Hi Phil.
I have built a small tri in aluminium, and sailed it from PNG to
Australia and have the plans and aluminium to build a 40' Crowther
sailing fishing boat. The bottom fell out of the fishing and I
couldn't afford to continue at the time. Aluminium is a good fast easy
material to build in and tough in case of a collision but is harder to
make fair, heavier, noisier and colder in winter. The noise and cold
can be overcome with insulation, but this means even more weight. The
main points in welding it are cleanliness and no draughts. Spend some
decent money on a good three phase welder for stability of arc and get
the right tools for cutting cleaning and grinding. Don't drip sweat on
your work face or you will have a messy weld to cut out and reweld
with extra distortion. Also don't allow any of your skin to be exposed
to welding arcs or you will get severe sunburn; don't use any carbon
black containing material such as black rubber found in tyres next to
aluminium or you will get electrolysis; avoid crevices or you can get
crevice corrosion. I just priced aluminium and it is close to twice
the price of building the skin in composite.
I think a Harry really needs to be 60'/40' before it becomes
worthwhile in aluminium. The stiffness of aluminium is not as good as
its strength in the thinner sheets compared with cored composites.
Consider scantlings for a 50'/33'Harry. Probably get away with 3mm
sheet for aluminium but 4mm is more usual. Aluminium has an sg of 2.6
which makes a 4mm sheet 10kg /m2 or 3mm 7.5kg compare with 12mm
polyprop core, 600gglass skins plus resin comes to less than 4kg/m2.
Add the weight of internal skinning and insulation gives at least
another kg for the aluminium. This comes to double the weight for the
skins. For crossbeams, carbon has a tensile strength of about 600MPa
compared with Aluminium alloys of around 150MPa and aluminium weighs 3
times as much. The actual useful strengths are probably closer than
this as aluminium can be a bit more forgiving and the carbon needs off
axis reinforcing, but there is still a large weight advantage.
Going to 60/40 for a Harry the scantlings for aluminium would still
be pretty similar but would increase slightly for the composites and
the skin would be a smaller component of the overall displacement. I
figure the aluminium boat would be in the order of 30% more weight for
a 60/40 Harry for the bare boat. There would be no substitute for a
carbon mast.
Even though I have the aluminium, I am going for composites for a
50'33' Harry as I think it will be lighter, easier, quieter, better
insulated and reasonably resilient. I plan to use my 5m long plates of
aluminium as part of the 15m table for building the composite panels.
I reckon I could just about make a composite panel in the time taken
to weld the three plates together if I was building a boat with them.
Another aspect is to consider the carbon footprint of aluminium
compared with composites. I am embarrassed to own such a large
quantity of congealed electricity and plan to sell it when/if the
market improves.
On avoiding hobby horsing, have you looked at the reverse stems of
some of the new commercial Scandinavian boats. They reckon on, I
think, about 20% reduction in fuel and more comfort. The A class cats
are another example. The theory as I have been rabbiting on about
since I built my tri with a bow bulb nearly 30 years ago, is that you
want to meet the wave early and low so that you increase the time of
impact to allow a gentle rise, rather than with an over hanging stem,
you meet the wave all at once. Also, if the overhanging bow has a bit
of a flare, the bow goes in to the wave and then is pushed up again
vertically, with the wedge shape that the wave sees. If you have more
bouyancy down low, the bow rises from hydrostatic lift more gently. If
the flared bow actually goes under water, then you are in big trouble
as you have a flat area to grab the water. Another problem with a
flared bow, is that as the bow depresses, the water line increases,
water resistance increases, the apparent wind may increase and you are
over. With a reversed stem with more bouyancy below, the resistance
actually gets less and you can use the bow going under as the point
that you start to ease sails. The bouyancy lower down also helps in
driving the boat as the resistance to pitch movement is started
immediately and there is little momentum built up to lead to oscillations.
The Harry as well has a flat rocker, which increases resistance to
rocking motion, but doesn't have the flat area aft of some boats which
also increase resistance. Harrys also have different length hulls
which avoids having harmonics with waves lengths as there will be at
least one hull that is the wrong length for the harmonics.
Some say that you should avoid any weight in the ends and concentrate
all the weight in the middle. I can't really understand that reasoning
as it makes it easier for the boat to pitch. I would prefer more even
distribution of the load, increasing the moments of inertia and
slowing the period of pitching.
regards,
Robert
adva--- In harryproa@yahoogrou
>
> I've been trying to learn about how bow shape effects hobby horsing,
> and stumbled onto a series of pages about this boat:
>
> http://www.setsail.
>
> Which is obviously very much not the kind of boat normally discussed on
> this forum, or proa_file, or any of the others. It does talk about
> using aluminum to build the boat though, and I haven't really seen much
> discussion about this in regard to a harry. Rob mentioned in this post:
>
> http://au.groups.
>
> That the new boats will have flat bottoms...and thinking about that
> post and using sheet aluminum as a building material, it got me
> wondering about using aluminum for a harry.
>
> Right off the bat, I'll say that I think if it was left unpainted, it
> would take the already space ship-like appearance of a harry and make
> it look like a bad 1950's sci fi movie prop. Yech.
>
> But, reading these:
>
> http://www.setsail.
> http://www.setsail.
>
> it got me wondering. The KSS system that is being developed for the
> boats sounds like it'll be lots easier than using strip planking. And
> it seems like building using aluminum would be even easier than KSS.
> Plus it sounds like maintenance would be easier.
>
> I have no idea how to estimate cost on this to get that comparison, and
> I don't know if this would actually be any easier, but I thought I'd
> ask the group since I couldn't really find any discussion in the past
> on this topic.
>
> Phil
>
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