I don't care what type of hull you have, if the rock is sharp and
tough, then you are going to have a rip in the skin.
A welded hull would not have the dissimilar metals or the stress
concentrations of a pop riveted one.
I don't think I would like the puncture proofing of a 3mm ply skin,
unless I had kevlar on the outside. Possibly you could use kevlar as
your outer skin. Certainly gluing , rather than screwing reduces the
corrosion and point loading.
I have done a fair bit of welding of aluminium boats. I built a small
tri out of aluminium and sailed it from PNG to Australia, and I
researched pretty thoroughly the pros and cons. I assure you there is
a corrosion and point stress problems in marine use unless a lot of
care is taken. Have you looked at aluminium crossbeams on old beach cats?
Skin on frame is a sound technique but be careful of the choice of
materials.
--- In harryproa@yahoogrou
wrote:
>
> The beauty of the design Rob is working on for us is that the hulls
> will need little or no lining, Col is going to get some figures to
> gether re weight per square mtr for comparison, im not saying this
> method is for me , more that when new things come along people can be
> dismissive or suspiscious (me included) and that a open mind is one
> that learns.
>
> Its a funny thing in this world but very often things arent done a
> certain way because thats the best way but because thats how
> everybody else is doing it , im sure Rob is very familar with that
> promoting a design that makes eminent sense but meets with huge
> paranoia and resistance .
>
> Im also sure thatRob is familar with people that say something wont
> work and state a very plausable reason why but at the end of the day
> his designs are out there sailing despite the reasonings to the
> opposite.
>
> When looking at something as different as this i think we need to
> realise that not everything will be obvious at first look. Rob just
> realized something quite significant recently re the load carrying of
> Proas, now if it only just occured to he who has spent a significant
> portion of the last ten years thinking "proa" then ill keep
> investigating, for fun and edification .
>
> Just to add a little to the pot, what if the skins were 3mm ply what
> weight then .Remember the function of the skin is now only a
> waterproof barrier. What if the skin was pure fibreglass, same or
> better point loading no need for epoxy as with ply do it up on a
> table to size . Col has had one example in the water for two decades
> still going strong, although this had the frame welded which would
> actually detract from its strength. the skins can actually be glassed
> to the stringer frames so little or no screw fastening is possible.
>
> Come on guys lets do a little brain storming !
>
> What do you think would happen with this type of construction in a
> collision versus a cored hull. Not a dinky weight dropped on a panel
> that bounces, 3-4 tonn of boat doing 10knts doesent bounce when
> striking a pointy bit of rock you get a long rip .
>
> Cheers Tim
> --- In harryproa@yahoogrou
> wrote:
> >
> > Looked at he website, and the internal frames are covered with some
> > kind of skin. Any skin has to add at least 1kg /m2.If you have to
> have
> > an inner skin, it may as well be part of the sandwich. You are also
> > hiding the frames and can't check for corrosion easily. Stainless
> > steel into alloy can cause corrosion. The stainless steel screws
> into
> > the ply , unless as Rob pointed out, you drill and presaturate the
> > hole, are a means of getting rot. The chines still need to be
> rounded,
> > glassed and faired, though the glassing could be part of the final
> > exterior glasing This seems a a fair amount of work and risk to me.
> > --- In harryproa@yahoogrou
> > >
> > > Not sure how you do one and a half sheets of double bias. I am
> > > assuming the extra sandwich thickness and glass are to eliminate
> the
> > > internal framing
> > >
> > > Looking at the weights
> > > 20mm core 1.6kg/m2; glass 1.8kg/m2; resin-depending on type of
> core,
> > > effort in edge joining, holes, shaping cuts, scrim for polyprop
> > > honeycomb` .9-1.5kg/m2. This gives a total of 4.3-4.9kg/m2
> > > With internal frames, this could possibly reduce to 3.2kgm2. I was
> > > wondering how the internal frames are covered-That could add
> > > significantly more weight
> > > Epoxy saturated 6mm marine ply is in the order of 4.5kg/m2. The
> ply
> > > still needs almost .5 kg abrasion and surface cracking protection
> on
> > > the outside, and still needs the inner frame.
