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Old 12-08-2015, 17:21   #61
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Re: Steel Hull?

Corten for a hull still needs to be coated...look at shipping containers, which aren't supposed to be constantly or deliberately immersed (harebrained contests excepted). They get multiple coats of epoxy, in and out. There's no reason to be frightened of steel, as long as one follows the rules. Meticulous preparation, good quality modern coatings, a little maintenance, in that order.
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Old 12-08-2015, 17:23   #62
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Re: Steel Hull?

I was joking about the Monel...for that money I'd get Tensen to build me a composite hull, and coat it in steel armour plate.
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Old 12-08-2015, 18:44   #63
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Re: Steel Hull?

I should have mentioned this before, check out the Metal Boat Society if one is interesting metal boats, https://metalboatsociety.org/. They have a forum that I think non members can read, Metal Boat ‚ÄĘ Index page.

Later,
Dan
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Old 12-08-2015, 21:29   #64
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Re: Steel Hull?

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Originally Posted by Reefmagnet View Post
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.
Really? Where was that demonstrated?
We're talking around four or five times thicker, so a total of around 20 to 30 mm (about an inch, for Americans).
How is that nonsensically thick? It's quite a typical hull thickness - have a look at cored composite hulls.

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Plus there's always carbon fibre which can be as strong as steel for the same thickness.
Stronger, not "as strong".

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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.
Steel has superior ductility, but the steel used for hulls doesn't have superior hardness - it's very similar to that of carbon fibre epoxy composites.

Hardness alone doesn't determine abrasion resistance. Carbon fibres are harder than aramid fibres, for example, but aramid composites have superior abrasion resistance.

Ductility doesn't translate to toughness (impact resistance to being holed) - strength and ductility *together* do.
So an extremely strong material like carbon fibre can be tougher than steel despite lower ductility, adjusted for density.

If you want ductility, make your hull out of gold - good luck surviving impacts with that

Neither hardness nor ductility have any impact on strength.
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Old 12-08-2015, 23:30   #65
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Re: Steel Hull?

In assessing the strength and toughness of composites versus metals one should be mindful that composites are composites ie. a bunch of usually strong, and very flexible, threads given stiffness by a resin binder. Consequently the crush resistance of the composite tends to be largely provided by the resin binder.

Steel is an alloy of the element iron which is comparitively soft until alloyed with carbon compounds and other elemental materials. Can this be done with gold to raise it's hardness?
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Old 13-08-2015, 01:07   #66
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Re: Steel Hull?

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Originally Posted by Tensen View Post
<snip>
Neither hardness nor ductility have any impact on strength.
To keep this boat related, in regards to ductility there's a rather large ocean liner lying on the bottom of the Atlantic that could be said to have proved otherwise. There were also a number of WWII Liberty ships that met similar fates as a result of brittle fracture. As for resistance to abrasion aka hardness I'd say it's pretty well known to be the Achilles heel of fibreglass.

Perhaps I'm mistaken but "strength of materials" to me includes tensile (and/or compression), hardness and impact strengths. In fact I used to work in a materials testing laboratory in a past career and I'm pretty sure we had a tensile tester, Charpy impact tester and Vickers hardness tester all in constant regular use. We exclusively tested common metals used in construction.

One point to note is that the Vickers hardness test method is used because it allows better localized hardness testing (due to the very small test area) over, say, Rockwell or Brinell.

Again - just my opinion - but when it comes to boat hulls, impact and abrasion resistance are two very desirable properties.
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Old 13-08-2015, 01:13   #67
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Re: Steel Hull?

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In assessing the strength and toughness of composites versus metals one should be mindful that composites are composites ie. a bunch of usually strong, and very flexible, threads given stiffness by a resin binder. Consequently the crush resistance of the composite tends to be largely provided by the resin binder.

