I intend to use an NPC to make her attack a vessel. The NPC is capable of using Metamorphosis as a multiclass Ardent 7, manifester level 11. My idea was to make her metamorph into a flying creature, fly directly above the vessel and then turn into a huge rock, crushing the deck and part of the hull.
The vessel in question is a Dromond, I use "Stormwrack", for sinking the vessel 15 hull sections (each a 10 feet cube) would have to be crushed, each with hardness 5 and 80 hp. The Dromond has a 20 feet by 100 feet space. The whole ship has AC -3, attacking a hull section is AC 2.
My problem is to assess the power of my imagined kind of attack. Metamorphosis says:
You can also use this power to assume the form of an inanimate object. You gain the object’s hardness and retain your own hit points. You can take the shape of almost any simple object you can think of, such as a chair, a sword, or a rug. (...) You cannot use this power to assume the form of a psionic item or a magic item, or any object with a hardness of 15 or higher. (...)As an inanimate object, you lose all mobility. You retain your normal senses and your abi lity to speak. You can manifest a power if you make a Concentration check (DC 20 + power level); however, doing so ends the duration of this power. If you take damage while in the form of an object,your actual body also takes damage (but the object’s hardness, if any, protects you). Expanded Psionics Handbook, p. 117
Stone does have hardness 8, so turning into a rock is possible. The NPC would take falling damage minus 8 points of hardness. But how much damage would the ship take? The NPC could turn into a 11 HD creature - but there seems to be no limit regarding objects.
I see the following possibility to limit the Size of objects you can turn into using Metamorphosis: The power specifies that the manifester retains her hp. An inch of stone in a 5x5 feet square has 15 hp. (PHB, p.166) The character has 117 hp. So you could say she could turn into a stone slab of 5 feet by 5 feet 7 inches thick. This slab would have 105 hp. You could transform it into a stone ball of roughly 3 feet diameter. This is a massive boulder - but just how massive in terms of damage?
A granite ball of 3 feet diameter would weigh roughly 1,16 tons. This is more than 11 times 200 lb, so the DMG section for falling objects applies: This boulder causes 11d6 points of damage on a 10 foot fall, for a 100 feet fall 20 d6 damage would be applied to the ship and 9d6 of damage to the NPC (minus 8 pts of hardness).
While this seems reasonable, I am still wondering: Are any of the figures given above limits inherent to the power or could you just use Metamorphosis to say "I turn into a mountain" and bury the ship? Or simply put: How big a rock can this character turn into using Metamorphosis?