If it was supposedly big enough to launch b29's, it also big enough to have a decent radarinstallation and a couple of TBF's to keep away these nasty jerry warships
What would happen if KM Tirpitz shows up on a dark and stormy night and pumps several hundred 38cm shells into this giant ice cube? Would the mid Atlantic airfield become crushed ice?
The Pykrete had supposedly good resisteance to damage. The story is that in a meeting Mountbatten shot a cube of ice with his revolver and it shattered. He then shot a cube of pykrete and the bullet richocheted.
Also, weren't the Germans too scared to put the Tirpitz out to sea after the Bismarck was sunk?
Yes but we aren't talking about pistol bullets. German 38cm/52 projectiles weighed about 800 kg and could penetrate about 16 inches of armor plate at a range of 20,000 yards. I suspect they would penetrate pykrete just fine.
Submarine torpedoes would also be a threat. I suppose you could hang an old fashioned anti-torpedo net all the way around supported by floating buoys.