Stern rake doesn´t seem weak to me. Pehraps a matter of opinion or personal interests but to me it may even seem a bit too powerful. A bad angle and it may seem weak, as it should but in a good angle, to me it seems that sometimes it is too powerful. A large ship can measure about 40 or 50 meters and if i am not mistaken, i saw rakes where at the distance of about 2 large vessels, a rake withy grape can be devastating in game.
I do believe grape should not be so effective at 100 mts. at less than 50mts i would think it would be a absolute nightmare to the enemy deck, if marines were getting ready to board.
Also, pehraps a 5th rate rake should not be so effective to a SOL as it is in game at present time. A rake made by a SOL to another SOL at short range could be devastating and well... A SOL against a 5th rate would only need a broadside to be devastating.
As for the moral, absoluly a good sugestion. I would love to see it.
To sink the vessel, i also agree with your sugestion, leaks (enough to win the pumps) and boarding then deciding to scuttle. I would add fire (out of control and maybe the explosion).