Harpoons work better if you go with either a Wound Point system, or use something similar.
Actual Injuries (optional rule):
Whenever a character takes damage from an attack, and his resulting hit point total is less than his Constitution score, the attack is a solid strike, delivering a considerable bodily injury. While normal hit point damage might simply represent scrapes, bruises, and tiny cuts, this type of attack leaves broken bones, bleeding gashes, or gaping wounds. Normally this has no mechanical effect beyond the normal effect of the damage. However, for some weapons, this Wound Damage is where a creature is vulnerable to special attacks.
Harpoon:
Dmg: 1d10, Rng Incrmnt: 10 ft., Crit: 20/x2.
When using the optional Actual Injuries rule, whenever a harpoon does Wound Damage to a creature, or whenever the harpoon strikes a critical hit, the target must succeed a Reflex save (DC 15) or be impaled on the harpoon. As long as the attacker maintains a hold on a rope or chain attached to the harpoon, an impaled creature functions similar to a grappled creature. It must succeed an opposed grapple check to move away from the creature holding the harpoon line, unless the wielder chooses to let the impaled creature move freely.
The creature wielding the harpoon with a tow line can perform a specialized Bull Rush maneuver from range. If he succeeds in the Bull Rush attempt, the impaled creature moves toward the harpoon-wielder in the most direct path possible, a distance equal 1 ft. for every point the harpoon-wielder won the opposed Strength check by, minimum 5 ft. The impaled creature must succeed a Reflex save or Balance check (DC 10) or fall prone when moved in this way.
The impaled creature may attempt to control his foe's movement in the same ways as above, but the harpoon wielder can choose to simply release the tow line. A rope tow line has hardness 2, 5 hp. A chain tow line has hardness 10, 10 hp. A magical harpoon's magical enhancements to hardness and hp also apply to the weapon's tow line, if they were enchanted together.
Removing the harpoon requires a Heal check (DC 5), which takes a move action, and deals an addtional 1 damage to the impaled creature. If the check succeeds by 5 or more, removing the spear deals no extra damage.
No clear rules can be applied to the disruption a long rope may cause by crossing a battlefield, but players and GM should be aware of the hazard caused by a tow line.