Wreck Hunting

[The Chairman]

I thought almost all cannons were cast of brass or it close cousin, bronze? I don't think I have ever seen a cast iron cannon, although many of today's "guns" are iron and not brass/bronze. It was known rather early on that iron and the sea did not mix well.

Seems like side scan sonar is the ticket for wooden boats, but only if they are higher than their surroundings. Other than that, and it's a lot of guess work and diving to locate one.

 

[WreckWriter]

Originally posted by NetDoc
I thought almost all cannons were cast of brass or it close cousin, bronze? I don't think I have ever seen a cast iron cannon, although many of today's "guns" are iron and not brass/bronze. It was known rather early on that iron and the sea did not mix well.

Seems like side scan sonar is the ticket for wooden boats, but only if they are higher than their surroundings. Other than that, and it's a lot of guess work and diving to locate one.

Actually most cannon are iron, brass/bronze are the rare (and valuable) exception. If you had visited John Pennecamp park while you were in the Keys (shore side park) you would have seen a bunch of big iron guns off the HMS Winchester that they have on display.

I still pick the mag if I could only have one for wooden wrecks. In Southern waters a wooden ship only has profile for a few years.

Tom

 

[aue-mike]

Actually, magnetometers pick up anomalies in the Earth's magnetic field, not strictly ferrous metals. You can detect ballast piles as the rocks that make up the ballast piles have a particular magnetic alignment. When the rocks were formed "back in the day", the crystals were "frozen" to align with magnetic north at the time (not necessarily true north due to periodic pole reversal). Anyway, if all of these rocks were lined up correctly, it amplifies the magnetic signature. If they are scattered as they usually are, they tend to cancel each other out. This example could be explained with bricks or other natural material that has a magnetic signature as well. In any case, wood wrecks do not produce a big anomaly (registered in gammas). Metal wrecks, with much more material that disturbs the natural magnetic field, are easier to locate. Orientation, as mentioned above, either excentuates or reduces the anomaly. If a big metal wreck is laying north-south, it amplifies the reading; if it is east-west, not so much.
We have been able to pick up big wrecks (steamers, etc.) as far as 1/4-1/2 mile away with a mag.
The advantages of a mag are speed; with a bow mounted coil you can run full speed. Disadvantages are attentuation (due to water depth) and you still don't know what you are getting - you are only provided gamma data with which to make an educated guess. To cancel out attentuation, you can tow a unit but then you lose the advantage of speed. With a bow mounted coil, you also have to consider any error that is being produced by your own boat (e.g., engine).
Side scan is great for casting a good image of an object. The new digital units are fantastic. No more induced seasickness due to ozone produced from paper machines (it gives you a terrible headache). With a 600khz fish, you can get a great idea of what is on the bottom. They even have 1200khz fish which can "see" through the sediment! However, the higher the res, the lower the effective range of the fish. Typically, most people use 100khz for wide range swaths (+/- 1/2 mile) and 600 khz for detail. Check out the CSS Florida and USS Cumberland that we imaged with a 600khz fish on the AUE website.
The advantages with SSS are that you can get a fairly good idea at what you may be diving before even getting wet. Disadvantages is speed: since you are towing a SS fish, *maximum* tow speed is 4kts.
So, there are pros and cons to each system. Some manufacturers produce a mag/scan, which is a towfish with both applications.
I prefer the mag as you can cover a lot of ground and with good hang numbers, find what you are looking for without having to deploy/tow/recover a SS unit. A SS is a wonderful toy though...

Cheers,
Mike

 

[WreckWriter]

aue-mike once bubbled...
Actually, magnetometers pick up anomalies in the Earth's magnetic field, not strictly ferrous metals.

I stand corrected. Thanks for the detailed explanation Mike!

Tom

 

[aue-mike]

It is a popular misconception that a magnetometer only picks up metal (i.e., "metal detector"). I learned a lot of this stuff from my buddy who I worked with up north who is a real wiz with all this stuff.

There are a few web pages out there that detail mags and how you can even wrap your own coils.

For example:

Here is a page with some good, basic information:
http://www.portup.com/~dfount/proton.htm

Here is a page that details how you can build your own, and how to wrap basic coils:
http://members.aol.com/alka1/ProMag.html

These are actually pretty basic instruments and can easily be crafted in a garage. You can even run this all thru an inexpensive laptop and also input GPS data so you can replay your surveys at home to see if you missed anything.

If you have a known target and can predict its metal mass, there are formulas you can use to determine the expected anomaly size. Then you can use this to assess at what range you can get a positive disturbance and then plot out survey lanes based on that information. If you pursue this in a logical and scientific manner, coupled with basic research, you eliminate a lot of the luck required to find a wreck.

Cheers,
Mike

 

[tchil01]

Mike

Thanks for the great info. It looks like I have a project for this winter...

Tom posted a great article about you and your group. Very impressive. So far as a fairly new diver most of my research has been reading anything and everything I can on wrecks, marine history, technical information and biographies of diving pioneers. It is so much more enjoyable to dive a wreck if you know what it is, why it’s there, and how it got there.

So far I don’t think I have “China Fever”, but the to me the ultimate in diving has to be finding an unknown wreck and being the first person in maybe centuries to be on that ship.

Anyway, again, thanks for the great information and I look forward to following your future explorations.

Ty

 

[gs1]

Hi, would anybody be able to give me some info on volunteer wreck hunting work thanks:1poke:

 

[Tom Yerian]

From what I understand about side scan vs. magnometer, is that with side-scan you end up diving on a lot of rocks and such, but the benefit is your not digging excitedly to uncover the washing machine buried under 3 feet of sand!

If your looking to truely find treasure, you'd want both. If your looking to do a lot of digging, magnometer is the way to go, if your looking to find stuff to dive on, with the hopes of finding a wreck... I'd go side scan.

I've actually decided that I'd rather have a side-scan than a boat... Finding a boat willing to take me and my side-scan aboard shouldn't be too tough!

Get your own boat! Running patterns in the ocean and plotting, using GPS takes a little more than using a buddies boat once in awhile, The expense of trying to hire a dependable boat and capt. Plus, the most important issue, secreacy!!Don't expect you have a secrete with a friend, or honesty either! Also, the same goes for hireing a boat and capt! Your own boat, your own scan, and only you know what you find, and where you found it!!!! Get the picture?...Capt Tom tyerian@charter.net (write)
t

 

[DiverBuoy]

Wow. Sounds like you've been shafted before in a deal gone sour.