How effective were U-boats during WWII?

[Sarge McSarge]

It all depends on how you look at the stats.

In Clay Blair's "Hitler's U-boat war - The hunted 1942-1945" the there is a statistic mentioned. It deals with the period Sept -42 till May -45 and so does not include either of the so called "happy times", but still.

He claims that during this period 953 convoys comprising 43 526 ships sailed on the North and Mid Atlantic runs. 272 of these were sunk!! by u-boats (that is 0.6%)

He further claims that during the entire war there was 2919 ships sunk by u-boats. I havent been able to find any figure of total sailings, single or in convoy but the 1% sunk does not seem that unrealistic.

There was not 43,526 ships but 43,526 individual voyages. Was this 45 ships each doing every one of 953 convoys or 90 ships doing 476 convoys?? 272 does not look so bad when you look at it this way. If the sinking percentage was 1% per voyage then 10 voyages puts your odds up to 10%

A better way of looking at it is to look at the percentage of capacity sunk.
The total allied merchant shipping tonnage available in the Atlantic and Med was about 67 million tons. This includes about 38 million tons constructed during the war as well as what existed at the start. The U boats sunk about 14.5 million tons so it is closer to 25% of total and about half of what was available at the start of the war.

Sarge