Steel is an alloy of the element iron which is comparitively soft until alloyed with carbon compounds and other elemental materials. Can this be done with gold to raise it's hardness?
Gold is alloyed with other metals (e.g. silver, copper) to increase it's strength and hardness. For example, 24 carat (i.e. pure gold) is just about unusable for jewelry. 22 carat gold can be used but is still reasonably soft. The usual alloy is 18 carat which is only around 75% gold.
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Old 13-08-2015, 01:15   #68
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Re: Steel Hull?

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Originally Posted by micah719 View Post
Corten for a hull still needs to be coated...look at shipping containers, which aren't supposed to be constantly or deliberately immersed (harebrained contests excepted). They get multiple coats of epoxy, in and out. There's no reason to be frightened of steel, as long as one follows the rules. Meticulous preparation, good quality modern coatings, a little maintenance, in that order.
Yes, some structures are unpainted e.g. Sydney Tower. Boats are always painted. I just wondered how it compares to a typical mild steel build as a boat building material because I've seen the odd corten boat advertised.
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Old 13-08-2015, 03:33   #69
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Re: Steel Hull?

We don't need to look far afield to see the successful application of FRP's, the automobile industry. Increasingly high-end sportscars have CF main structures, and they protect the occupants well on impact.

What is different in that world is that these structures are designed to absorb omni-directional impact energy, without taking into account environmental issues like keeping a separation intact from one side of the material to another, e.g. keep the water separated from the air. The energy of the impact incorporates mostly the sacrificial nature of the construction in cars, but with boat-hulls this cannot be the case, as you want to stay afloat.

There is no ideal material, it seems. Boats are built in a way that makes it difficult or impossible to inspect hull-integrity, or even de the necessary maintenance to preserve that integrity, you cannot practically take your boat apart every couple of years. Drop a steel bolt in an aluminium hull and it could eat its' way to the other side in months. Problems are hidden by shiny coats of paint, possible disaster-scenario's are plentyful.
The one thing that remains is that a metal hull can be repaired to its' original strength in a relatively simple manner.

An FRP hull cannot. One can repair visually perfect, but once the fibres are broken, they are just that. Forever a weak point, unless extensive repair is undertaken. And that is hoping that the hull was laid-up properly in the first place.

It would be interesting to have access to the archives of an ibsurance company: They should be able to produce figures on risk-factors per mile traveled.

Best,

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Old 13-08-2015, 03:41   #70
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Re: Steel Hull?

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Originally Posted by Reefmagnet View Post
To keep this boat related, in regards to ductility there's a rather large ocean liner lying on the bottom of the Atlantic that could be said to have proved otherwise. There were also a number of WWII Liberty ships that met similar fates as a result of brittle fracture. As for resistance to abrasion aka hardness I'd say it's pretty well known to be the Achilles heel of fibreglass.

Perhaps I'm mistaken but "strength of materials" to me includes tensile (and/or compression), hardness and impact strengths. In fact I used to work in a materials testing laboratory in a past career and I'm pretty sure we had a tensile tester, Charpy impact tester and Vickers hardness tester all in constant regular use. We exclusively tested common metals used in construction.
What it is to you is of little interest. Strength is strength, ductility is ductility, hardness is hardness. These are established terms.

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Again - just my opinion - but when it comes to boat hulls, impact and abrasion resistance are two very desirable properties.
Toughness ("impact resistance") is very important, and abrasion resistance is not unimportant, but strength, stiffness, weight, and cost are all considerably more important than abrasion resistance in my opinion.

Ductility has its positive aspects - it allows you to see and feel areas of the hull that are under greater strain than they were originally, and so give you warning of failure, but on the other hand you could view it as a bit of a waste if you could choose higher strength instead (a theoretical thought only).

Toughness increases with strength, and with ductility, and if I had to choose only one of either strength or ductility I'd choose strength every time (and in real life modern hulls do get made of very strong, brittle materials, but never from very weak, ductile materials).

In any case, the discussion directed at me is drifting a bit from the point that brought me here, which is that, given equal mass, FRPs are stronger than steel.
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Old 13-08-2015, 04:26   #71
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Re: Steel Hull?

