Steel Hull?

[leftbrainstuff]

Originally Posted by SUSailor

I guess I should have been more clear with my question. Steel vs Fibreglass. Any advantage to steel over fibreglass in a collision ?

Toughness and impact resistance are the characteristics of interest in terms of a collision with a whale.

I quick mental calc shows a thin steel hull and a 1980s era thick fiberglass hull will have similar toughness when subjected to a mostly uniformly distributed load like a whale strike.

A steel hull will out perform a composite hull when subjected to a piercing type impact. Like the corner of a container. Hard to imagine a whale being able to pierce a hull like this.

Steel also has much greater abrasion resistance than composites. Although kevlar has impressive impact and abrasion resistance. Again hard to see a whale striking a sufficient number of times to abrade the hull. Think 100s or thousands of strikes.

I'm more concerned about container strikes than whale strikes in terms of potential hull breaches.

A directional magnetometer might be useful for detecting a container at a suitable distance. Forward facing sonar wont work at the air water interface.

I've seen numerous anecdotes that red bottom paint might aggravate whales. I don't know of any scientific evidence to support this hypothesis.

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[transmitterdan]

If you are worried about losing the boat then loss of keel and/or rudder are a much higher priority than worrying about holing the boat. The design of the attachment of these appendages is much more important than hull material choice.

How many times have you read about a mono-hull sail boat lost due to a hole punctured in the hull material and all else was intact?

How many times have you heard of a loss due to keel or rudder failure?

[micah719]

Tenset, the folks that go for steel boats want something attainable, not pie-in-the-sky theoretical that nobody can afford, and nobody builds anyway. The old objections to steel are answered in these links, as though they already haven't been in this thread...we're talking past each other. If I had the money to afford a composite wunder-boat, I'd simply use it build a bigger boat, still in steel.

The analogy of the eggshell wasn't meant to be a strawman, and I should have spent more time explaining where I was going with it. I won't, now....the info has already been presented along with the arguments, and I'm too tired and busy to rehash it all ad nauseum with someone who seems to just be wanting to make a point. It was already conceded, several posts ago.

Please come to Boatdesign.net forums, start a thread, and we'll have it out there if it's a material design discussion worth having. In the meantime, people will still build ships, cars, tanks, buildings, machines and hammers out of steel...all materials have good and bad points, all boats are compromises, not all people want or need the same thing, and hopefully they'll make informed decisions rather than falling for marketing hype and follow the Bernay's trail.

Here is another raft of information including much from folks that have designed, built, owned, sailed and maintained steel boats. They don't argue that steel is king to the exclusion of all else, and they don't do that for any other material either. You don't have to listen to them, but keep plugging composites without regarding other considerations for real-world boats that real people want and can have, then you'll find the favour returned.

Just a designer, builder and owner:
Steel yacht building questions

Just another designer, etc:
Good Old Boat - Is there a metal yacht in your future? article

Yet another designer....:
Metal Boats For Blue Water - Kasten Marine Design, Inc.

Guess....and he cites even more:
Cold Hard Steel | | PassageMaker

Article about a boat called Joshua, used to belong to some Fench dude called Bernard:
BERNARD MOITESSIER: What Really Happened to Joshua | Sailfeed

30 ways to sink a boat. Most of them just extra dumb things, but also a couple that steel boats don't mortally fear:
30 Ways To Sink a Boat { ...and 29 to prevent it } | Boating Magazine

And, something non-boat.......comparison of composite vs. steel, in something to do with repeated impacts. A bit remote, but I put it in here to draw attention to the chief advantage which you are desperately ignoring:
BulletProofMe Level III Armor Plate | Tactical Gun Review

And finally, a spiel about insurance; 7 years old, but interesting nonetheless. Consider the implications....and the insurance experts may please chip in and expand, or make a dedicated thread; it's probably worth it (.pdf warning):
http://davismarine.com/articles/Yach...20Coverage.pdf

[Pauls]

The posts by Tensen show a lack of understanding of the dynamics of impact resistance. His reasoning on the subject is one dimensional - namely that ultimate strength determines impact resistance. Ultimate strength is only one factor in impact resistance.

