The harpax was a ballista-launched grappling device, not an ordinary hand-thrown hook. Its projectile carried a line that could catch an opposing vessel and help pull the ships together. The concept resembles a more elaborate corvus because both systems convert a naval engagement into a boarding action. The weapon suited a Roman preference for bringing infantry-style combat onto the enemy's deck. A later clarification distinguishes the large bolt-like harpax from a conventional grapnel.
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The Roman harpax as a stand-off boarding weapon
A discussion of the harpax shows how Roman forces used a projectile and cable to turn a sea fight into the close combat they preferred.