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Old 24 July 2012, 00:52   #1
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Finding a lost propeller with Echo sounder ?

A friend of mine lost a relatively expensive SS propeller (roundabout 1200 euros), from a outboard. Its a small prop about 15 inch diam. The search area is relatively narrow(within 200 x 200 meters), depth is less than 20 m. The water is not clear so having a diver searching it
without accurate position would be a huge job.

But, how about using a echo sounder to locate it first? What type of kit(leisure level...) would be needed to find such a small piece, or is it a totally mad idea?

I think the sea bed is solid rock with a unknown layer (guess pretty thick) of sediment on top
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Old 24 July 2012, 01:23   #2
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You won't see it with any normal depth sounder. You need a magentometer (which will vastly exceed the value of the prop unfortunately)

I had friends who found a hydroplane prop which came off when the shaft broke at 150mph. They don't go far, you need a precise location when it fell off though.
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Old 24 July 2012, 02:21   #3
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Captjack, what about downscan or sidescan from Hummingbird/Lowrance/Simrad?
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Old 24 July 2012, 04:08   #4
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I have Raymarine HD Sonar on BP & can easily see lobster pots, small rocks & wreck debris. It depends greatly on the nature of the bottom. If it's firm sand or flat rock you would pick it up with a high end setup, if it's soft mud or weed, forget it. Even if you do see a "lump" you won't be able to differentiate between metal & a rock or any other crap down there, it might mean a lot of "bounces" for the diver. If it was my prop & I had the time & inclination, I'd be diving & doing circular rope searches. Find a diver with a rebreather, at 20m he should be good for 3 hours
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Old 24 July 2012, 06:04   #5
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Without Navy equipment it's a bit like this......




I take it insurance is out the question?
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Old 24 July 2012, 06:12   #6
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Had a mildly comical search mission yesterday trying to find a mooring stone that hadn't been used for 10yrs. A pal was paddling the 2.3 with one oar, I was hanging over the transom with my face in the oggin directing him through a snorkel. The viz in the river wasn't good. After many passes over where we knew the stone to be, Lee heard a splash, looked around and I'd gone. A few secs later I emerged, chest deep standing on the thing.
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Old 24 July 2012, 11:10   #7
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Drop a weighted down line in the center or the area, and have the diver use a reel to spiral out from there. Doubt you'll find it with any kind of consumer electronics commonly on boats.

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Old 24 July 2012, 11:47   #8
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Quote:
Originally Posted by ncp View Post
Captjack, what about downscan or sidescan from Hummingbird/Lowrance/Simrad?
In mud, no way. Even the 800 hz HDS system doesn't have enough resolution. If the bottom were hard packed sand, maybe there'd be enough of a signal to see it. Assuming it wasn't lumpy with rocks around it. The object needs to be big enough that the wavelength of the transducer signal has not spread out (as it spreads from the cone of projection) to be longer than the size of the object.

With sidescan you are looking for "shadows" and a prop on the bottom is horrible at casting a clear shadow. A shoebox would be easier to see since its got a hard edge. The prop doesn't have a hard straight edge to cast a good shadow.
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Old 24 July 2012, 12:02   #9
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It is possible, been there and got the t shirt.

You need to be a very experienced sonar operator and you need a good resolution, but other than that you just need patience.
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Old 24 July 2012, 12:14   #10
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It is possible, been there and got the t shirt.

You need to be a very experienced sonar operator and you need a good resolution, but other than that you just need patience.
Very flat water is also required

As a diver I could search the 200m x 200m area with line faster than I could find it any other way.
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