Part of the following is based on basic physics. And part of it is based on the experience of working with another engineer who specializes in impact resistance testing. His company owns a specialized cannon, ultra high speed cameras and a lot of instrumentation to capture and record what happens at impact. Engineering teams come from all over the world to study the impact resistance of various structures.

If the only thing that counted was ultimate strength then the engineering would be elementary. None of the large amounts of time and money spent studying this subject would be expended, cause we'd know it all. Funny, the companies that spend the money don't think they know it all.

An impact is kinetic energy delivered to a structure. The energy is determined by mass and velocity.

A structure, to withstand an impact, must be able to absorb the energy or transmit it to something else.

Time is a direct factor in how the energy is received. The stiffer the structure the higher the peak loads will be because the amount of time available to absorb the energy is short. A more flexible structure will deflect more during the impact. This increases the amount of time it has to absorb the energy and thus the peak load is reduced. Twice the deflection = half the peak load. Does that mean that a structure only half as strong, but twice as flexible, will take the same impact as the stronger but more rigid structure? Yes it does.

The point here is that we are dealing with structures, not just the properties of the materials with which they're made.

The ability of the steel hull in the pix posted earlier to bend and absorb the impact when it was rammed allowed an extended time period for the energy of the impact to be absorbed. A brittle hull (like fiberglass) could not dent in in this manner. Instead of yielding to the force, giving and allowing more time to absorb the energy the glass hull would deflect only a small amount, peak loads would climb to levels exceeding the strength of the material, and the glass would shatter. And probably the people down below would drown.

One last point, and it is an important one. Fiberglass is not a homogenous material. It is composed of a large number of glass fibers. Each is its own structural member, and each is supported in a relatively weak resin. (Very weak, in the case of polyester) In a situation like a boat pounding on a rocky beach or reef, wave action will pound the boat over and over again on the rock. With glass, any significant impact will result in microfractures in the resin supporting the glass in the area of the impact. With one, or a few impacts this might not create visible damage. But the structure is gradually fracturing internally and becoming weaker with each impact. It becomes softer and softer. (This is what happens with older dinghies. They get "soft". It's microfracturing of the resin). Eventually the structure is weakened enough by the repeated impact that the hull breaks thru.

Steel is much better in this regard. As many experienced people have noted on this thread, in the real world it is far better at surviving impact. When a proposed theory does not match observable reality then it is time to try a new theory. That is called the scientific method.

Another factor not mentioned

[Tensen]

Originally Posted by micah719

...all materials have good and bad points, all boats are compromises, not all people want or need the same thing...

I've never even suggested anything otherwise.

They don't argue that steel is king to the exclusion of all else...

There have been quite a few in this thread who do though.

I haven't suggested to anyone they should build or buy a hull of any particular material - I came here to dispell the pervasive myth among the uniformed that if a hull is FRP it must necessarily be weaker and less tough than a steel hull - something which is entirely untrue.

As for your links, post something written by an engineer and I'll read it.

[Tensen]

Originally Posted by Pauls

The posts by Tensen show a lack of understanding of the dynamics of impact resistance. His reasoning on the subject is one dimensional - namely that ultimate strength determines impact resistance. Ultimate strength is only one factor in impact resistance.

Actually Pauls, it's you who doesn't understand the topic at hand.
And I have not said strength is the only factor in impact resistance/toughness (in fact I've repeatedly said here that ductility is also a factor).

If the only thing that counted was ultimate strength then the engineering would be elementary.

Would it? Please tell all of us engineers which material would be the answer to everyone's problems?

Time is a direct factor in how the energy is received. The stiffer the structure the higher the peak loads will be because the amount of time available to absorb the energy is short. A more flexible structure will deflect more during the impact. This increases the amount of time it has to absorb the energy and thus the peak load is reduced.
Twice the deflection = half the peak load. Does that mean that a structure only half as strong, but twice as flexible, will take the same impact as the stronger but more rigid structure? Yes it does.

The point here is that we are dealing with structures, not just the properties of the materials with which they're made.

