Vixen
Penultimate Amazing
I was just going to ignore this, but as you asked about it, I'll respond. "Double-headed hammer" is a colloquial term for bar shot, in which multiple projectiles are connected by one or more iron bars. This, along with the closely related chainshot, is known as dismantling shot, and was used mainly to attempt to damage enemy vessels' masts, sails, and rigging.
As for Nelson's always following a broadside of "cannon shot" (presumably Vixen means roundshot, or ball) with a broadside of dismantling shot, she just pulled that out of an orifice, as usual. First, Nelson never told his captains what type of ammunition to fire; each captain used his own judgment based on Royal Navy doctrine and the tactical situation. Second, dismantling shot was usually fired from a ship's lighter guns, as a 32lb bar or chainshot was unlikely to cause significantly more damage than a 12lb shot, as either one would have likely torn through any rope, sail, or spar encountered. But a 32lb ball striking the hull of an enemy vessel would have almost certainly caused far more damage than a 12lb ball.
Round shot, bar shot and tiered grapeshot. The point was to attack the beam, then the rigging and then the crew. So the claim that no way could there be more than one cause of the Estonia sinking is misconceived if the sinking was a military operation.
No way was it 'a few strong waves pounding the bow'.