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Hi Alan,
It's really good you're looking at Stachiw, which is
the bible for viewports. I don't mean to be overly critical, this is probably
just interpretation (just like with the real bible, no?) I looked up page 460,
and found a rather dire warning that if you're using a flange you need to
back it up with a bonded acrylic ring. I agree with your option
#1, but I think options #2 and #3 really have to be combined. Call it option
2.5, which consists of machining down the flange AND backing it up with a
bonded ring.
For those without the book, the dire warning is:
"In an actual application of free-blown hemispheres with flanges, it was
found that when the heel backup ring was eliminated the magnitude of tensile and
compressive meridional stresses increased 1000 and 64 percent, respectively,
although in an attempt to compensate for the expected increase in stress level
the thickness of the wall was doubled. (Figure
11.90)." Figure 11.90 then shows two ways to use a
flange, labeled "proper" and "improper". The proper one does indeed have a
thinner flange, but crucially it also has a bonded acrylic support ring
inside that flange.
My recommendation would be to do just that, rather than
assume the dome can be backed up with fiberglass instead of bonded
acrylic. Or else just take the proven K250 approach. On the K250 the flange area
is machined off entirely, in other words Kittredge went with option #1. But for
hold-down convenience he added a bonded acrylic ring on the outer surface
of the dome. The external ring is over and above the structural
requirements, it's just to give you a handy hold-down on a dome that's already
strong enough without it.
thanks,
Alec
Hi Alec,
Thanks for your concern. I'm open to any
cross-examination of what I'm doing.
The idea of partially elliminating the flange comes
from Stachiw's book page 460, as one
of the solutions to the problem of bending stresses
at the heel. Apparently there are 3
options. !/ removing the flange. 2/ glueing a
supporting ring to the heel or 3/ partially
removing the heel so that the heel curvature &
hence stresses are lessened.
I'm intending to partially remove the flange to the
point where I still have sufficient flange to
retain the dome & also support the
heel curvature with fiberglass & acrylic glue rather than a glued on
ring.
If I retain the flange its the easiest way to
enable me to hold the dome down. I didn't like other ways that
impede your vision.
The guy blowing the dome has the Stachiw book &
we discussed annealing when I put my order in.
I want to clamp my dome to a thick steel plate
& test it separate from the sub first.
Would be sad if it failed, I'd probably say
something like "BUM" as the rope that suspended it suddenly got
heavier.
Alan
----- Original Message -----
Sent: Thursday, February 18, 2010 2:56
PM
Subject: RE: [PSUBS-MAILIST] Hatch
pressure
Hi Alan,
Uh-oh, problem. Although it sounds entirely reasonable,
that isn't a safe hold-down method. It's essential the flange should be
machined entirely off because it's the area of maximum stress in a blown
dome. You could only really use the proposed method in an ambient
boat. I'll let Greg weigh in as the resident expert, but if he misses
this post please be sure to contact him proactively. This is a serious safety
issue, not a detail.
I'm no acrylic expert, but be aware there are other
details that set a submersible dome quite apart from what your local supplier
may be used to fabricating. Like the annealing protocol, for instance,
which as I understand it can have drastic consequences. Viewports are
an area in which something may look OK but not be OK.
thanks,
Alec
Thanks Carsten & Alec, thats great
information.
I'll add a pressure equalizing valve. If I had a
manually operated valve it may
cause problems If I forgot to open
it.
I'm not building a K250, the dome on order is
being blown 550mm O/D & from 35mm
thick acrylic. I've asked them to trim the flange
to 15mm wide & 15mm thick & will have
a retaining ring holding it down.
I was concerned that maybe I'd gone a bit thin to
support the upward lifting force of the
dome but it calculates out to about 1 pound
for every inch of flange just at below the surface.
Thats providing as Carsten said, the barametric
pressure isn't higher than outside the sub.
Alan
----- Original Message -----
Sent: Thursday, February 18, 2010 9:24
AM
Subject: RE: [PSUBS-MAILIST] Hatch
pressure
Hi Alan,
If it's a K-250 you're building, here's your other
data point. I just weighed the hatch, and it's 23.5 kg. So it would be 6.5
kg buoyant.
Alec
Hi thanks people,
I was having trouble understanding this. I just put plastic wrap loosly
over the top of a glass & submerged it.
Some of the wrap bulged up
but you could push parts down below the level
of the top of the glass.
( This proves you right )
I am going with a dome hatch, but
haven't designed the locking mechanism yet.
The dome will be close to 30kg of boyancy
before its mounted.
Alan
----- Original Message -----
Sent: Thursday, February 18, 2010
6:51 AM
Subject: RE: [PSUBS-MAILIST] Hatch
pressure
First weigh the hatch in air. Then calculate the
displacement of the hatch, which is the weight of the water that would
fill an equivalent volume. Subtract the displacement from the weight. If
you get a negative number, the hatch will tend to "float" as you describe.
I doubt very much that would be the case for any conventional elliptical
steel hatch. In the case of an acrylic dome hatch like the K-250, I'd
recommend doing the math because I'm less sure of the outcome
-- those might be slightly buoyant.
Alec
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Hi all,
I've brought up this subject before, but are
tackling it from a differant angle.
How much pressure is exerted upward on a
hatch at the moment where the
submarine
just submerges below the water? This is the
point of most force before external water
pressure helps close it.
If you took your submarine, filled it with
water & hung it upside down, would this be the
same amount of force, or close to
it.
In wich case if you have a design like Franks
flying saucer that angles up to the hatch,
you'd have a lot more force acting upward on
the hatch than a K250 where the lifting force
would be spread more evenly along the
hull.
Am I thinking right?
Alan
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