The ability of the steel hull in the pix posted earlier to bend and absorb the impact when it was rammed allowed an extended time period for the energy of the impact to be absorbed. A brittle hull (like fiberglass) could not dent in in this manner.

Your understanding of material properties is simply plain wrong: stiffness (or "rigidity/flexibility" as you've called it) is *not* related to brittleness/ductility. They are two entirely separate properties.

When building ships, high stiffness is desired.
And note that, even adjusted for density, although carbon fibre composites are stiffer than steel, steel is stiffer than E-glass composites.

Now that you're aware of that fact, will be be changing your advocacy to E-glass composites? I don't think so.

Ask any naval architect: the stiffer the better (and I repeat, this does not refer to ductility).

Also note that the carbon fibre composites to steel ratio of strength is greater than the ratio of stiffness, which, as you've helpfully pointed out (and yes, as an engineer I am aware of the basic physics of collisions, believe it or not) means carbon fibre composites are superior in impact resistance in yet another aspect.

Fiberglass is not a homogenous material.

A fact which is very much in it's favour when it comes to weight vs performance (strength, toughness), as I've explained previously.

[FamilyVan]

I just had to reread the ops question, I forgot what this thread was about with all of this back and forth about the engineering properties of steel.

I'm not sure what the answer to that debate is, but I used to be a navigator on an ice breaker, would punch right through 4 feet of ice at 10 knots. It was made out of steel, not fibreglass, not carbon fibre.

If I'm going to be running into stuff (such as the OPs whale), I'll take steel.

As it turns out, I've never been rammed by a whale and I make an effort to turn around objects in my path with my sailboat, so I'm okay with fibreglass. Very thick fibreglass in case I run into something by accident.

Tensel, naval architect guy, have you ever run into something solid with a boat? Not read about it, but actually piled 10 or 20 or 100 or 3000 tons of boat up onto a pile of rocks or some other really solid object, another boat in or above your weight class?

Glass has lots of great attributes , which is why its so popular, but I don't think anybody who has run into a lot of things is out shopping for a glass boat because they believe its going to survive a collision better than steel.

Ever seen a fibreglass tugboat, not the cutsie tugboat replicas that are popular with some cruisers, but a working tugboat designed for running into ships all day?


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[micah719]

Strength/weight wise only, I'd be tempted to go cold-moulded. In some ways I will, since my modified boat is composite with a steel skin. Timber, the original boatbuilding composite, is still competitive....in its area of speciality. Grass, in the form of bamboo, has amazing properties, and it grows free. Research pseudosasa amabilis.

The OP asked about whales....an improbable impact, but if you "win" that lottery, a potentially game-changing one. This is safety engineering and risk assessment. Nutting out a cruising boat is more an art, since it has so many variables. Think of it as a poor-man's spaceship, and no Houston to call on when problems come. We have to cover the important bases now, or we end up climbing a ladder onto a cargo ship if we're lucky, and our floating home gone. That is xtill a fail in my book.

Steady posted a thread about whale impacts. First amongst the examples were the Robertsons.....they didn't hit a whale, they were hit by whales, Orcas, several tons each behind a bony head doing 30 knots. Dougal, the son, noticed a wounded Orca as they scrambled over the side in the precious couple of minutes they had. So the Orca died, sad; but so would they if they had continued to rely on externals. Douglas decided after being missed repeatedly by ships that they'd self rescue by rowing, living off the sea. At that point, a Japanese fisher rescued them. Douglas also learned by this that Japanese are kindly humans, not the brutal monsters he'd met in WW2. In a way, they learnt and grew more by being sunk than they would have by continuing undisturbed! But given the choice...

More likely for a cruiser is running aground, or bashing against a bommie or a dock...or having some dolt in a harbour bash into them. The result could easily be the same as one of those rare whale or container impacts. Look at the 30th way to sink amongst my links above. Using materials and tools aboard, or whatever is available in some primitive harbour, with no library aboard or specialised knowledge of proprietary composites, with a battery welder and some bits of scrounged steel a resourceful but not exceptional cruiser can make a repair as good as the original, of damage that is very hard to inflict in the first place. The other objections to steel hulls, namely weight and corrosion, also fall apart. The engineers (=designers, builders & owners) linked above explain this. I'll listen more to someone who has dared do something and proven it with serious loss at stake, than a theoretician with no skin in the game, but has dollars to gain if I do it their way.

