Home' Work Boat World : December 2016 Contents 12 December 2016 WORK BOAT WORLD
Alternatives to steel
Cutting-edge developments affecting the work boat world, By ANDREW BAIRD
The Bow Wave
To this day, whenever someone suggests building a naval vessel
out of aluminium another person will chime up with the “fact”
that aluminium burns.
The story harks back to the Falklands War and the sinking of
the HMS ‘Sheffield’. The guided-missile destroyer was said to have
been lost because the superstructure “caught fire” and that the use
of aluminium was to blame. Leaving aside the point that the vessel
was built fro m steel, it has been pro ven in numerous studies by
both the Royal Navy and US Navy that the primary danger from
aluminium, compared to steel, is that its lower melting point
causes structural instability.
Anyone who has seen the photo of the aluminium catamaran
‘Swift’ that was hit by an anti-ship missile off Yemen recently will
observe that whilst there is clear evidence of deformation from
heat, the aluminium did not burn. The fact that the vessel is still
floating and that no crew were killed is certainly a testament to the
design and proof that the use of aluminium won’t result in a vessel
that can’t survive a fight.
The fact that composites burn hasn’t seen militaries deterred
from using them, and certainly hasn’t been accompanied by the
same volume of vitriolic complaints as with aluminium. There is
something slightly amusing about this attitude considering the
history of large composite patrol craft has been anything but fire
free with the most recent notable incident being the combustion
of KRI ‘Klewang’ a 63-metre carbon co mposite trimaran for the
Indonesian Navy that caught fire four weeks after launch in late
2012. Whilst there are still rumours of foul play surrounding the
cause of the blaze, the complete destruction of the vessel didn’t
deter the navy from continuing with the programme.
Carbon fibre, anyone?
Carbon fibre has traditionally being shunned by the
commercial maritime world due its relatively high cost, consigning
it to the playgrounds of the well-financed navies and the high
performance toys of multi-millionaires. Increased production of
carbon fibre in China, albeit of lower quality and with
correspondingly lower strengths than its European or American
alternatives, is making its way into international supply chains.
Whilst these fibres might not be suitable for high performance
aircraft where every kilogram of material matters, these lower cost
fibres could well be used in marine superstructures.
The idea of constructing a superstructure out of a lighter
material isnt new. Steel ships have had aluminium superstructures
for decades and the safety benefits of the resultant lowered centre
of gravity are well understood in addition. The reduction in overall
displacement and the consequent reduction in fuel consumption
are equally well understood.
Perhaps the arrival of these lower cost carbon fibres could
herald the dawn of large composite superstructures? The skills
involved in producing a structure made of composites are quite
different to those needed for aluminium or steel, but that need not
put an end to the idea. It isn’t uncommon for one yard to build a
hull whilst another is contracted to build the superstructure, even
if they are of the same material, so it should be no issue for this to
occur for composites as well.
High density polyethylene
The other material that I’m surprised hasn’t had as much
success is high density polyethylene (HDPE). More appropriate for
smaller craft up to a maximum length of around 15 metres or so,
HDPE is growing in popularity amongst small workboat operators,
with a 14-metre pilot boat built in Turkey having been reviewed in
the pages of the September issue of this magazine.
Having spoken with a couple of builders who have been
working with HDPE, they tell me that construction costs for HDPE
boats are comparable to aluminium in terms of materials and
labour. The main selling point seems to be that maintenance is
significantly reduced over the life of the craft and that repairs are
simpler and cheaper to complete than on aluminium.
Black HDPE has an operational life of 50 years with minimal
degradation o ver that period. It doesn’t suffer the “pitting” that
afflicts aluminium that isn’t properly cared for and, if for so me
reason the vessel is catastrophically damaged the material is
completely recyclable with very low energy input.
The temperatures needed to repair HDPE are significantly lower
than those for aluminium and can be achieved in even
rudimentary ways. According to one builder, an old wire coat
hanger, held in a flame until it turns red can melt and rework
HDPE sufficiently to form a bond, or rework material around a
crack or puncture.
Even bullets aren’t an issue. Standard 14-18mm HDPE plate can
stop 9mm rounds with little deformation. While high velocity
rounds from an M-16 or an AK-47 will pass through the plate, the
heat transferred is sufficient to cause the HDPE to melt and refill
the hole. This same ammunition will pass easily through
aluminium, and even steel if the range is short enough but the
holes will stay open allowing water to pass through.
A final bit of positivity about HDPE is that it is virtually
impervious to marine growth which is slower than on steel or
aluminium because weeds and barnacles can’t get a sufficient grip
on the hull. This smooth surface also makes it easier to clean with
one owner mentioning that on the rare occasion that he has to
inspect the hull of his vessels, a wipe with the hand is all that’s
needed to remove growth. No anti-fouls or scraping needed.
Any comments, or perhaps you’ve come across something interesting?
Feel free to conta ct me at the. bow.wave@gmail. co m
The aluminium catarmaran ‘HSV Swift’ before that telling day in Yemen
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