Good points Redcoat. Germany building 100 U-Boats instead of Bismark and Tirpitz would have alerted Britain to possible German intentions, but as vonManstein39 says, Britain also had to watch Italy and Japan building their battleships, so chances are she would have continued with her own battleship program. More than likely Britain would have excellerated her Fleet Air Arm/Royal Navy anti-submarine warfare studies to counter-act the U-Boat threat. But she may not have openly challenged the Germans over the U-Boats.

Afterall, when Germany finally revealed the existence of the Luftwaffe, the new and larger Wehrmacht, etc. what protests were made by France and Britain were weak and fell on deaf ears. If Germany had wanted to build U-Boats instead of battleships, she would have done so regardless of world opinion.

And an extra 100 subs at the start of the war would have seriously threatened not only Britain's supply lines but also threatened the effectiveness of the Royal Navy.

Imagine if, instead of using these additional subs to attack supply ships, these subs were sent to sink Britains battleships, cruisers, and aircraft carriers, or used to just bottle up the Royal Navy in port, allowing the Kreigsmarine's other ships free run of the world's oceans.

With the Royal Navy having suffered either major ship losses to U-Boats, or being penned up in port. Britains supply lines would have been open to attack from all quarters. This could easily have led Britain to being brought to her knees, and sueing for peace.

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There seems to be something wrong with our bloody ships today.
- Admiral Sir David Beatty, after the destruction of the HMS Queen Mary at the Battle of Jutland, May 1916