[Pelagic]

Originally Posted by Tensen

When building ships, high stiffness is desired.
And note that, even adjusted for density, although carbon fibre composites are stiffer than steel, steel is stiffer than E-glass composites.

I would disagree with you on that point as NA's and structural engineers design flexibility into ships so that they can hog and sag and twist in heavy weather...Google this and you will see some telling videos

[Tensen]

Originally Posted by FamilyVan

Glass has lots of great attributes , which is why its so popular, but I don't think anybody who has run into a lot of things is out shopping for a glass boat because they believe its going to survive a collision better than steel.

Basing these beliefs on the fact that one hull is steel and one is FRP is the problem - the choice between material alone isn't enough to determine which hull will survive collision better.

Originally Posted by micah719

The engineers (=designers, builders & owners) linked above explain this.

Designers, builders, and owners are not (necessarily) engineers. Years of study and an engineering degree makes you an engineer.

I'll listen more to someone who has dared do something and proven it with serious loss at stake, than a theoretician with no skin in the game, but has dollars to gain if I do it their way.

What an absurd view. Engineers design what their clients want. They have absolutely nothing to gain by persuading people to do things "their way" (whatever that means - engineers design by science first, and personal preferences second).

Originally Posted by Pelagic

I would disagree with you on that point as NA's and structural engineers design flexibility into ships so that they can hog and sag and twist in heavy weather...Google this and you will see some telling videos

Actually we do our best to reduce hogging and sagging to acceptable levels.

[FamilyVan]

Originally Posted by Tensen

Basing these beliefs on the fact that one hull is steel and one is FRP is the problem - the choice between material alone isn't enough to determine which hull will survive collision better.



Designers, builders, and owners are not (necessarily) engineers. Years of study and an engineering degree makes you an engineer.

What an absurd view. Engineers design what their clients want. They have absolutely nothing to gain by persuading people to do things "their way" (whatever that means - engineers design by science first, and personal preferences second).



Actually we do our best to reduce hogging and sagging to acceptable levels.

So, I was right, you've never actually run into anything?

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[micah719]

[Pelagic]

Originally Posted by Tensen

Basing these beliefs on the fact that one hull is steel and one is FRP is the problem - the choice between material alone isn't enough to determine which hull will survive collision better.



Designers, builders, and owners are not (necessarily) engineers. Years of study and an engineering degree makes you an engineer.

What an absurd view. Engineers design what their clients want. They have absolutely nothing to gain by persuading people to do things "their way" (whatever that means - engineers design by science first, and personal preferences second).



Actually we do our best to reduce hogging and sagging to acceptable levels.

Of course, structural flexing of hull in heavy weather, must be to acceptable levels so that sheer stresses are well below failure.
But do you agree that this "designed flexibility" is part of the structural engineering for heavy weather forces?

[Lepke]

When I was a commercial fisherman, I saw a large steel commercial boat in drydock. It had been leaving a harbor between jetties, lost power and was slammed stern first against the jetties. It pounded there about 30 minutes before the Coast Guard could get a line to them and tow them off the jetty. The drive shaft (about 3") was bent 90°, rudder ripped away and lots of dented steel, but only a minor leak where the stern bearing housing was forced out of shape by the bent shaft. Probably no other boat building material would have survived.

[Pauls]

Rational discussion, which most of the members of this forum desire, is not possible unless both sides of the discussion make a genuine effort to understand what the other side says. Tensen, I am genuinely not trying to give you a hard time. You come across as argumentative and defensive, at least to me. Objectively, you did not understand and reply to my last post. My post dealt primarily with basic physics of kinetics, which is the starting point for any discussion of impact resistance. You did not respond to this at all.

What you have done is given this forum a great example of an individual who is going to argue their point regardless of any and all evidence to the contrary.

As a working engineer I assume you are familiar with design reviews where your peers critique and criticize your work. In the companies I've worked for an inability/unwillingness to understand others would be very career limiting.