If statement not comparing two valeus - python

I'm making a Battleship game and when placing the boats they sometimes overlap, the if statements in the function are there to prevent this but aren't helping

I've tried changing the if statements implication:

import random
# Here I create a grid

a = ["a","_","_","_","_","_","_","_","_","_","_"]
b = ["b","_","_","_","_","_","_","_","_","_","_"]
c = ["c","_","_","_","_","_","_","_","_","_","_"]
d = ["d","_","_","_","_","_","_","_","_","_","_"]
e = ["e","_","_","_","_","_","_","_","_","_","_"]
f = ["f","_","_","_","_","_","_","_","_","_","_"]
g = ["g","_","_","_","_","_","_","_","_","_","_"]
h = ["h","_","_","_","_","_","_","_","_","_","_"]
i = ["i","_","_","_","_","_","_","_","_","_","_"]
j = ["j","_","_","_","_","_","_","_","_","_","_"]
letters = [a,b,c,d,e,f,g,h,i,j]

# This is just asphetic

def board(line_0, Letters):
  print
  print line_0
  for i in Letters:
    print " %s %s %s %s %s %s %s %s %s %s %s" % (i[0],i[1],i[2],i[3],i[4],i[5],i[6],i[7],i[8],i[9],i[10])

#Here's the function
# And what I'm trying to do is to position boat on the grid 

def place_boat(letters, length, BT, Name):
  while len(BT) != length:
    AC_y = random.randint(0,9)
    AC_x = random.randint(0,9)
    if letters[AC_y][AC_x] == "_":  #  These if statements
      rotation = random.randint(0,1)
      if rotation == 0:
        if AC_y >= 5:
          del BT[:]
          for i in range(0, length):
            if letters[AC_y - i][AC_x] == "_":      # Don't work
              BT.append(str(AC_y - i) + str(AC_x))
            else:  
              break            
        if len(BT) != length:
          del BT[:]
          if AC_y <= 6 and AC_y != 0:
            for i in range(0, length):
              if letters[AC_y + i][AC_x] == "_":    # Since they end up
                BT.append(str(AC_y + i) + str(AC_x))
              else:  
                break             
      else:
        if len(BT) != length:
          del BT[:]
          if AC_x >= 5:
            for i in range(0, length):
              if letters[AC_y][AC_x - i] == "_": # reassining multiple values
                BT.append(str(AC_y) + str(AC_x - i))
              else:  
                break             
        if len(BT) != length:
          del BT[:]
          if AC_x <= 6 and AC_y != 0:
            for i in range(0, length):
              if letters[AC_y][AC_x + i] == "_":     # to the same coordinate
                BT.append(str(AC_y) + str(AC_x + i))
              else:  
                break             
  for i in BT:
    letters[int(BT[BT.index(i)][0]) - 1][int(BT[BT.index(i)][1])] = Name
  print BT
  board(line_0, letters)
  return BT
  return letters

#Here are the lists that represent the boat I want to create

AC = []     #   
Bat = []    #   
Cru = []    #   
Sub_1 = []  #   
Sub_2 = []  #   
Des_1 = []  #   
Des_2 = []  #   

#Here I call the function for the individual boats

place_boat(letters, 5, AC, "A")
place_boat(letters, 4, Bat, "B")
place_boat(letters, 3, Cru, "C")
place_boat(letters, 3, Sub_1, "S")
place_boat(letters, 3, Sub_2, "$")
place_boat(letters, 2, Des_1, "D")
place_boat(letters, 2, Des_2, "T")

I expect to have a final grid that displais a grid with randomlly placed and not overlaping boats everythime, example:

  1 2 3 4 5 6 7 8 9 10
a _ _ _ _ _ _ _ _ T _
b _ _ C _ _ _ _ _ T _
c _ _ C _ _ _ _ _ _ _
d _ _ C _ _ B B B B _
e _ _ _ _ _ _ _ _ _ _
f D D _ _ _ $ $ $ _ _
g _ _ _ _ _ _ _ _ _ _
h _ _ _ _ _ _ _ _ _ _
i _ _ A A A A A _ _ _
j _ _ _ _ _ S S S _ _

But instead recieve one with overlaping boats, example:

  1 2 3 4 5 6 7 8 9 10
a _ _ _ _ _ _ _ _ _ _
b _ _ T T _ _ A _ _ _
c _ _ C _ _ _ A _ _ _
d _ _ C _ _ B B B B _
e _ _ _ _ _ _ A _ _ _
f _ _ _ _ _ $ $ $ _ _
g _ _ _ D _ _ _ _ _ _
h _ _ _ D S S _ _ _ _
i _ _ _ _ _ _ _ _ _ _
j _ _ _ _ _ _ _ _ _ _