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