GRP v Aluminium?

[mobeydick]

What actually happens when an aluminium boat hits a reef? I can understand that a steel boat boat might dent, and need panel beating/painting (quickly) and possibly welding to repair. A GRP would eventually crack, be penetrated and ground down to bits.

Would an aluminium boat generally be strong enough not to be penetrated? I assume it would get dented. Then what - how would you repair?

MD

 

[Ional]

Boatyard Soprimar in Lisbon repaired a circular ding, (caused by being blown onto an angled corner around a pile, mooring up in a gale). Used aluminium filler, polished you cant see a thing. We would have bashed it out with a hammer from the inside but our Ovni's anchor well has a double skin, so the inner plate was inaccessible. The aluminium there is really thick, cant believe it would puncture much hitting an object at yacht speed. If it did it could be welded by experienced operators like Soprimar who repair ferries and military boats all made of aluminium.

 

[Colvic Watson]

The real problem with wood, steel and aluminium is that all three materials are desperately trying to return to their component elements whereas GRP is happy to stay pretty much as it is. It's the potential hidden problems in all three that would worry me. But I guess with proper management and constant vigilance it's OK. The collision resilience of steel or aluminium must be a nice thing to have.

 

[westernman]

One trouble with wood and GRP for any serious offshore work is fire. If it gets hold, then nothing is capable of stopping it. Certainly not that dinky fire pump you have or the toy fire extinquishers.

Also with metal you will survive hitting a partly sumerged container. With GRP you almost certainly won't.

 

[PeterWright]

Even steel will burn, if you get it hot enough, that's why stuctural steel in any large building has to be clad in insulation to limit the rate at which it heats up. Mind you, any humans inside a steel yacht which has reached the ignition temperature for steel will already be burned to a crisp.

Aluminium, however, has a much lower ignition temperature and, once burning, can't be extinguished by water - it burns very fiercely, as the unfortunate troops on HMS Sheffield learned, to their cost, in the Falklands war. I for one would not like to be aboard an aluminium yacht in the event of a serious fire.

 

[westernman]

Even steel will burn, if you get it hot enough, that's why stuctural steel in any large building has to be clad in insulation to limit the rate at which it heats up. Mind you, any humans inside a steel yacht which has reached the ignition temperature for steel will already be burned to a crisp.

Aluminium, however, has a much lower ignition temperature and, once burning, can't be extinguished by water - it burns very fiercely, as the unfortunate troops on HMS Sheffield learned, to their cost, in the Falklands war. I for one would not like to be aboard an aluminium yacht in the event of a serious fire.

Igniting an aluminium yacht is very hard to do. But you are right, once it is burning all is lost.
Igniting a steel yacht is even harder to do.

Igniting a GRP yacht is unfortunately far too easy - just check out the number of GRP yachts which have burnt compared to metal ones.

Did the World Trade Center collapse because the steel structure burnt or because it melted?

 

[TimBennet]

Aluminium, however, has a much lower ignition temperature and, once burning, can't be extinguished by water - it burns very fiercely, as the unfortunate troops on HMS Sheffield learned, to their cost, in the Falklands war.

There are no records of aluminium burning in any yacht fire. The theoretical temperature required for combustion is so high that the aluminium has melted and flowed away long before it catches fire. If the combustion point wasn't much, much higher than the melting point, then it would be impossible to weld, as welding it would set it on fire.

The guff about HMS Sheffield was a totally misinformed journalist speculating about something he knew nothing about and the official report into the sinking went to considerable length to demonstrate that at no time did any aluminium 'catch fire'. I'm amazed that 30 years on this ludicrous idea is still being regurgitated.

The problem of aluminium (and construction steel) is that it looses its rigidity and strength when hot, leading to collapse. All the fire codes for metal boat construction aim to prevent the structure over heating, not the material burning.

Any yacht fire that would compromise the structure would render the yacht untenable during the conflagration. Even if the boat was evacuated during the fire, it would only remain floating if every through hull had been turned off before the crew left. Otherwise the rubber piping would burn through and the boat would probably sink before re-entry was possible.

 

[PeterWright]

While I agree that aluminium does not catch fire as readily as grp, it's fsr from difficult to get it going with an electrical fire, for example, and, once alight, next to impossible to extinguish, Add water and you get a metal water reaction going which releases hydrogen in large quantities, adding an explosion risk to the hazards.

I agree that steel is much more difficult to ignite than aluminium, and I thought I made that clear in my earlier post, in fact so difficult that I don't think the occupants of a steel yacht will be worrying when their hull ignites.

While I suspect the probability of an Al yacht catching fire is less than a GRP one, I'm not sure how much less. Given the relative abundance of GRP hulls compared with Al ones, I would expect to hear far more reports of GRP yacht fires even if the probability of ignition were the same for both materials.

As to the World Trade Centre, I assume it was constructed to a code which provided insulation to the structural steel to protect them from ignition for a sufficient period to permit evacuation, given a conservatively assessed fire loading. I don't imagine that fire loading included tens of tons of aviation fuel on one side of a couple of floors. Nonetheless, as I recall the technical assessment of the event which I read many years ago, the structural members on the floors where the fire was at its hottest failed neither through melting nor burning, but as a result of the reduction in yield stress from the increased temperature, well before either melting or ignition temperatures were reached.

 

[TimBennet]

While I agree that aluminium does not catch fire as readily as grp, it's fsr from difficult to get it going with an electrical fire, for example, and, once alight, next to impossible to extinguish, Add water and you get a metal water reaction going which releases hydrogen in large quantities, adding an explosion risk to the hazards.

Simply not true. Aluminium in the form of extrusions and sheets used in yacht construction it is all but impossible to get to burn. Even if you set about it with an oxyacetylene torch, it won't burn. HMS Sheffield didn't burn with the temperature of an exorcet missile.

The behaviour of aluminium foil and powder in a fire, bears no relationship to the behaviour of the metal when formed into structural members.

Aluminium 'catching fire' is simply not a concern to an aluminium boat owner. At all.

 

[PeterWright]

Hi Tim,

While I have not personally heard of an aluminium yacht burning, I have heard of aliminium fires in other situations and have seen the consequences of one. All metals except noble metals can burnon air, and their ignition temperatures vary widely - as low as room temperature for sodium and potassium. Where the ignition temperature exceeds the melting point, this does not mean the whole structure has to be liquid before the fire starts - very local heating can start a fire in a small part of a structure.

Indeed, for this reason it is impossible to weld aluminium in air - the weld pool must be protected from the air by the shield gas until it fuses.

If you have had access to an MOD report on the loss of the Sheffield then you know more about it than me. As far as I am aware, no analysis of this event has made it to the public domain, beyond the newspaper reports which you rightly surmise are my only source. If they are untrue, I apologise for repeating them, but I have never seen a denial of this by the MOD. If you are able to share some more accurate report with me by pm, I would be most grateful.

Peter Wright

 

[TQA]

Well here are some pics of an aluminium yacht that caught fire in Prickly Bay Grenada June 2013. It was a few boats over from me and it went from a small fire under the cabin floor to a burnt out hull in about three hours. It was a year old 80-foot Jongert. The mast was carbon fiber and the first pic captures the moment the mast burnt through. The burnt out hull was plugged and patched with plywood and is anchored meters away from me at the moment awaiting a suitable scrapyard deal to recycle the metals left.

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