High Grade Steel, heavy diesel engines are just the start.
These 300 U-boats either get reinforced concrete pens or get bombed out of existence.
I’d say Doenitz’s dreams were about as unlikely to happen as the Kriegsmarine Plan Z.
http://en.wikipedia.org/wiki/Plan_Z
I suggest you re-read my posts and then be so kind and point out exactly where I claimed 300 U-boats was in any way realistically possible before the outbreak of the war.
They aren't just steel tubes, you know!!! Thats the least of it.
Its all the stuff that goes in, the equipment, torpedoes, etc. If you have people building this, then they aren't building something else, and there were desperate shortages of things like rubber and copper which they made heavy use of.
Of course they are just steel tubes, just look at they way they are constructed

. But yes U-boats need a lot more resources and raw materials then just high-grade steel that I do agree with.
What I do not agree with is that those very same resources, raw materials and industries that are needed to produce these U-boats, are not needed to either restock the German army’s supply of artillery shells and rifle cartridges. We aren’t talking about the conflicting need for resources that was between U-boats, tanks, aeroplanes and artillery (well at least I’m not).
Why do I not believe so? They weren’t in OTL, so why should they be in any other potential TL? We aren’t talking about daily expenditure ammunition of the scale of Verdun or Somme since there were no actual large scale fighting between mid October –39 and early May –40.
The resources and raw materials needed to produce enough supplies to the German army so that it’s stocks of ammunition and shells are not dangerously low after the Polish campaign, were comparatively small. And could have used those very same stocks of resources and raw materials between May and September –39 as Germany did OTL between October-39 and January –40. The industries needed for this is not needed by the U-boat industry, and does not use the same manpower, could have increased its production, they were producing at much less then their full capacity OTL.