On Thu, 10 Dec 2009 05:03:59 -0600, cavelamb
wrote:
Bruce In Bangkok wrote:
On Wed, 9 Dec 2009 18:16:52 -0700, "Bob La Londe"
wrote:
"Bruce In Bangkok" wrote in message
...
On Wed, 9 Dec 2009 13:08:20 -0800 (PST), Bob La Londe
wrote:
On Nov 29, 6:28 pm, "Bob La Londe" wrote:
I find it interesting that a lot of the small boat aluminum designs seem
to
say the main hull should be 3/16 and yet .100 is used in a heck of a lot
of
commercially available small boats. .125 is considered heavy duty. Is it
just CYA?
Anyway, getting back to my original point. A lot of commercially
available boats seem to be made out of a lot light sheet than the
designers are reccomending in the boat plans they are selling.
http://www.seaarkboats.com/boat.php?...&boat=Big+Easy
For example: The link above is to a medium V design 24 footer. The
specs says .125 thick sheet. I can't buy a plan to build a boat that
size designed for .125 sheet. Most of the ones I have seen want to
spec .1875 which basically means .190. Now why is that? Is it just
the typical over building to cover your dearie aere, or are all those
commercial boat builders building inadequate boats and damn the
liability?
I wonder whether you are comparing designs that could be in one case
simply a covering attached to a structural frame where the skin only
keeps the water out while the structure provides the strength versus a
design where the skin provides some of the total structure strength?
Think of the original kayak design versus a stitch and glue dinghy
with no internal frames for exaggerations of this concept.
I remember reading that some of the British Steel 70-footers found
dented plating around the bow after leaving the Southern Ocean. the
answer was to add frames to the bow section for the next year's race,
not increase the thickness of the plating. In other words the
thickness of the plating was not considered a factor in the strength
of the bow portions. The frames provided the strength while the
plating kept the water out.
Well, it seems except for S&G conversions most of the aluminum plans I have
seen call for building a frame. Atleast in the 20 foot size range. Here is
an example from the Glen-L design catalog. 230 Max HP. The Sea Ark in my
previous post is rated at 225 so similar in stresses.
http://glen-l.com/designs/hankinson/...snakeshtr.html
SNAKE SHOOTER VEE between 17-1/2' and nearly 23'
Plating
1/8" 250 sq. ft.
3/16" 200 sq. ft.
1/4" 40 sq. ft.
3/8" 10 sq. ft.
Based on a once over that looks like 3/16 plating for the hull bottom and
1/8 for the sides to me. I have not looked at just one or two designs.
I don't think you can tell from the materials list. The plywood boat
for example uses 1/4" for the bottom planking and 3/8" for side
planking. Looking at the aluminum list it appears that 1/8" is for
bottom and 3/16" for side, which assumes that the two versions are at
least similar in design.
Given that Glenn-L makes a meal of saying that anyone that can weld
can build one of these boats I would suggest that thickness of plating
may well be selected from sizes that the designer expects that
"everyone" can have some hope of welding successfully.
Again assuming that the basic design is at least similar the aluminum
thickness doesn't seem to relate to the plywood sizes given that
aluminum has an ultimate tensile strength of somewhere around 90 MPa
while plywood is in the region of 30. If it were simply strength
bottom aluminum plating would be in the region of 0.083, based on
plywood strength.
Cheers,
Bruce
(bruceinbangkokatgmaildotcom)
If they are using 1/4" ply for the bottom, it must be double layered.
In plywood construction I would expect the bottom planking to always be heavier
than the sides.
That's where the heaviest stresses would be imposed.
I'm going only by the materials list that Glenn-L supplies which
doesn't go into detail and simply says something like Plywood, 1/4", X
sheets,bottom planking.
However... The aluminum version doesn't not specify what the various
thickness' they list are used for and there is a noticeable difference
in quantities of say, 1/8th sheet and 1/4" plywood, so I doubt that it
is a case of apples and oranges.
There are no construction drawings and the photographs are mainly
included to show you what a beautiful boat you can build so details
aren't detailed enough to make a reasonable assessment of how the boat
is built, other then a general idea of the frames and stringers.
But you are correct and normally the bottom is stronger.
Cheers,
Bruce
(bruceinbangkokatgmaildotcom)