Re: Hits on PoW and Bismarck
Posted: Tue Oct 16, 2018 12:34 am
Turning a properly designed spin-stabilized projectile 'sideways' is actually quite difficult to do insofar as it generally requires a rather large and sustained force exerted from a location that creates a significant offset between the center of mass and the center of pressure. Water can do this because the force from the liquid is exerted fairly well forward on the bullet, and the couple is significant and sustained. In the case of impact, unless the bullet is significantly yawed to begin with, the force required to rotate the bullet along its longitudinal axis is so large, and the effective couple so small, that the projectile will generally suffer a structural breakup before it rotates very much.
In the case of the Bismarck damage -- and for that matter other cases of damage as well -- the size of the hole, once it exceeds a certain area, really has fairly little to do with the total quantity of water taken aboard, although it can of course quite dramatically affect the rate at which flooding occurs. Probably anything over 10" (250mm) or so would have been sufficient to overcome any practical dewatering capacity, meaning (in practical terms) that plugging would be the only real answer. Certainly plugging a large hole is more difficult than occluding a smaller one, especially if the ship is at speed (or wishes to proceed at speed after stopping temporarily for repairs) in which case dynamic effects , rather than purely hydrostatic issues, must be addressed as well. What's easy to fix in a shipyard can be very difficult to fix underwater while at sea.
returning to photographic issues, the whole picture in this case (no pun intended) is rendered somewhat problematical insofar as Bismarck was the victim of a very large number of incoming rounds, a fair proportion of which were probably significantly yawed at impact due to striking the water as 'shorts' . This means that entry/exit holes are often somewhat geometrically unusual, rendering it often very difficult or impossible to assign a given hole or holes to any specific timeframe, whether that be measured in minutes or days, with any degree of certainty.
Bill Jurens
In the case of the Bismarck damage -- and for that matter other cases of damage as well -- the size of the hole, once it exceeds a certain area, really has fairly little to do with the total quantity of water taken aboard, although it can of course quite dramatically affect the rate at which flooding occurs. Probably anything over 10" (250mm) or so would have been sufficient to overcome any practical dewatering capacity, meaning (in practical terms) that plugging would be the only real answer. Certainly plugging a large hole is more difficult than occluding a smaller one, especially if the ship is at speed (or wishes to proceed at speed after stopping temporarily for repairs) in which case dynamic effects , rather than purely hydrostatic issues, must be addressed as well. What's easy to fix in a shipyard can be very difficult to fix underwater while at sea.
returning to photographic issues, the whole picture in this case (no pun intended) is rendered somewhat problematical insofar as Bismarck was the victim of a very large number of incoming rounds, a fair proportion of which were probably significantly yawed at impact due to striking the water as 'shorts' . This means that entry/exit holes are often somewhat geometrically unusual, rendering it often very difficult or impossible to assign a given hole or holes to any specific timeframe, whether that be measured in minutes or days, with any degree of certainty.
Bill Jurens