The situation contemplates engaging with a huge or gargantuan ship, by means such as an 18th level Rune Knight using the Runic Juggernaut feature as described in Tasha's Cauldron of Everything, pg 46 (also applicable for a large rowboat, in which case a ballista/harpoon as described in Storm King’s Thunder pg 133, similar to an Oaken Bolter’s harpoon attack as described in Mordenkainen's Tome of Foes pg 126).
I am assuming that a vehicle like a ship is not a creature, as that appears to be dealt with in other questions regarding the effects of spells that target creatures but not objects etc.
According to chapter 9 Combat, section Grappling, PHB pg 195, it states (italics mine):
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. //snip// The target of your grapple must be no more than one size larger than you and must be within your reach. //snip// A grappled creature can use its action to escape.
Further according to Appendix A: Conditions, PHB pg 290 it states (italics mine):
Conditions alter a creature’s capabilities in a variety of ways and can arise as a result of a spell, a class feature, a monster’s attack, or other effect.
According to chapter 9 Combat, section Making an Attack PHB pg 192, it states (italics mine):
- Choosing a target. Pick a target within your attack’s range: a creature, an object, or a location.
According to Objects section, DMG pg 246, it states:
For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
According to Appendix A: Of Ships and the Sea, section Components, in Ghosts of Saltmarsh (“GoS”) pg 186 states (italics mine):
A ship is composed of different components, each of which comprises multiple objects.
In all the example stat blocks of ships in GoS, despite all of the examples being immune to most condtions, none of them have immunity to the grappled condition (or the restrained condition, for that matter).
According to the description of a Kraken’s Tentacle attack MM pg 197, it states (italics mine):
Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained.
Which is different from a infernal war machine Scavenger’s Grappling Claw, which according to its stat block Descent into Avernus pg 219 it states (italics mine):
Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: The target is grappled (escape DC 12). If the target is a creature, it is restrained until the grapple ends.
It appears that an attack would (presumably) be targeting the hull component of a ship, either as (the equivalent of) an object, or a location, however it is unclear which.
It also appears that while the rules for grappling in PHB refer to targeting only creatures, the rules elsewhere strongly imply that items other than creatures can be grappled.
It is not clear whether the rules elsewhere (eg. Kraken, Scavenger, harpoon) can only be used by non player characters, or whether a player character wielding a harpoon/ballista against a large (possibly huge?) ship, or a huge Rune Knight against a huge or gargantuan ship, can make use of the grappling rules. Like this.
Accordingly, can a player character grapple a ship?