Port Phillip Heads. The rock has been responsible for a large number of shipwrecks and strandings.

Corsair Rock is a submerged rock pinnacle that marks the south-western extremity of the submerged Nepean Bank, the sand and reef shallows that extend from Point Nepean and the drying Nepean Reef through to Corsair Rock. It is a flat-topped rock 6 metres in diameter, with 3.5 metres of water over it. Its location is indicated by a swirl above it on the flood tide and breaking short seas and whitewater on a strong ebb tide.

When conditions are favourable the kelp-covered top of the rock may be seen from the surface. When conditions such as this exist diving around Corsair Rock is at its best.

Corsair Rock is sometimes called Outer Corsair, while, heading towards Point Nepean, a shallower and broader patch on the inside part of the Nepean Bank, about 25 metres across and 150 metres long on a north-south axis, is known as Inner Corsair. The wreck of the RMS Australia (1904) lies on this part of the reef at a depth of 3 to 4 metres.

The Rip & Tides Warning: Always keep an eye on sea conditions throughout any shore or boat dive within "The Rip" (aka "The Heads"). This is a dangerous stretch of water, where Bass Straight meets Port Phillip, which has claimed many ships and lives. Please read the warnings on the web page diving-the-rip before diving or snorkelling this site.

Corsair Rock History

It had been known among ship pilots and experienced mariners that a hazard existed in this area, when ships entering The Heads believing they had sufficient clearway suddenly struck an uncharted and submerged obstacle. Two such vessels were the William Salthouse (1841) and the Isabella Watson (1852). Both had struck Corsair Rock prior to its gazettal on 7 November 1853. The inward bound Ontario (1853) was following the French ship Marie through Port Phillip Heads when both struck Corsair Rock just seven days after its gazettal. The unfortunate Ontario broke up and sank almost immediately while the Marie was able to be run aground and was later refloated.

While it is popularly believed that Corsair Rock was named after the wreck of the pilot cutter Corsair was wrecked on it, in fact it was in the Corsair that pilots first located and charted the position of the notorious obstacle in October 1853. The Corsair was not wrecked on Nepean Reef until 19 years later, in 1874.

The difficulties of surveying and navigating this area of The Rip have meant that shipping is simply warned to keep clear of this area by authorities, though smaller boats can traverse the shallow channel. The RMS Australia wreck was blasted to keep this channel between Corsair Rock and Nepean Reef open for fishing vessels. The beacon on Beacon Rock and the Westfall beacon, on Point Nepean, are navigation markers to provide a transit for Corsair Rock and warn mariners of the location of the hazard.

Crayfish Dive Site
Crayfish Dive Site | © Ian Scholey

Divers have the opportunity to catch Southern Rock Lobster (aka Crayfish) at this dive site. Remember your catch bag, current Victorian Recreational Fishing Licence, rock lobster measure, and cray tags. Once you get back to the dive boat, or shore, make sure you clip the tail and tag your Crayfish as per Fisheries requirements. Please abide by all current fishing regulations if you intend to catch crays. See article-catching-crayfish for practical cray hunting advice from The Scuba Doctor, plus melbourne-cray-dives for a list of other crayfish dive sites near Melbourne. For tips on cooking your Crays, please see article-cooking-crayfish.

Boon Wurrung / Bunurong country
Boon Wurrung / Bunurong country

Traditional Owners — This dive site is in the traditional Country of the Boon Wurrung / Bunurong people of the Kulin Nation. This truly ancient Country includes parts of Port Phillip, from the Werribee River in the north-west, down to Wilson's Promontory in the south-east, including the Mornington Peninsula, French Island and Phillip Island, plus Western Port. We wish to acknowledge the Boon Wurrung as Traditional Owners. We pay respect to their Ancestors and their Elders, past, present and emerging. We acknowledge Bunjil the Creator Spirit of this beautiful land, who travels as an eagle, and Waarn, who protects the waterways and travels as a crow, and thank them for continuing to watch over this Country today and beyond.


Corsair Rock Location Map

Latitude: 38° 18.017′ S   (38.300283° S / 38° 18′ 1.02″ S)
Longitude: 144° 38.481′ E   (144.64135° E / 144° 38′ 28.86″ E)

Datum: WGS84 | Google Map
Added: 2012-07-22 09:00:00 GMT, Last updated: 2022-05-07 23:46:08 GMT
Source: GPS
Nearest Neighbour: Campbell, 122 m, bearing 284°, WNW
Depth: 3 to 30 m.
Dive only on: SWE.

DISCLAIMER: No claim is made by The Scuba Doctor as to the accuracy of the dive site coordinates listed here. Should anyone decide to use these GPS marks to locate and dive on a site, they do so entirely at their own risk. Always verify against other sources.

The marks come from numerous sources including commercial operators, independent dive clubs, reference works, and active divers. Some are known to be accurate, while others may not be. Some GPS marks may even have come from maps using the AGD66 datum, and thus may need be converted to the WGS84 datum. To distinguish between the possible accuracy of the dive site marks, we've tried to give each mark a source of GPS, Google Earth, or unknown.


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