How effective were WW1 Uboats vs Big ships?

I was wondering - were Uboats really very effective against big targets (since no torpedo protection existed yet; after all, one Uboat in first months of the war succefully sank three cruisers at once)? Were 1-2 torpedoes enough to sink any warship? If they really could have provided naval supremacy after sinking big ships in several major engagement (maybe ambush, maybe HSF used as a lure, etc), then Germans made a very costly mistake by focusing on sinking merchant navy instead of Royal navy.