Hull Construction Materials
- Thread starter KiwiRob
- Start date
John Fedup
The Bunker Group
There are two reasons why aluminum isn't preferred as a hull material.
The first reason is that aluminum warships, suffered crack from metal fatigue, the second is that should the ship burn the aluminum melts at a lower temperature and suffers structural collapse, it also conducts heat well and this helps spread the blaze.
And don't forget corrosion, especially pitting and severe issues with dissimilar metals. They work fin in the civilian and commercial world but put them in service and start pushing them and you have problems.
John Fedup
The Bunker Group
Galvanic corrosion can be an issue with aluminium but careful construction can eliminate this type of corrosion. For naval ships steel is best but for many commercial displacement hulls, the weight savings can offer substantial fuel savings and for builders the lighter weight makes construction easier. This is why yacht builders have embraced aluminum for custom builds.
Most of the cracking issues with aluminium in warships relates to mixing it with steel - they have different characteristics and the two work against each other, usually opening up cracks near where the ally meets the steel. A pure aluminium structure won't usually suffer in the same way.
As to fires, aluminium doesn't *burn* in anything like normal fire temperatures but it does lose structural strength at lower temperatures.
Extreme example, USS Belknap - but it's not every day you get run over by a carrier, showered with JP5 then set on fire.
As to fires, aluminium doesn't *burn* in anything like normal fire temperatures but it does lose structural strength at lower temperatures.
Extreme example, USS Belknap - but it's not every day you get run over by a carrier, showered with JP5 then set on fire.
Aluminium hulls do not handle rough conditions well, well depending on hull form and active stabilisation they can perform quite well in the rough stuff but the issue is it breaks them far more quickly than it would a steel hull. For naval vessels I would pretty much always prefer smaller vessels, composite, larger ones steel and leave the aluminium to the commercial operators who know what they are doing and don't go out in the rough stuff, pleasure craft and government agency craft. Never ever for naval unless you like replacing you ships frequently and enjoy huge repair bills.
The only qualification i would make reference an aluminium hull is i do not know how a wave piercing multi hull or SES design would fare in comparison to a mono hull. If the selected hull form could mitigate some or most of the fatigue issues related to slamming then you could be on a winner.
I just don't know enough about it to say it would be better or not.
I just don't know enough about it to say it would be better or not.
Yacht builders, yes. However, this is hardly commercial. The problems with getting A-60 protection on an aluminium hull are significant and add greatly to the cost. For the vast majority of commercial ships mild steel (with som HT in some ships) is the preferred option given the durability of the material and relative ease of fabrication
Given the difference in material properties you are not going to see a large commercial vessel built in aluminium where a high deadweight for vessel size is sought.
For super yachts aluminium and composites offer a great advantage for speed and relative fuel burn as the design speed, however, the recent loss of the aluminium super yacht off cairns is a pretty good indicator of the fire resistance of such designs
Following the Falklands war & the loss of HMS Sheffield, the UK RN allegedly produced a report, based on technical analysis from experts & the commentary of those who survived various attacks / actions.
In the report there were several key points raised about ship design & the materials used during construction.
ONE of those was the use of Aluminium, which was 'frowned upon' in future designs, due to issues relating to a low melting point, the ability to transfer heat to adjacent spaces & starting secondary fires, listed amongst the factors.
It is always a set of scales when designing a ship as weight & structural integrity must be balanced against the speed capability of the ship, & its available space / structure to fit & support various pieces of equipment / weapons systems.
The preference in major war fighting vessels (Frigates & larger, greater than 2,500 tonnes in weight), is that while steel is heavier, the overall structural integrity & cost of the materials, mixed with thru life costs / ease of maintenance & actual life-expectancy of the material, all translate to make steel a more viable & durable option.
However, it should be noted that where 'a need for speed' is a driving factor in the capabilities of the vessel, Aluminium becomes a more attractive option, due to the weight savings that can be had.
SA
Building large vessels out of aluminium poses structural challengers (by this I mean in the order of 100 m plus). If you desire the same uplift (mass) capacity life will get very interesting as you need much more structure for the same durability. Materials such as composites may resolve some of these issues but at massive cost on large platforms...... essentially this may simply not be worth it.
On the commercial side A-60, A-30 and A-0 protection is very difficult on aluminium involving insulation of both sides of a bulkhead and special treatment of all penetrations. expensive and difficult. Quite often addition fixed fires systems area also required and all this adds weight and complexity.
Finally I fully agree on the durability and repair issues. Campsites would be a nightmare for you average ship yard to repair. Aluminium is common but repair has to be very careful not to build in issues as part of the process (lots of NDT please) and it is costly.
Mild steel is quite a forgiving material.
And it is also much easier to identify and monitor corrosion on steel than on aluminium., in particular when it has been painted. A painted steel structure will show rust stains where aluminium will not rather the pain will likely lift off as a sheet after the damage has been done.
the road runner
Active Member
As always you guys have raised some good points.
Can i ask what you think of the Austal LCS being of good value for the US Navy?
Is it that the US Navy has crunched the figures on fuel burn over the cost of building in Alu in favour of the LCS?
I assume these ships have a 20 year life span and curious to see what issues these ships have over the 10 year mark.(cracked hulls ,shagged IE like the RAN patrol boats)?
Can i ask what you think of the Austal LCS being of good value for the US Navy?
Is it that the US Navy has crunched the figures on fuel burn over the cost of building in Alu in favour of the LCS?
I assume these ships have a 20 year life span and curious to see what issues these ships have over the 10 year mark.(cracked hulls ,shagged IE like the RAN patrol boats)?