The effects of underwater explosions are two-fold:
(1) The expanding bubble of a contact underwater explosion has walls that move out at a speed faster than the speed of sound in water, so that it expands in all directions, REGARDLESS OF ANY VENTING, because the force on one side of the explosion knows nothing about the things going on at the other side of the explosion. When the force has expended itself, the bubble collapses and then re-expands if in an open symmetrical situation. However, next to the ship, the bubble portion that moved toward the hull has torn it open and otherwise disrupted it. On top of this, if there was an air-filled void next to the outer hull, the collapse of the bubble on that side will not occur, so that the collapsing roughly-hemispherical bubble wall on the opposite side *away from the ship* will meet no opposition when it reaches the center of the explosion and forms a "shaped water jet" (exactly like the jet of a HEAT tank-killing projectile, but here made of slower, much more massive water in a somewhat less focussed jet) that blasts into the hull at right-angles to the hull right through the hole opened up by the original bubble expansion. This second WHAM! now hits the next bulkhead in line, which is usually water/oil backed, reinforcing it, but will still tear it open. Now the jet must accelerate a large volume of inert liquid that was in the filled void, which dissipates and dilutes at least some of its energy when it hits the next bulkhead on the far side of the liquid layer (there may be two or more such liquid-filled middle layers back-to-back, as in IOWA and SOUTH DAKOTA in their original design), which in most ships in the primary "torpedo bulkhead" and is usually much thicker in an attempt to stand up to this blast (there should always be at least one air-filled void inboard of the inner-most torpedo bulkhead to allow it to stretch backward without damaging internal equipment and to prevent leaks, assuming it works and is not itself torn open--BISMARCK failed to have this innermost air-filled void, which is another bad design feature). In no case will any kind of venting have any positive effect on the original damage done, though it may reduce some secondary damage to bulkheads/decks at the top of the anti-torpedo system as the system is squeezed by the torpedo explosion.
(2) Another major problem is the high speed of the original bubble expansion,
which creates a powerful shock wave in the water around the explosion and
carries quite a way (it can be heard hundreds of miles away!), with intense
concussive effects on nearby objects (even if a fish did not have a swim
bladder, its body may be reduced to jelly by a nearby high explosive detonation).
This is the primary purpose of the outermost void in most anti-torpedo
systems, which will "decouple" a shock wave in water and, though it might
leak afterwards, reduce the damage to the ship from a non-contact (usually
a mine in this case) underwater detonation. ONLY IF THE EXPLOSION IS SO
WEAK THAT THE ORIGINAL BUBBLE FORMED CANNOT REACH THE FAR (INNER) SIDE
OF THE AIR-FILLED OUTER VOID WILL THE EXISTANCE OF SUCH A VOID IN A CONTACT
UNDERWATER (TORPEDO OR MINE) EXPLOSION BE OF ANY USE IN WEAKENING THE EXPLOSIONS
EFFECTS ON THE SHIP, BECAUSE IF THE BUBBLE REACHES IT, IT WILL *ALWAYS*
BE DESTROYED BY THE BUBBLE'S BLAST (NO METAL WALL IS STRONG ENOUGH, EVEN
IF BACKED UP BY A LIQUID LAYER BEHIND IT) AND THE FOLLOWING WATER JET WILL
HAVE ONE LESS OBSTACLE TO BLAST THROUGH TO REACH THE SHIP'S "VITALS." Also,
the shock effects that move through the ship due to the hit and cause wide-spread
damage throughout the ship are in no way lessened by any venting--venting
occurs well after the primary damage is all over due to the rather slow
motion of air and liquid compared to the explosive's high-speed blast effects
and the compressed, focussed water jet.