Hi Marc,

to my knowledge, information on schnorchel design are scarce and widespread, although mentioned in several sources :
1. Prof. Gabler's book Submarine Design, Bernard&Graefe Verlag, Koblenz 1986, ISBN 3-7637-0124-9 confirms your story (p. 72 - 80, showing also some design aspects
2.Eberhard Rossler's book, Geschichte des deutschen Ubootbaus Band2, Bernard&Graefe Verlag, Koblenz 1986, ISBN 3-7637-5802-8 also confirms this (p. 328 - 338), stating that for the German designs, the idea was taken up by professor Walter (later designer of the Walter-Uboote using hydrogen peroxide) in the srping of '43. After consulting with donitz, apparently the design was carried out in Kiel by some of Walter's group : Gabler and Heep. The few pages in this book contain excerpts from letters between Donitz and Walter, which is probably what you are looking for, initial design variants and which Uboats were initially equipped.
3.a dutch book, titled ' Wegduiken' ' De Nederlandse Onderzeedienst 1906 - 1966' , a limit print by the dutch submarine service published by the ' Zuid-Hollandse Uitgevers Maatschappij', no ISBN number (didn't exist yet !) has a few pages describing the existance and a picture of a snorting O21 in 1940.

There are a number of drawings on schnorchels in different books.

When visiting the Bunderarchiv records in Freiburg (Germany) in ' 87, my colleague ' Uboot-buff' and I discovered a 17 page post-war report (dated 30.4.1947) on schnorchel design, development and experiences written by Dipl. Ing. H. Oelfken, a well known German submarine designer addressing technical and operational aspects, including a patent application for a streamlined schnorchel, dated 18.4.'45 .

If this would be of any interest I could try to get you more information.

best regards,