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Old 11-08-2015, 06:59   #46
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Re: Steel Hull?

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To the same budget? Of course not. Steel is by far the cheapest material.

To the same mass though - any day you like.

(No, I rarely design in FRP, our firm does mainly aluminium designs, occasionally steel.)
Tensen, interesting thoughts, I'd like to hear more about how your extra tough FRP boat would be layed up. I'm guessing you are not using polyester and e glass? Solid or core?

Give me the hard core engineering figures and we have a discussion. At the moment we just have some very vague statements that don't do your university degree any justice.

Have a look at this and let me know the flaws in his figures. To me it shows a clear advantage in toughness to the metals, with much higher area under the stress strain graphs. Even correcting them for density should still show an advantage to the aluminium and less so for the steel. I am asking this to improve my knowledge of material science as it relates to boats, not to justify my own bias, so I appreciate any extra light you can throw on the behaviour of these materials under local impact and failure. Cheers
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Old 11-08-2015, 08:59   #47
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Re: Steel Hull?

If we are going to consider exotic composite structures.... how about Titanium hulls
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Old 11-08-2015, 11:07   #48
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Re: Steel Hull?

Is there anyone out there with a monel hull? That would be about the happiest sailor on the seas....
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Old 11-08-2015, 14:02   #49
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Re: Steel Hull?

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Some time ago I read an article about an individual who was sailing from the Panama Canal to the UK. Sometime during his crossing he is believed to have hit a whale while under auto-pilot. His boat was sunk and he spent some time on a life raft before being rescued (this was in the time before sat phones, EPIRBS, PLB's etc.).

How big of a concern is this type of incident during a ocean crossing? Second would a steel hull prevent this type of incident from happening, or is it just a risk you accept on a crossing?
I think it would be sweet to own a steel boat. They can just take so much punishment, and even if you breakem they are easily enough fixed. Always been a fan of steel boats. I think a steel expedition sailboat would be the most desirable boat I could own.

Fibreglass just isn't that great at high latitudes. Operating in the arctic or somewhere cool like that, glass would be abraded by the ice and nearly impossible to repair with the tools at a native fishing village. But I bet you could find a welder!

Having said that, I intend for my next several boats to be wood.

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Old 11-08-2015, 18:55   #50
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Re: Steel Hull?

A titanium hull would be awsome. There was actually one built for a titanium supplier in Japan in the 80's I think with apparently great reviews. Of course the cost would be incredibly high because the welding would be a nightmare. At least until the Navy figured out how to friction weld hulls (research is currently underway to do this).

there have been a few Monel hulls built, but they are very expensive, and electrolysis is a major concern.
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Old 11-08-2015, 19:22   #51
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Re: Steel Hull?

We have two steel boats.

When in Newfoundland I get to sail with whales occasionally.

Here is a pic of one attacking a wooden boat.

Oh the horrors!
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Old 11-08-2015, 19:25   #52
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Re: Steel Hull?

I found some weak spots in my smaller boat and replated a bit. The first two where tough because I was too timid. The third was a relative piece of cake because I put on my man pants and had at it.

Still taking a cutoff wheel to you hull feels a bit like a self amputation of a beloved limb.

Fixing the bloody paint was as big a job as the welding.

The problem was a leaking ventilator in an inaccessible area of the lazarette. I found the two other spots, same root cause earlier. But then finally located the leak and the last rotted spot.

Somewhere I read that steel will warn you of a problem with a small leak, that can not be ignored.

I've never had a leak, but I can see the wisdom in those words.

We have a steel hull house boat in our yard. No zincs, none. It came from elsewhere for a bottom paint job. The hull has hundreds of holes, some the size of dimes. You could clearly see the active electrolysis is areas as those holes were bright shiny bare metal, no rust at all.

It is a miracle the boat floated. And it had the wee smallest of bilge pumps. It is a total loss.

But this was a case of pure and simple gross neglect.
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Old 11-08-2015, 19:36   #53
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Re: Steel Hull?