> > > Costs of the foam sandwich depends on type of core, type of
> resin,
> > > and very much source of materials: for 20mm core $35-70/m2; resin
> > > $10-20/m2; glass $10-15/m2 I think you would find the costs of
> going
> > > with the ply would be similar or slightly more with the cost of
> the
> > > frame. I must admit the simple meccano approach to the frames
> appeals,
> > > but for the shape of a Harry, I don't see an advantage. Simply
> make
> > > some big flat sheets,with judicious leaving out of glass where you
> > > want to bend, pull or push them into shape - possibly a little
> extra
> > > shaping on the ends- and the basic hulls are there, except for the
> > > main bulkheads where the crossbeam loads are concentrated. I
> reckon a
> > > Visionarry hull up to topsides could be done with less than 4m2 of
> > > external fairing and much of the internal work will be covered by
> > > internal furniture. Anyway, thats what I am aiming at. I hate
> adding
> > > good materials, only to sand it off again. Don't know how much
> fairing
> > > is required on the aluminium frame set up
> > > Robert
> > >
> > >
> > >
> > >
> > > --- In harryproa@yahoogrou
> wrote:
> > > >
> > > > G'day,
> > > >
> > > > Col is a very smart bloke, has been designing boats for yonks
> and has
> > > > always been full of ideas, most of them good.
> Experimentation is a
> > > > good thing and so is caution , so talk to Col and at least a
> couple
> > > > of people who have built with it. Maybe build his little canoe
> as a
> > > > test. Then do the numbers and decide which suits you best.
> > > >
> > > > I worry about corrosion of alloy and screw attachment of ply,
> which
> > > > often rots around fastenings unless they are individually over
> > > > drilled, filled with glue and redrilled. I also think
> > > > glass/ply/alloy will be heavier, maybe more expensive and
> require
> > > > more finishing. Could be wrong on all counts.
> > > >
> > > > If you talk to Col, say hi from me, tell him your boat will
> almost
> > > > certainly be 20mm foam or Polycore with one and a half layers
> of 600
> > > > double bias each side and some serious strength required
> around the
> > > > mast and the beams, but no other bulkheads. Surface area of
> > > > Visionarry lee hull is 57 sq m. I will be very interested to
> see
> > > > what he suggests, and how it competes with the panel build
> method.
> > > > Also ask him about the large flat)ish) cabin roof and floor
> areas
> > > >
> > > > regards,
> > > >
> > > > Rob
> > > >
> > > > On Wed, Apr 9, 2008 at 3:40 PM, Tim Barker <clairebarker5@
> wrote:
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > Hi Rob/ all
> > > > > Visited Col Clifford today , as much to see his radial
> engine as
> > > have a
> > > > > yack about build methods, very interesting guy and very
> cluey.
> > > > >
> > > > > What are your thoughts about the alloy internal frame
> system , as a
> > > > > person who has done quite a bit of metal fabrication it makes
> > > sense to
> > > > > me and offers some build speed advantages as far as i can
> see .
> > > Coupled
> > > > > with the idea of glassed ply skins it should be very cost
> effective
> > > > > tough and simple to build.
> > > > >
> > > > > For those on the site who arent familiar the method involves
> > standard
> > > > > ally extrusions and cast ally fittings which allow the frame
> to be
> > > > > fabricated from ally without welding, it is then skinned in
> ply or
> > > > > composite however the skin basically only has to act as a
> > waterproof
> > > > > membrane not as a structural member and also to hold the
> > structure in
> > > > > tension, light strong simple . WWW.ccplans.
> > > > >
> > > > > What are the various opinions out there.I myself am very
> > > suspiscious of
> > > > > ply or timber however i know that this is a fairly basless
> > predjudice
> > > > > given modern methods and materials hence the ongoing
> > investigation of
> > > > > different methods and materials.
> > > > >
> > > > > Coupled with the relative costs of some of the composite
> cores
> > on the
> > > > > market and the slowly dawning realization that using these
> > composites
> > > > > may result in a craft not much lighter (if at all) but
> > substantially
> > > > > more expensive than a craft using ply skins has certainly
> eroded my
> > > > > predjudices.
> > > > >
> > > > > Cheers Tim
> > > > >
> > > > >
> > > > >
> > > >
> > >
> >
>
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