Hi James,
I'm also planning on using sectional spherical viewport domes.
While I've not built any, I'll share what I've found:
1) You could use a neoprene gasket instead of the sealant.
Also, in the cross-section, where the viewport, the retaining ring
and the frame meet, there is a little triangle shaped void, you could
place an O-ring in there to stop water entering the viewport frame
and retain the dome.
Alternatively, the "flat gasket" you've currently drawn (between
the viewport and retaining ring) could be extended so that it's
sandwiched between the frame and retaining ring as well as
sandwiched between the viewport and retaining. This would prevent
water entering the frame. Figure 11.76 on page 524 of the Stachiw
book has diagrams of both implementations.
2) Yes, you should have an over hang so the inside viewport is no
where near the edge of the fame. The view port will compress
slight under operating conditions, fattening the base a little.
If the base moves over the edge, then there exists the conditions
for notch cracking to start, which means replacing the viewport.
In non-normal operating conditions, i.e. the viewport is failing,
the base will get much bigger, if it goes over the edge, then
notch cracking will make the dome fail much quicker.
I think there is a hint at how much over hang to have in the
Stachiw book, but I can't find it just now.
3) I've had a hard time finding information about how to design
the viewport frame/housing. Most are designed thick so that the
pressure hull moving as it dives deeper will not deform the
viewport frame/housing/seat. If the frame deforms, then the
Acrylic will not be against a flat surface and all sorts of other
problems will come into effect from that.
With a sectional spherical viewport the side walls of the
frame/housing/seat will be under pressure as the force pressing
on the dome presses the flat surfaces of the dome against the
frame. From looking at examples in the Stachiw book, all the
frame walls seems to be at last the same thickness of the
Acrylic, in some case 2x or 3x.
As for kicking the dome out in an emergency, my guts say that
isn't going to work so easily. Alec Smythe had a hydrolic
piston system for opening his dome in an emergency. Maybe
make the dome a hatch, so that the frame swings open and things
like retaining rings, O-rings, sealant and gaskets won't be
an issue. Will the dome be above the water when it's getting
kicked off?
I know at least one person who has/had a sub where the dome
didn't have any retaining device other than water pressure.
Cheers,
Ian.
-----Original Message-----
From: James Frankland <james@guernseysubmarine.com>
Sent: Aug 26, 2008 10:02 AM
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] Dome Calculations
Hello Everyone
Progress on my sub has been slow recently, but i have managed to get a
few
things done including a change to the possible design for the removable
front dome mounting system that i am adding instead of the 15" flat port
specified on the plans.
I have made some changes to my original design as a result of reading
Staichows book and with some advice from members of the group.
http://www.guernseysubmarine.com/index_files/Page12229.htm
I would really appreciate if you could take the time to have a look and
let me know what you think of this idea.
Points to note\questions.
1. The "O" ring groove has gone and been replaced with a 1mm bead of
sikaflex sealant. Bearing in mind the idea of this design change is so
the dome can come out in an emergency, do you think this amount of
sealant
will stick it permenantly and prevent removal, even with a good kick
from
the inside? Could make it thinner, 0.5mm?
2. The inside diameter of the ring meets exactly the inside diameter of
the dome. (The 32mm bit on the second picture). Should it be a bit
larger? 35mm?
3. I do not have the ability to do FEA on this ring, so based on the
"rule of thumb" that says you should replace the amount of material you
remove, i have calculated volumes of the ring and the removed piece.
The
figures are shown at the bottom of the page. The endcaps are spun and
thicker than specified on the plans. 13mm at the rims ranging to about
10mm at the centre. So, exact calculations for volume of this are
difficult. If based on the worst case scenario of 13mm all over, the
volume of weld in ring does not cover the "rule of thumb". Should i add
an extra 10mm to the thickness of the ring, bearing in mind the endcaps
are already quite a bit thicker than needed.
Does that make sense? I've not done one of these before so i think its
better to ask silly questions now and avoid mistakes or accidents in the
future.
Again, this is still just an idea at this stage and could all change,
depending on how things turn out.
I appreciate any advice, good or bad.
Regards
James
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