Exactly hpeer many do not realize that repairing the structural integrity or plating of an older steel hull is much easier to achieve with 100% success, than any other material, especially FRP.

On super yachts we regularly cut a large hole in the side of the hulls to change out the Generators. Section is welded back on, X-rayed and faired, they are as good as new.
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Old 12-08-2015, 05:53   #54
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Re: Steel Hull?

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Originally Posted by Snowpetrel View Post
Tensen, interesting thoughts, I'd like to hear more about how your extra tough FRP boat would be layed up. I'm guessing you are not using polyester and e glass? Solid or core?

Give me the hard core engineering figures and we have a discussion. At the moment we just have some very vague statements that don't do your university degree any justice.

Have a look at this and let me know the flaws in his figures. To me it shows a clear advantage in toughness to the metals, with much higher area under the stress strain graphs. Even correcting them for density should still show an advantage to the aluminium and less so for the steel. I am asking this to improve my knowledge of material science as it relates to boats, not to justify my own bias, so I appreciate any extra light you can throw on the behaviour of these materials under local impact and failure. Cheers
If you're in a position to need my advice you aren't in a position to judge whether I do my qualifications justice.

Your steel here has a yield strength of about 200 MPa, and ultimate strength of 400 MPa.

The FRP shown here is apparently the weakest one the author of the image could find (CSM E-glass in polyester, with a low fibre percentage?).
Even working with that, once we adjust it for density we get the equivalent of about 800 MPa (using the image as a basis).

Far stronger than the same mass of steel.

There are a huge range of composites (an infinite number really) and they range up to high performance ones like carbon fibre in epoxy, where we might expect strengths of 1600 MPa before adjusting for density.

If we adjust for density we'd get an equivalent of around 7800 MPa for an equal mass! Almost 20 times higher than steel's 400 MPa.

Aluminium comes out looking a lot better than steel, but still pales in comparison to composites. Even titanium comes nowhere near.

Of course, the downside is cost - you pay for the performance you get. Steel's greatest strength is its price.
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Old 12-08-2015, 07:00   #55
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Re: Steel Hull?

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Is there anyone out there with a monel hull? That would be about the happiest sailor on the seas....
Yes, it has been tried. The result was a financial and engineering disaster. Monel is incompatible with most other metals when immersed in sea water.
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Old 12-08-2015, 07:43   #56
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Re: Steel Hull?

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Originally Posted by Tensen View Post
If you're in a position to need my advice you aren't in a position to judge whether I do my qualifications justice.

Your steel here has a yield strength of about 200 MPa, and ultimate strength of 400 MPa.

The FRP shown here is apparently the weakest one the author of the image could find (CSM E-glass in polyester, with a low fibre percentage?).
Even working with that, once we adjust it for density we get the equivalent of about 800 MPa (using the image as a basis).

Far stronger than the same mass of steel.

There are a huge range of composites (an infinite number really) and they range up to high performance ones like carbon fibre in epoxy, where we might expect strengths of 1600 MPa before adjusting for density.

If we adjust for density we'd get an equivalent of around 7800 MPa for an equal mass! Almost 20 times higher than steel's 400 MPa.

Aluminium comes out looking a lot better than steel, but still pales in comparison to composites. Even titanium comes nowhere near.

Of course, the downside is cost - you pay for the performance you get. Steel's greatest strength is its price.
Thanks Tensen, makes sense, although I am not sure that the extra Mpa in all cases would compensate for the extra energy required as the metals yeild?



To use this example. Crudely the toughness is the area under the stress strain curve.

So roughly unadjusted for weight the figures for toughness look something like this?

CFRP carbon fibre 2500*1.5/2=1875
GFRP Glass fibre 500*1.8/2=450
Steel 250*16=4000 (very simplified calcs, 16% elongation)
Alloy =3000? in comparsion

To correct for density using something like 1.5 SG for the plastics, 2.7 for alloy and 7.8 for steel would give us.

