Thank you all for your replies.
Garlicdesign is spot on with his hints and obviously knows the answer.
My take on the worst building disaster of the Dreadnought era was the French Danton class Battleships. (
https://en.wikipedia.org/wiki/French_battleship_Danton )
The French had made wrong conclusions about the Russo-Japanese war and the Tsushima battle. While the Dreadnought was completed and commissioned 02/12/1906, the Dantons were not even laid down till 1907. France built six battleships that were completely obsolete before they were even laid down. As GD pointed out, the six ships also tied up a lot of building resources and meant that the next real Dreadnought type (Courbet Class) ships did not get laid down till 1910-11.
I think that qualifies as a building disaster of epic proportions.
It also gave me a bit of time to do a couple of kitbashes to show what may have been.
First drawing is Alvamas original Danton Class with the 4x12" 12 x 9.4"
Second drawing is of the proposed change to make the twin 9.4" into single 12"
Third drawing is my take on what Danton may have looked like if the French had gone with the lozenge arrangement the Germans and Japanese started with (12x12")
The French then had 36 twin 9.4" turrets to do something with. The next drawing is just one possibility of how the turrets could have been used.