Re: [PSUBS-MAILIST] Re: Stiffiners on Heads

Brian

ABS rules in the chapter on hull stress analysis has a section for stiffened conical frustums. The equations are are harder to implement compared to cylinders or semi-elliptical or hemispherical heads so I did not implement them in the current ABS hull spreadsheet. I believe the solution Vance suggests that of using the large diameter with the same stiffener spacing would be a conservative approach. The actual maximum depth would be something between the results for the small diameter and larger diameter. If you want a more accurate answer, code the ABS equations or get someone to do a comprehensive FEA of hull. Coding the equations would be better as they account for all the major failure modes. A simple FEA analysis were you look for stress hot spots using a linear model will not cover items such as fabrication out of roundness de-rating of the strength or stiffener tripping or a sensitivity to the number of lobes in overall buckling. I don't have time to do this update to the sheet but it would be nice if we could get someone to volunteer to add a tab for these stiffended conical sections to the ABS hull spreadsheet.

Cliff

Cliff Redus
Redus Engineering
USA Office: 830-663-6445
USA mobile: 830-931-1280
cliffordredus@sbcglobal.com

From: Brian Cox <ojaivalleybeefarm@dslextreme.com>
To: personal_submersibles@psubs.org
Sent: Mon, November 2, 2009 9:12:14 AM
Subject: RE: [PSUBS-MAILIST] Re: Stiffiners on Heads

Cliff,

In the areas where your sub has cones that reduce in size from one diameter to another, are those transitions farely straight forward as far as stressses? In other words is it simply a matter of calculating smaller diameter cylinders to arrive at the ultimate stresses? The reason I ask is because I'm going to have a transition from a 31" diameter to a 51" diameter which will most likely be a 45 degree sloped cone.

Brian

-----Original Message-----
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org]On Behalf Of Cliff Redus
Sent: Monday, November 02, 2009 5:42 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Re: Stiffiners on Heads

I concur with Daniel on the rib issue on heads. Radial stiffiners on heads do not increase design depth. Just for fun, a few years back when I did the hull design for the R300, I did some FEA experiments adding radial stiffiners of different designs to try and increase depth. On this boat, the semi elliptical head on the forward end of the pressure hull has the same thickness as the stiffened cylindrical section (0.25 inches). What I found was that every different stiffener design I tried on head actually reduced the heads ability to equally distributed the load. Also it showed that for an elliptical head the highest stress area is in the transition between the short cylindrical skirt and the elliptical portion also known as the knuckle. The only way I could increase the design depth for a give head diameter was to go to a thicker head or a hemispherical head.

Cliff

From: Daniel Lance <lanceind@earthlink.net>
To: personal_submersibles@psubs.org
Sent: Sun, November 1, 2009 10:42:36 PM
Subject: [PSUBS-MAILIST] Re: Greg Cottrell's Sub

Jon,

Greg will be first one to admit that if he had it to do over again he would not put stiffening ribs on the heads of his sub. He readily explains that when he began constructing his sub in the late 1980s his enthusiasm for building out paced his knowledge of the engineering principles involved. When we did the refit and added tee bar stiffeners internally in the cylinder section theoretically we increased his dive capability. In reality he still had the problem of the external ribs on the hemi heads.They are undesirable for two reasons. #1 The heads should be allowed to compress in a uniform manner. The ribs prevent that. #2 The ribs were not welded on using full penetration welds. If there is a weld failure any number of unpleasant things could occur. In the end we decided it was not cost effective to remove them . Greg is currently building a new sub incorporating all the lessons he has learned a! long the way.

Stiffeners on cylinders are fine. On elliptical heads ,hemi spheres and spheres they are not. Did anyone see stiffeners on the DeepWorkers or on the hemi-heads of the Aquarius?

This issue illustrates the potential hazard of designing a sub just by looking thru the Psubs photo gallery or the archive. Because some one else did it and took a picture of it does not mean it is an acceptable method. Unfortunately as a group we lack a peer review mechanism to sort photo gallery or archive entries into categories that could prevent this type of misunderstanding .

Dan Lance