Despite what all the realists say. You can beat a lone flower on the surface if your sneaky enough, the flower has 4 weaknesses we can exploit.
Its slow compared to most escorts
Its lighty armed (about the same fire power as your Uboat)
Its lightly armoured
Its as blind as a bat from the rear.
Preperation.
1) First off prepare an electric eel in one of your fore tubes to run at about 3 meters set to magnetic or 1 meter on impact make sure the tube is opened too.
2) Through out this attack you should always keep moving at ahead standard as full and flank will effect your aim with the deck gun.
3) Have some crew assigned to dammage control just in case.
Approach.
The best approach is to get on his tail whist you are submerged, surface behind him at about 3000 meters distance - he wont see you even in broad daylight, flowers (bizarly) are visually blind on their 180 degrees.
This gives you time to prepare deckgun (assign crew, load gun and take careful aim)
Attack.
The flower will be alerted as soon as you fire off the first shot and will start to swing around to return fire with his main gun, giving you a few vital seconds to deal some heavy blows with AP shells whist he is defensless, as he turns, the gun will start to fire but will likley miss (due to him turning)
when he does turn around to face you, you are still quite safe at a distance as you are facing him head on, which gives the Uboat its smallest profile and thus a tricky target for his gunner. that said his shot will now become more accurate.
What happens next depends on how the flower behaves, if he heads right for you, you can lanch that torpedo (we prepared earlier) in to his bow, while your crew continues to attack with the deck gun -if on the hand he he zig-zags or alters course in anyway then dont bother.
If the flower manages to close to about 1500 meters its time to cut your losses, knuckle to port or starboard whilest crash diving. (do this at 2000m if in a type IX) You'll get a pasting from his charges, but its probably not going to be fatal if you get go deep enough.
But all being well you should have destroyed or at least mangled him pretty badly. Most of the time i have tried this I have succeded in crippling the the flower to the point where he can no longer engage me while im submerged.
Its worth a try because if anything its garranteed to get your pulse racing.
This will not work on any destroyer they are way too deadly! If there are any other armed ships around (including merchies) dont risk it!
I also dont belive it can be done if you play at 100% realism, (feel free to prove me wrong) quite simply, your reload times will be too slow - but i dont expect those players to try this tactic anyways as it probably to 'gamey' for them.
Its risky but it can be done!
Happy hunting.