How much ferrous metal is on an average Spanish shipwreck?

[Marine Archaeology Int.]

Dear TH community: How much ferrous material do you think is on an average Spanish shipwreck? Lets use as a model a regular 600 ton (Atocha size) vessel and lets assume it is all pretty much in one place. Lets also make a distinction if their cannons were all bronze or iron. Thank you very much for your collective wisdom and knowledge in this matter.

MAI

 

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[Jason in Enid]

Have you tried researching Spanish shipbuilding of the 17 and 18th centuries? You don't specify the type of ship you are inquiring about.

 

[Au_Dreamers]

If it's all pretty much in one place you will get pretty distinctive mag hits!

Anchors would be huge down to rudder materials, straps for masts, rigging, cargo...

When I went to Canaveral a few years back to look at the replica "galleon" I walked slowly around looking at all the fixtures and fittings thinking of them being torn apart and strewn over acres of ocean. The flotsam would have been incredible.

With a modern mag you're sure to find debris areas and even a poor operator would find a ship "pretty much in one place."

my 2 cents

 

[Salvor6]

Can you put that in tons Au Dreamers?

 

[Marine Archaeology Int.]

Thank you Jason. Our targets are early XVII century Spanish vessels between 450 and 700 tons.

 

[Marine Archaeology Int.]

Thank you Au Dreamer. We are designing a computer model for a drone application. Iron mass is a key component. Like Salvor6 says, we are looking for a number in pounds of tons to use as an average in such a model.

 

[smokeythecat]

Would depend on the number of iron cannons on it, of course bronze cannons could have been used. I doubt there would be any set number.

 

[enrada]

My 2 cents

Potential ferrous articles;

3 ferrous anchors, varying sizes, best bowyer down to stern anchor(weights might be found in Treasure Divers Guide)
ferrous cannon balls, bronze cannons(no magnetic value)
1 main cast iron cooking pot, 200 to 300 pounds. Because of size would seriously affect magnetic lines of deviation from a magnetometer.

As a former computer operator and programmer, I would be careful about trying to put an exact value of nanotesla(nT) of deviation from a magnetometer. If you are in the general area of your research criteria, I would be checking out almost any anomaly.

At one time I had a small manual published by Geometrics(I think) which gave approx readings and other details about mags. Lots of good info.
I am going to try and attach a little info from the company.
/Users/Administrator/Desktop/Geometrics Knowledgebase : Will the magnetometer find gold?.webarchive
Not sure that was helpful, maybe go to <Geometrics.com>

 

[Marine Archaeology Int.]

Thank you Enrada. We will follow on your Geometric data suggestion.

 

[ropesfish]

Some resources that may be of interest.

Magnetometry | Geometrics.com



Articles | Marine Magnetics

The Spanish Galleons

and this one, while it discusses English ships in particular can give you some insight on the anchors required aboard:
https://anthropology.tamu.edu/wp-content/uploads/sites/11/2016/07/Jobling-MA1993.pdf

Best of luck to you.

I should have added this- https://www.quora.com/What-was-the-...anchor-in-a-16th-or-17th-century-sailing-ship

 

[stevemc]

Also some ships of that era used iron spikes, I would assume mostly on internal framing. Some ships used all bronze spikes and nails. But there were many with iron spikes, that would set off a mag.

 

[ivan salis]

bronze cannons were quite costly and were highly prized because of the fact that they did not rust like cast iron ones did and thus they were much less likely to blow up in battle.....often only high value treasure vessels carried all bronze guns ,,once a "prize vessel" was taken any bronze guns on it was stripped

 

[Marine Archaeology Int.]

Thank you Ropefish. Impecable your information as always. I completed the readings and videos of all your material and learnt a lot in the process. For what I see here and judging other responses, no one seems to know exactly what was the number (tons) of iron on an historical vessel. A calculation of sorts can be deducted though by adding the weight of cannons and anchors, and then guessing what all the rest of the paraphernalia could weight. I will defer here to the wisdom of Salvor6 who wrote to me saying that as a general rule, a 500 ton Spanish galleon was carrying 100 ton if all its cannon were iron, and about 50 tons if they were bronze. This seems to make sense as I add all the rest of the information I was able to gather in this forum and others.

In terms of designing a magnetometer survey model, our own assumptions are that 50% of the iron material could still be in relatively small area (the main wreck site: 300' by 200'). Worst case scenario (or best depending on your point of view) the shipwreck carried all bronze cannons, which means 50 tons of iron, which means that 50% of that, or 25 tons of iron, could be spread around the main wreck site. This could help others with the calculations as to how wide their marine survey lines should be, or if drone or aerial (fix wing of helicopter) are a viable alternative for any given site.

Thanks to everybody for helping with this matter.

 

[enrada]

MAI
Ropefish's chart shows 1nT which seems to me to be a very small anomaly on a mag led or even paper print out. Although I don't pretend to be an expert, you might have a back ground noise of say 37,000 nT at the equator and a 1nt is a really small amount to see. On top of any diurnal drift that there might be.
After you get the drone up and working, it would be really interesting to run an underwater mag parallel to the drone search for comparison.
Not trying to be negative, just bringing up things that you might consult an expert about.
Keep us posted.

 

[Marine Archaeology Int.]

Thank you Enrada. My understanding is that a 1nT is not a small anomaly. Current magnetometers have a sensibility 20 time larger (meaning that they could detect up to 0.05nT at 100'). Not sure about your 37,000 nT noise though (I will check this out). In our case, we do have other systems that can measure other type of natural anomalies and therefore filter them out. However, I never expected the so-called "back ground noise" to be so large (37,000 nt will be the equivalent to a 37,000 ton iron object at 100' distance). Maybe there is an error here.

 

[enrada]

OK
I found the map but I can't figure out how to post it here. I'll post tomorrow. I have a Macbook Pro and I have tried to select all, drag and drop etc but no luck.

 

[Salvor6]

Here is a map of the world magnetic model. View attachment WMM2015_F_MERC.pdf

Click on the map for a larger image.
enrada is correct.

I am a mag specialist. My mag reads the earth's magnetic intensity at any given location. In central Florida it reads 45000nT. it is the CHANGE in intensity that the mag reads when it goes over a ferrous object that disrupts the local field.

 

[Salvor6]

Here is my crew doing a mag survey of an area 10 miles out in the Gulf:

View attachment 1524211

View attachment 1524213 View attachment 1524214 View attachment 1524215

 

[Marine Archaeology Int.]

Here is a map of the world magnetic model. View attachment 1524210

Click on the map for a larger image.
enrada is correct.

I am a mag specialist. My mag reads the earth's magnetic intensity at any given location. In central Florida it reads 45000nT. it is the CHANGE in intensity that the mag reads when it goes over a ferrous object that disrupts the local field.

This makes sense Salvor6. Thank you!

 

[enrada]

MAI


Hopefully this attachment will open in a world map with nT background noise.
reply if not.