CFRP 1875*5.2=9750
GRFP 450*5.2=2340
Steel =4000
Alloy 3000*2.9=8700

I guess this does show that a full carbon laminate is pretty tough. Somehow it doesn't seem intuitive and I guess that a huge number of other factors come into play like sheer strength, notch toughness, after rupture, the effects of welds, stringers and bulkheads. The calcs here are laughably crude. You can tell I never went to varsity. I'm not sure if the carbon laminate is balanced, I'm assuming it is otherwise it's not really a fair test.

My guess is that most people aren't going to build a carbon layup at the same weight as a steel hull, so in the real world I'm sticking with my metal boats, but I'll stop sneering at pansy carbon fibre boats

Just out of interest a 3.2 mm steel hull would therefore equate to about a 16mm thick solid FRP hull and about 8.7mm alloy in this example and 4 mm would be more like 20mm of frp, and 11.6mm of alloy, for the same weight.

Feel free to dispute my figures and assumptions.

Cheers

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Old 12-08-2015, 13:19   #57
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Re: Steel Hull?

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Of course, the downside is cost - you pay for the performance you get. Steel's greatest strength is its price.
Or you could rephrase that as : " steel has the greatest strength/impact performance per cost of any boat building material in popular use"

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Old 12-08-2015, 16:05   #58
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Re: Steel Hull?

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Just out of interest a 3.2 mm steel hull would therefore equate to about a 16mm thick solid FRP hull and about 8.7mm alloy in this example and 4 mm would be more like 20mm of frp, and 11.6mm of alloy, for the same weight.

Feel free to dispute my figures and assumptions.

Cheers

Ben

No, your post looks pretty much correct. A 20 mm FRP hull isn't uncommon, although that would be cored.

The figures we've discussed don't take cores into account, which make composites look even better in comparison.

I think we intuitively evaluate similar size things when comparing properties, and doing it that way steel is very strong, but when you adjust for density in roles where the extra width isn't critical (like ship building) composites become king.

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Or you could rephrase that as : " steel has the greatest strength/impact performance per cost of any boat building material in popular use"

Very true. There's no such thing a a free lunch.
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Old 12-08-2015, 16:40   #59
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Re: Steel Hull?

All this talk of tensile strength is a wild goose chase. As has been demonstrated already, to match the weight of steel, fibreglass construction would need to be nonsensically thick. Plus there's always carbon fibre which can be as strong as steel for the same thickness.

BUT...
steel has superior ductility and hardness. In the real world this translates to impact resistance and abrasion resistance. Both of these properties are inferior in composites and are critical factors for the overall "strength" of a hull.

I just happened to be at the boatyard yesterday, and thinking if this thread, I wandered over to an abandoned "steelie" and snapped a couple of photos.



Not the first one of these I've seen where the owner has cut some holes in the bilge and then walked away.

A typical clue is anchor well / chain locker perforation corrosion.



A quick look inside the bilge usually reveals the horrible truth







That's my concern with steel. Its stronger in all aspects except durability. Unprotected from the marine environment, it simply dissolves. Coating protection systems need to be carefully applied to ensure long term durability and even then the coating will eventually start to degrade if not maintained.

Having said that, firstly, there is a very nice looking fibreglass boat beside us that is essentially junk as it has delaminated coring in both deck and hull. Secondly, I see the occasional boat constructed from Corten steel (steel that forms a protective oxide and can be left unpainted). Has anyone feedback on the long term resistance to corrosion from this material?
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Old 12-08-2015, 17:00   #60
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Re: Steel Hull?

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Yes, it has been tried. The result was a financial and engineering disaster. Monel is incompatible with most other metals when immersed in sea water.
One Monel boat dissolved itself thanks to a being steel framed with a Monel shell. There have been other solid Monel boats built at extraordinary cost that didn't suffer this problem. But it is a crazy expensive undertaking. Far more expensive than a titanium boat.
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