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Discussion Starter #1 (Edited)
Cardiff BSAC fitted a new side scan sonar to the club rib about a month ago, but up until last weekend we've only had the chance to try it out in Cardiff Bay. With the forecast for last weekend looking flat calm, it began looking like it could be a great weekend to see if we could get some decent images and maybe some new sites to dive around the Haven.

In order to try and pre-plan some new possible dive sites for the weekend, Rhian and I had spent an evening on the internet, researching wrecks in the Milford Haven area and cross referencing co-ordinates we had with information Rhian had gathered over the years. We came up with a number of possibilities which we transferred into the GPS.

We both took the day off on Friday, leaving Cardiff by 9.30am to try and get an afternoon on the water, checking out the new sites. Arriving at midday we launched from Gelliswick and headed off up the Haven to try and get some images of the Sunderland flying boat near Pembroke Dock. The co-ordinates we had should have been correct within about 10 metres and even the 100 metre exclusion zone for diving, fishing or anchoring, showed on the chart plotter that we were in the right place, but we could get no images at all.

This wasn't a very promising start, making us wonder if the sonar was working as well as we hoped. We decided to give up on the Sunderland and go and try something that we had previously dived and new what the wreck looked like. The first of these is the Collier, just past the new gas terminal as you head down the Haven.

We past over the wreck co-ordinates with some trepidation, wondering if we would see anything at all, we were relieved to get this image scrolling down the screen.




The side scan image as the name suggests, is a scan either side of the boat, in the picture above you can see the boats track up the centre of the screen with the result of the scan scrolling down as you track across the wreck site. In this scan, the scale across the bottom of the picture shows the beam is set to scan 50 metres either side of the boat. The scan on the left is just flat sand but, on the right you can see the definite outline of the flattened wreck.

With renewed enthusiasm we headed the short distance to the wreck of the Behar and ran our scans again. The detail on the screen is generally better than I can reproduce here, and of course is a moving image. The Behar is also a very broken and flattened wreck except for the steel structure standing off the bottom in the centre of this image.



You will notice that this scan looks different from the scan of the Collier. This time we selected the down scanning sonar instead of side scan. As you can see it produces a similar type of scan to the the echo sounder or fish finder images that most of us are used to, but with a bit more detail in the scan. This as we were to find later, is much better for shotting a wreck, as the side scan is not so good at looking directly below the boat.

From this wreck it is just a short hop to the wreck of the Dakotion, the largest and most intact wreck in the Haven. We decided on the way there, to try and find the partial remains of the Caroline. We knew it wasn't far from the northern most point of the Dakotion, but never been able to find it with the old echo sounder. On our first pass we spotted it with some other strange anomaly not far away which is yet to be investigated.



This shot was taken just using the side scan on the left hand side, the boats track is on the far right of the image. For those of you wondering about the Lat & Long positions on the images, it reads the position of the boat at this time, which will be the top of the image on the centreline or the far right for down scanning.

By continuing the scan from the Caroline on the same heading within about 50 metres we came across the Dakotion.



From the length of the shadow on the left of the Dakotion, with the post production viewing software I'm using to produce these images, it is possible to measure the the height of the wreck off the bottom. This part of the wreck stands about 5 metres high.

At this point we thought we were practised enough to try and find one of the little dived wrecks that had been alluded to on the internet. This was described as HMS Pilot Boat no.10, and positioned not far from the southerly end of the Dakotion. We found what looked like a selection of pebbles on the bottom, but dismissed this as being too small a target to be a wreck. Not even considering the image worth saving, we moved on to the wreck of the Thor just a few hundred metres away.

Having done quite a comprehensive survey on the Thor, and feeling we knew it better than some of the other wrecks in the Haven, we were surprised to see how small the wreck looked in the scan.



We thought that if the Thor only looks this big on the screen, then the small disturbance on the scan of the Pilot Boat might be worth a dive after all. The position of the disturbance was saved for later.

The next more adventurous part of the mission, was to try and find the wreck of the Balholm, a small passenger ferry which sank in 1979 near Linney Head. We had already tried to locate this wreck on a previous occasion but even after half an hour this time with the side scan, we could find no trace of the wreck. We did notice that the water visibility was extremely good outside the Haven though.

The afternoon was coming to an end, so just time to check out the Landing Craft, which sank in the Haven. The flat bottomed landing craft lies upside down with the forward ramp partly broken off.



This particular shot was actually taken on Sunday while 2 pairs of divers were on it. If you look closely you can see the 4 of them and their bubble streams causing shadows across the left of the image. The partial image on the right is the same wreck but because the boats track was almost over the top of it, a small part of the wreck shows up on the right side.

There were a few more images recorded over the weekend. The next one is the Highland Holm which lies in about 30 metres of water. With greater depth and choppy seas the images start to deteriorate quite noticeably, but we've yet to try the sonar out on anything deeper than this.




The final image is of the Lochshiel or Whisky Wreck, which lies on the side of Thorn Island in the Haven. I've dived this on quite a few occasions, but never realised how in line the remains of the hull actually was.



The unit also has the facility to save the current screen at the touch of a button, This also gives some great quick results if you happen to catch something interesting. Here's just a couple of snaps from the weekend of the landing craft, if you look closely you can see the two divers on the wreck in the first picture and later another 2 on the wreck and 2 on their accent.


This has just been a chance to show off our new piece of kit to other divers who may be looking to upgrade their boat equipment. I'm sure this equipment has many limitations. With rough seas and greater depth, the quality of the scans tends to deteriorate. For myself and a number of other club members that have had the chance to use it, we found the result impressive and greatly simplified finding and shotting a site.

Safe Diving

Mark

The unit we were using is the Humminbird 997c SI which has recently been upgraded to the 998c SI

The Post production software is called Humviewer
 

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Discussion Starter #4
The scan of the highland Holm was at 28.4 metres. We've only had the one weekend out in open water with it so far. The side scan transducer is transom mounted so, I think the images would get quite small at depth. The information that comes with the unit gives 50 metres deep as the maximum for clear side scans with the supplied transducer. They also do a through the hull transducer as an upgrade which increases the power.

I did see a thread on the Humminbird forum where a number of owners were trying to post decent images from the greatest depth. I think they were getting results down to 70 metres.

I've seen a few home made tow fish on various web sites with the high power transducer fitted inside. This gives a much more stable scan in choppy seas and can be set to tow at adjustable depths, getting you closer to the target. It just seems a lot of hard work if your only using a rhib.

We will probably get back out with the unit again in a few weeks. (After we've all got over an arduous trip to the Red Sea next week, eruptions permitting)
I'll post any new information here.

Happy Diving

Mark
 

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We will probably get back out with the unit again in a few weeks. (After we've all got over an arduous trip to the Red Sea next week, eruptions permitting)
I'll post any new information here.
Thanks Mark.....we've been toying with going the sidescan route for a little while....one of our crew is keen on towfish whereas I quite like the idea of transom mount (less hassle option on the Rib IMO), so any further info as /when you get it would be great....

Oh, and tough break having to go to the Red Sea ;)

Dan
 

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Just a quick tip, its worth buying a spare transom mount bracket if its the same one as used on the 797....They are designed to flip up if they hit any debris but they dont they just snap at the arm!!
 

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Oz Ninja
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Hi,
Does anyone have a sonar scan of HMS Prince Leopold off the Nab Tower or HMS Prince Philippe in the Minches Scotland?
Regards
 
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