On Thu, 10 Dec 2009 19:02:23 -0700, "Bob La Londe"
wrote:

"Bruce In Bangkok" wrote in message


I suppose the most logical question is "why". Yes, you can bend
aluminum at a sharper angle and have a less radiused bend by thinning
the aluminum at the fold line but what is the necessity for this
fabrication?

I'm not sure what you mean by a "racing pad" but I can't see any
correlation between sharp and speed. Unless you mean that the hard
chines have a sharp edge whereby they are more effective in combating
lateral forces?

On certain types of boats there is a narrow "pad" along the keel about a
foot wide. This will vary based on design, weight, and over all size of the
boat. At high speeds the boat is running mostly on this pad. Far be it
from me to explain the dynamics of it, (I'm just parroting what the speed
junkies say here) but there are a couple things that can improve speed. I
certainly do not know all the design characteristics that contribute to
optimum design, but one of them is supposed to be the sharpness of the edge
of this pad. Perhaps it has something to due with the way it sheds water or
breaks the surface tension of the water. Another is a rough rather than
smooth finish to the bottom of the pad. That most definitely has to do with
breaking the surface tension of the water. Like a shark skin bathing suit
or the finish on certain high tech racing sail boats.

I agree, that if one is determined to use this sort of bend that one
can probably design something that necessitates it but it is doubtful
that it is really necessary.

Well, obviously if the goal does not involve eeking 1 more MPH out of your
hull's top speed there's no point. LOL. Pun intended.

In addition, I do not believe that, providing you are talking about
lengths usually found in boat building, say chine logs, that run the
length of the boat that warping will not be a problem.

I think it would be much less of a problem than with two separate pieces
joined to make the same edge.

You say you can
see some warping in a 3 ft. piece... What about 45 ft. chines on a
sail boat?

Ah, see. Now I know you didn't understand what I wrote. I saw that kind of
warping in pieces that long that were welded together to form the edge. Not
in a single piece thinned and folded. Big difference. My apologies for not
being clear. When you start with one piece that is bent to form an angle
its going to be much more rigid than two pieces stuck together. The
remaining metal that holds the two legs of the angle together will not allow
them to expand and contract at different rates due to vagaries in welding
technique. I was mostly trying to point out that after I had completed my
initial work pieces I experimented some to see what else worked and how it
worked.

My assessment, for whatever it is worth,

Its worth a lot. If I do not look at ideas from a different point of view I
can't see what's wrong with them or how to make them better. It forces me
to engage my brain.

is that "your" system is
certainly viable and, depending on the size of the weld bead, may even
be stronger then a simple bend, however... It takes more time to make
and will be more costly as well as offering more chances of error in
manufacture then alternate methods.

Like? Roller bending? Edge welding two pieces? Braking?

Well, I don't know what you can do with roller bending, but they use it to
put longitudinal creases in sheet for strength. Not sure how it would apply
in this case anyway. I guess it might be an alternative way to make my
thinned and folded edge instead of the brute force method I used before. It
might make doing this more practical on longer pieces.

Edge welding. No way. To do an ideal corner weld you have to weld it
twice. I covered that. Most commercial mass production builders don't, but
a lot of their boats crack too. Weld, back chip, weld.

Braking? Well see this one is arguable. A brake is a tried and true
method. It works and its fast. If you can afford one big enough to do the
job. At this time I can't. It will definitely not make a bend as sharp as
the process I describe.

I can setup a built rig to bend "thinned" metal. I am sure the weld will be
better than two pieces edge welded in less time. IN GENERAL... There is
some grey area about the actual deep corner of the inside of the bend. I
guess to resolve that I would need to weld up some more pieces and then
slice it up to see if I get those corner voids I fear. If it gets a void in
it then there is a very thin bit of aluminum on the outside which could
break through easily. Not an issue for one outing or even a dozen, but it
could create a pocket that could trap water. We know about trapped water
and aluminum over time. Big potential problem.

The ability to perform the process is not in question. The reason is in
question depending on your goals.

My reason for considering the idea is building an aluminum boat that will
out run a glass boat in the same size class. ie to take advantage of the
weight savings with out throwing them away on hull shape.

My biggest concern would be the stresses on this very sharp hard point.

A hard point (as I am sure you already know) is a corner, turn, or bend in
the design of a boat or other structure that may concentrate the stress from
external forces due to a fast or abrupt change in shape. That's why we
often see stress cracks in the gel coat of glass boats in the corners of the
top cap. Its also why we see cracks at the weld in the back corner of a lot
of older Trackers and some other boats or at the front corners of square jon
boats. (HAZ and alloy selection not withstanding.)

I wish that you could point me at a url showing this "speed pad" as I
can't seem to visualize it, nor can I find more then the words, i.e.,
has a speed pad. I am somewhat familiar with the hulls of both three
point hulls (the old speed boat hulls with the sponsons) and more
modern open ocean racing stuff, but the speed pad defeats me

True that corners can be stress risers, or hard points, but simply
having a sharp corner does not cause cracking, it simply provides a
location where, assuming that the item is incorrectly designed,
stresses will concentrate. So, if you have an item that is not
designed to resist the forces applied to it a sharp corner will be the
likely location for deformation to occur.

Again, as I previously wrote, if you want to use a bent/welded
structure then you will certainly be able to design a need for it :-)

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)