steel VS aluminum toughness at low temps?

The question was "for ice".

There, anything but metal is out of race.

Using high grade steel makes the cost of hull and deck equal to AL. (if not a bit higher). So, even the metal choice is irrelevant.
I would NOT recommend to narrow that down too close. Both have their pro´s and con´s and one should become familiar with the fact that every boat is a compromise.
Quite often the market situation makes one choice superior over the other. Being free to pick the "best" offer at a certain moment is a great advantage.

Regards
Richard

 

See wikipedia article on liberty ships:

"Problems

SS Jeremiah O'BrienEarly Liberty ships suffered hull and deck cracks, and a few were lost to such structural defects. During World War II, there were nearly 1,500 instances of significant brittle fractures. Twelve ships, including three of the 2,710 Liberties built, broke in half without warning, including the SS John P. Gaines,[5][6] which sank on 24 November 1943 with the loss of 10 lives. Suspicion fell on the shipyards who had often used inexperienced workers and new welding techniques to produce large numbers of ships in great haste. Constance Tipper of Cambridge University demonstrated that the fractures were not initiated by welding, but instead by the grade of steel used which suffered from embrittlement.[7] She discovered that the ships in the North Atlantic were exposed to temperatures that could fall below a critical point when the mechanism of failure changed from ductile to brittle (see ductile-brittle transition temperature, DBTT), and thus the hull could fracture relatively easily. The predominantly welded (as opposed to riveted) hull construction then allowed cracks to run for large distances unimpeded. One common type of crack nucleated at the square corner of a hatch which coincided with a welded seam, both the corner and the weld acting as stress concentrators. Furthermore, the ships were frequently grossly overloaded and some of the problems occurred during or after severe storms at sea that would have placed any ship at risk. Various reinforcements were applied to the Liberty ships to arrest the crack problems, and the successor design, the Victory ship, was stronger and less stiff to better deal with fatigue.

Several designs of mass-produced petroleum tankers were also produced, the most numerous being the T2 tanker series, with about 490 built between 1942 and the end of 1945."

Notwithstanding these failures, the production run was an amazing accomplishment.

 

Don't forget Titanic, split open like an old carrot .

 

I think if you ran any modern ship of similar size into a few million tonnes of basically immovable ice - I know what would give way and it would not be the ice.

And modern passenger ships exposed to the same length of damage as the Titanic would disappear below the ocean waves a lot quicker - with minimal chances of staying upright while they did so.

But of course these modern passenger ships have to comply with the SOLAS conventions which came about because of ............

 

I beg to differ,

today, watertight compartments are "watertight" , stand the pressure and do´nt end below deck level.

The damaged stability criteria are much much higher today than 1913.

Though your statement might be right for freighters.

Regards
Richard

 

Even though we today have a rule for passengers with double bottom, Maybe we are able to not let the ship sink but damn we would have really really bad injury.
The Pasiphae we did build at Landskronavarvet in the end of 1900. I think we did built this one close to the year of 1998.
Heard that she ran on coralls in almost full speed (about 27knots) she did a little scratch at the outer bottom, about 80 meter. Phu this was a hard one!
Looked like someone tried to open her with a canopener.

 

Richard

Titanic was designed (shipyard own choice) to stay afloat with 4 compartments flooded - and modern calculations show she would have. But I think 6 or 7 compartments were flooded. So the water came over the top of the bulkheads which did not reach high enough.

Passenger vessels after her (SOLAS regulations of 1948) had to stay afloat with 2 compartments flooded/

That is really the meaning behind my statement.

There is a very good paper on this in the 1997 Transactions of RINA.

It shows that the Titanic would easily meet (with only small detailed modifications) the 2 compartment passenger ship damaged stability requirements of SOLAS. With only few exceptions, she could also meet a three compartment damaged standard.

Interestingly, at the time the Titanic was designed the British Board of Trade did not have good rules for subdivision. In most countries it was up to the owners and shipyard. Up to 1914 Germany was a leader in these rules having brought them in after the loss of the ELBE in 1895. I think the designers of the Titanic used these German rules.

Not related to the Titanic - but in those days, some passenger ships like to leave Southampton with ZERO stability upright. They would only be stable at a small angle of loll. Thsi was because they wanted a minimum GM to have a very long and easy roll period.

 

Concur.


To make it short.

Yes the ELBE disaster lead to the first proper regulations worldwide.

 

...If you build a boat out of steel, you should use a very ductile structural steel wich will have no or very little low temperature cracking problems.

I can't remember exactly, (it's been decades...) but it has something to do the way the atoms on the cube of the molecules connect to each other.

...As one cube's corner atom connects to the next atom in the center of the cube in one type of steel, while in the other, the same atom connects to the surface center atom of the next cube...

This is a major part of the problem, choosing the wrong type of steel.
Before WWII we did not know about this problem!

There is another problem, when you weld two plates together, you are putting very high thermal stresses into the immediate area affected by the weld.

In an ideal situation, you would heat up the whole structure after welding and slowly cool it down to anneal the plates to a certain degree and to relieve the stress created by the welding.

As we know, this is practically impossible and certainly way too expensive to do this on a large ship. However, on smaller crafts, you may heat up the weld area with a large torch and check it with a so called "temp stick" 'till you have the right temperature.

This will reduce the welding stresses almost to perfection.

Aluminum is a completely different matter.

The heat from the welding will make the immediate area much softer and much weaker!

The aluminum you would use to fabricate the hull is a compromise between strength and corrosion resistance.

5086 aluminum is an affordable good boat building material.
Better yet is the 5383 (Sealium Sea-Aluminum!)
Corrosion resistance in salt water is the deciding factor.

For structural pieces, I like to use 6061-T6. (or 7050 or 7075?!)
Great to drill & tap & machine, strong and the 6061 comes in many variety of shapes.

However, over simplifying the problem:
Cold rolling a piece of aluminum makes it harder, heat from welding takes that hardness out of the material.

Most of these materials will be loaded in some sort of bending, where the "E" the Modulus of Elasticity is the major deciding factor.

This being said, the "E" is almost the same for all aluminums.

So what remains is the next variable in the deflection or bending equation:

This is the section modulus of the material.
...This is the shape, the thickness, the profile of the cross section!

Make the material thicker while maintaining the original weight and you will have a much stronger hull or superstructure!
Hence the advent of foam sandwitch-core materials between two strong plates.
Aluminum and ice will be a bad medicine!
One more thing to keep in mind. Cyclic loading and stress cracking.

Remember when the jet age started, we lost a few passenger jets because we had no idea about cyclic loading and stress fractures.

Ice brakers are built like battle tanks! You need many inches of solid steel to prevent the ice from opening up the hull like a tin can!
You can make the lower part of the hull, where you encounter ice, much thicker, but where the thicker and the thinner walls meet, you will have stress concentration.

So... there is no simple solution. Life is nothing but compromise after compromise!

Regards,

Stephen I. M.

 

Stephen,

did you read the thread?

 

Aluminum alloys are excellent low temp construction materials, that's why they use it to build aircraft. The normal ambient temp for large planes is -80 C.
You could also build a plane from steel, but for obvious reasons that would be stupid.
And you could build a ship from aluminum alloy if weight is your main concern (which it seldom is), but crushing ice with it is a bad idea.

The fast catamarans on the Adriatic are made of aircraft alloy. They never see water temps under 12 C., yet their bows are dented by waves to the extent that you can see the whole beam structure under the crumpled aluminum foil.

Using steel offers the advantage that you can weld together different alloys for each task and still have a homogeneous material without electrolysis problems.