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Old 15-09-2010, 06:47   #16
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Originally Posted by estarzinger View Post
There is a neat tool at SeamountsOnline where you can locate seamounts by ocean and depth.
That is a pretty neat site. Thanks for sharing.

Now all we need is a video "Seamounts Gone Wild"!
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Old 15-09-2010, 07:12   #17
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Usually with a sea mount, if an major earthquake strikes near one, the most usual change in depth would be caused by the collapse of one side or slope to the mount and this could cause a reduction in the height of the sea mount. It would also tend to spawn a tsunami.

Most sea mounts are in relatively deep water and it is unlikely that a yacht would actually run onto one, though not entirely impossible. There have been several vessels damaged by striking a shallow, uncharted sea mount, including the USS San Francisco (2005) and the SS Muirfield (1973).

There are an estimated 100,000 sea mounts, and only a fraction of them have been charted--about 13,000. Being volcanoes for the most part, sea mounts can grow, eventually some of them becoming proper islands.
Of course, the USS San Francisco (SSN-711) was operating at flank speed (>20 knots) and 500 feet below the surface when it hit...not exactly the realm most of us operate in.

And tsunamis at sea are pretty much a non-event. It's only when the displaced water from a sub-surface land slide reaches shallowing water does the wave become large.

For those that want maximum safety, I suppose a forward-scanning sonar and slow speeds (<3 knots or so, so you don't exceed the sonar range) would be the ticket.
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Old 15-09-2010, 07:45   #18
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To add a bit of fuel to the fire -- Here is a February 2004 note from Noonsite:

"An area to be avoided is at 35*22.25N 51*29.29W where a suspiciously shallow spot was reported by a yacht on passage to the Azores in the summer of 2003. The wreck of a sailing yacht was clearly visible on the bottom at this reported location, so until this report is fully investigated it is recommended to avoid this area especially in rough weather."

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Old 15-09-2010, 08:56   #19
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For the normal RTW sailor, the two spots to be careful are in the shelf to the south of Madagascar (more a continental shelf than a sea mount) and the sea mount I mentioned previously between Capetown and St Helena.

There has been some 'new island formation' in the Pacific; but that's also a bit different than seamounts - and you will usually have quite some warning of that well before you get near.

I can speak from personal experience about the seamount in the S Atlantic. I had carefully routed us around it, but on the day, it was really nice mild sailing (15-20kts) with very flat seas and we decided to just go over it to save some miles. Bad decision. As we were over it, we got one single 15' high perfect curl surfing wave, which swept the boat. Broke our wind wave and cockpit electronics, took off a dorade, and forces a couple gallons of salt water down our diesel tank breathers. We never knew for sure what caused this wave but guesses a landslide down the side of the seamount.
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Old 15-09-2010, 11:34   #20
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There has been some 'new island formation' in the Pacific; but that's also a bit different than seamounts - and you will usually have quite some warning of that well before you get near.
Generally speaking, many new island formations were once submerged sea mounts that have simply grown to be above water due to volcanic activity. One may of course have quite a bit of warning, and usually new islands are fairly promptly recorded. A danger is an eruption perhaps a century ago that raises a previously plotted sea mount to shallow enough depth, yet still submerged, to become a navigational hazard. Such an event could have gone unnoticed at the time and the new hazard never plotted or charted until some poor chap runs his ship or yacht onto it.
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Old 15-09-2010, 12:30   #21
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When you look at the small-scale ocean charts, it's fascinating to see the soundings are often aligned along the obvious path that some ship (likely oceanographic/research) made at some time in the past. It's amusing to see out in the open surrounded by great depths, a shipwreck symbol - where one of our hapless predescesors found a seamount the hard way
It's less amusing to see near one's intended path, a symbol indicating breakers, eddies or discoloured water; all of which could signify shoaling. These are often tagged with long-ago dates, or worse yet labelled PA (position approximate) or PD (position doubtful) - very helpful

Volcanic activity and coral growth can both create shoals out of the blue. We are still limited by our technology; we've come a long way from lead lines but even the latest airborne or space-based LIDAR systems are still confined to a relatively narrow path. There is no substitute for a good lookout, day and night.
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Old 15-09-2010, 12:31   #22
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Originally Posted by estarzinger View Post
For the normal RTW sailor, the two spots to be careful are in the shelf to the south of Madagascar (more a continental shelf than a sea mount) and the sea mount I mentioned previously between Capetown and St Helena.

There has been some 'new island formation' in the Pacific; but that's also a bit different than seamounts - and you will usually have quite some warning of that well before you get near.

I can speak from personal experience about the seamount in the S Atlantic. I had carefully routed us around it, but on the day, it was really nice mild sailing (15-20kts) with very flat seas and we decided to just go over it to save some miles. Bad decision. As we were over it, we got one single 15' high perfect curl surfing wave, which swept the boat. Broke our wind wave and cockpit electronics, took off a dorade, and forces a couple gallons of salt water down our diesel tank breathers. We never knew for sure what caused this wave but guesses a landslide down the side of the seamount.
This is the exact scenario that I am trying to avoid. Cruising along and all of a sudden the water depth goes from 1000' to 100' I suspected would have the effect of causing a breaking wave. Evans has shown me first hand experience with this. The thought is when planning a voyage across the Atlantic from North Carolina to the Azores are their some of these "shallow" spots that it is best to avoid. I thank those of you who have provided info and places to research.
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Old 15-09-2010, 13:03   #23
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t's amusing to see out in the open surrounded by great depths, a shipwreck symbol - where one of our hapless predecessors found a seamount the hard way[IMG]file:///C:/DOCUME%7E1/trf5/LOCALS%7E1/Temp/moz-screenshot.png[/IMG][IMG]file:///C:/DOCUME%7E1/trf5/LOCALS%7E1/Temp/moz-screenshot-1.png[/IMG]
Unfortunate but true. I suspect that is how a good many, if not most known, just barely submerged sea mounts have been discovered. Not something one would expect to find in an area that is normally 100s of fathoms deep. Silver lining to that cloud is that at least someone lived to tell the tale, else it would still be uncharted.
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Old 15-09-2010, 18:22   #24
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You can see them on the charts. Paper charts.

If you use electronics then look thru rasters first, as they will be seen on vectors at a specific scale only (at least on my plotter) (which feature of my plotter sucks BTW).

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Old 17-09-2010, 06:04   #25
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Google Earth has several of the Atlantic seamounts identified as well.
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