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In any case, the discussion directed at me is drifting a bit from the point that brought me here, which is that, given equal mass, FRPs are stronger than steel.
Certainly, but ultimate strength isn't a factor that trumps everything else. The original post was about a whale impact. Those a pretty rare, compared to other impacts which are much more frequent, such as connecting with docks, dinghies and raft-ups. There is also fatigue as the boat flexes. Steel's homogenous structure and indefinite fatigue characteristics at low yet constant strains are in contrast to frp's nature.

For impact strength, local ductility is more important than simply the strength of the whole, as Jack Verschuur pointed out. An eggshell is tremendously strong due to its shape but once that integrity is compromised the system fails. It is locally fragile. Human skin is amazingly difficult to penetrate because it gives, and returns to its original configuration; better than steel, and it is a composite. However, we aren't building coracles.

If the following technology really takes off, we might have a game-changing material. Note that its prime advantage, complementing its amazing strength, is its elasticity. Current composites aren't locally flexible. My fibreglass bow is flexible over its length, but a low radius bend will locally overstress the fibres in it and it will explode. If the microstructure was finer and the individual components more elastic, it would be more like....synthetic steel. Cumulative damage wouldn't limit the life of the whole item as long as the use stays within design limits.

AMSilk | high performance materials: Biosteel Spidersilk Fibers

The OP wanted to know which kind of hull was better for resisting impact. He didn't state a budget, but I suspect he was not merely after the theoretically strongest possible boat, but the most achievable under his circumstances. The means are similar for most people, what differs is their intent. Each material has its advantages and disadvantages, the art is in making the best compromise. Until we genetically engineer and breed species of self-replicating boats to fit individual purposes, we have to figure out what best fits our needs and either custom build, modify, or adjust our expectations to suit what is already available off the rack.

Most folks will choose off the rack and be fine with it....many will tweak and tune, and there will be the diehard individualists that insist on a tailored solution because their needs dictate a compromise without the limitations of someone else's compromise. What is wrong with combining the strength of composites and the toughness of steel? One can do this and still come in under budget.
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Old 13-08-2015, 06:05   #72
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Re: Steel Hull?

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In any case, the discussion directed at me is drifting a bit from the point that brought me here, which is that, given equal mass, FRPs are stronger than steel.
Does that make any difference in the real world? Not digging too deep onto the figures, would a FRP boat with hull of equal mass/m2 really still be floating after something like this? It takes an incredible amount of punishment to puncture a steel hull, is it really likely that some FRP would withstand the same treatment and still be watertight?

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Old 13-08-2015, 06:50   #73
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Re: Steel Hull?

Wood I own another steely, probably not and I only know one person on his second steely. The rest have stuck with Grp or gone to Grp, there is a reason and its called rust.

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Old 13-08-2015, 08:04   #74
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Re: Steel Hull?

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The myth is that steel is stronger than FRP. When compared pound for pound, FRPs are far stronger.
Again different opinion from another source:

"...about composites: in addition to choice of resins there are factors such as the working environment and the ability of the laminators."

In other words, steel as a material always remains steel as it is "machine-made" (!), while GRP quality may vary from terribly bad to perfect depending of above quoted factors.

++

IMHO, GRP or STEEL (or any other building material) choice should be based on a specific conditions related to each particular case.
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Old 13-08-2015, 08:12   #75
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Re: Steel Hull?

Here's a sad little story to inject into all this fun.


When our boat was built the guy who commissioned it was used to sailing steel boats. It was the late 70's, 1978 I think, and he really wanted a Swanson 42, but didn't trust this new-fangled fibreglass stuff. So he instructed the builder, a professional boatyard but not the original Swanson yard, to lay it up extra thick. So they did, they put at least another ton of resin and fibreglass into what was already, even by the standard of the day, a hugely overbuilt boat. So now he had a hull he could finally, rightly or wrongly, trust.


Unfortunately he spent so much money on the hull he could not afford to fit a deck, or cabin, or mast, or sails so he had to sell it.


On the upside, he didn't sink from hitting a whale, so that's good.


The moral of the story: Perspective, try to keep it.
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