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Re: [PSUBS-MAILIST] Hemisphere Seat Formula

That would be where I am. Trailer mods underway and a new truck in the driveway. Should be able to get wherever you end up. Let's stay in touch about it.
Vance



-----Original Message-----
From: Greg Cottrell <greg@precisionplastics.com>
To: personal_submersibles <personal_submersibles@psubs.org>
Sent: Wed, Jun 15, 2011 9:03 am
Subject: RE: [PSUBS-MAILIST] Hemisphere Seat Formula

The new sub is at about 50% and I’m shooting for dives in the early fall. If it works like it should I’ll be headed down to sunny Florida!
 
Greg Cottrell
Project Manager
 
 
greg@precisionplastics.com
http://www.precisionplastics.com
 
P please consider the environment before printing this email
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of vbra676539@aol.com
Sent: Wednesday, June 15, 2011 12:43 PM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Hemisphere Seat Formula
 
Truthfully, I don't think the dome ports will EVER develop cracks in their scheduled lifetime. The original design (o-ring in the center of the mating surface) is still running fine. The later ones, with the o'ring groove built into the retaining ring (with the mating surface serving as the third side and the whole thing captured by the acrylic) just don't have any hard points to worry about. Like you say, if it fails, it'll fail on the inboard side. O-rings are just to keep the water out on the surface. They mating surfaces do the sealing at depth and the o-ring takes a break.
 
I like the idea of a rodeo. Talked to Dan Lance and once to Dan H. about it. We'll be doing the major work on Gamma this summer, then start assembling after paint. Where are you on the new sub, and where do you want to dive?
 
Vance
 
-----Original Message-----
From: Greg Cottrell <greg@precisionplastics.com>
To: personal_submersibles <personal_submersibles@psubs.org>
Sent: Wed, Jun 15, 2011 8:38 am
Subject: RE: [PSUBS-MAILIST] Hemisphere Seat Formula
Vance,
 
As always, you provide a wealth of interesting info. Hopefully when your new sub and the one I’m currently building are done we’ll take ‘em out for a dance.
 
As far as the o-ring position is concerned, I think of it this way- if you’ve ever seen an old dome (or conical viewport) that is cracked up from stress, the cracks always form closest to the inside surface. This is where the stresses are highest and try to release near the unsupported inner surface. As you know, Perry engineers put the o-ring grooves close to the outside where those stresses are lowest. In my opinion, the dome is going to crack up down near the bottom long before cracks start to form around those o-rings.
 
 
Greg Cottrell
Project Manager
 
 
 
P please consider the environment before printing this email
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of vbra676539@aol.com
Sent: Wednesday, June 15, 2011 11:36 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Hemisphere Seat Formula
 
Greg,
 
You are correct, of course. Perry changed the design, however, for manufacturing reasons, rather than strength. The PC-8s and the PC-14s were 42" hulls and smaller, so they used the lighter design to keep things light, and to make the subs more than a few bucks less expensive.
 
Once they went to the PC-12 class and beyond, the viewport/frame assembly was used, as there was plenty of payload for the extra weight, fabrication was simpler, and maintenance much easier in the field. We could take a window assembly off in about three minutes, and put it back on just as quickly. It took more metal, as the welded reinforcement in the earlier boats and the retaining ring were essentially built as a bench assembly with the port (on the 12s, 16s & 18s), and a flat flange welded/machined in the forward hemi of the sub. Then the viewport was clamped in place with a big v-clamp like the battery pods. The load on the hull would have been quite different, as well. That said, they whole thing was still quite light, relatively. Two people could handle the assembly. We usually used more. Extra hands never hurt.
 
The earlier design (which also included the PC-15) was more of a pain as you had to float the port in mid-air, so to speak, set it on the o-ring, install the retaining ring (all the while trying to put a vacuum on the boat to help hold everything in place, keep all the little rubber bumpers in place under the retaining ring, start all those quarter-twenty bolts, and not spill your coffee or burn yourself with a cigarette). THEN you got to work with the torque wrench. More importantly, it was a big enough pain that casual paint maintenance happened less frequently, and so there were more problems for the annual overhauls (moisture would seep under the seat from the inside and sit on the o-ring and in the groove--no way to get it out except take the thing apart--and we didn't have a lot of time for that during the season).
 
Whew! Good thing we were all young!
 
Also, the original o-ring position was frowned upon by ABS, who decided the stresses on the acrylic MIGHT cause fracturing issues in multi-cycle usage. As a side note, three of the four 42" hulls are still in service with that style (an o-ring groove in the seating surface). No one has noted cracking or other issues. None. Nada. They work fine. Just like they always did. Gotta love those Perry engineers.
 
Vance
 
-----Original Message-----
From: Greg Cottrell <greg@precisionplastics.com>
To: personal_submersibles <personal_submersibles@psubs.org>
Sent: Wed, Jun 15, 2011 6:26 am
Subject: RE: [PSUBS-MAILIST] Hemisphere Seat Formula
You are correct. A tank head would require greater reinforcement if an
opening were made in it and capped with a small dome. The dome in this
example adds nothing to the strength of the hull.
 
Early Perry submarine sealing surfaces were amazingly light partly due to
the geometry of the dome- the dome essentially acted as part of the hull. A
full hemisphere doesn't do that unless it is joined to another hemisphere.
 
Imagine joining two full hemispheres together- you would then have a sphere
and all forces would be balanced. Now imaging joining two 120 degree domes-
without a strong ring to withstand powerful radial loads, the two domes
would collapse. In the case of the Early Perry submarine, the shallow dome
and the hull work in concert, and the reinforcement can be smaller.
 
Later Perry designs used a two part ring and don't really apply to this
example.
 
Greg Cottrell
Project Manager
 
 
greg@precisionplastics.com
http://www.precisionplastics.com
 
P please consider the environment before printing this email 
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
Sent: Wednesday, June 15, 2011 9:54 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Hemisphere Seat Formula
 
 
Greg,
 
I note that the K250 dome only has a 1/2inch seat welded to the conning 
tower.  I assume this is because the bulk of the force is transferred to 
the conning tower cylinder based on the manner of the configuration.  
Mounting a similar dome in a tank head would require a different and 
bulkier mount, if not only due to reinforcement requirements.
 
Jon
 
 
On 6/15/2011 9:30 AM, Greg Cottrell wrote:
> Jon,
> 
> The dome shape that exerts the highest radial loading should fall
somewhere
> around the midpoint between a flat disc and a full hemisphere. I imagine
> that the 90 degree segment exerts the peak load but I don't know how to
> demonstrate that mathematically.
> 
> 
> 
> Greg Cottrell
> Project Manager
 
 
 
 
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