Research on various forums, newsgroups etc. has indicated the article Eight New Weapons for RuneQuest by Paul Cardwell Jr on p.32-35 in issue No.22 of Different Worlds magazine (July 1982) as the preferred reference for the whip's weapon stats.
Since the topic keeps appearing on forums, I have taken the liberty of copying the most salient parts of the ‘whip’ section from that article here. I will obviously remove this post should anybody complain about copyright infringements but –hey!– the original article is from 38 years ago; we can almost consider this post as an ‘RPG history’ article of sorts.
The whip is a logical part of the gear of a herder, teamster, caravaneer, slave merchant, or even a general, all-purpose villain.
The whip is a weapon of fixed range. It is useless beyond its range, and it can only be used to attack a target at equal to or more than half its maximum range. Within that range, the whip is useless.
The whip will wrap around its target on any Special to hit roll. A wrapped victim cannot act. If a leg is caught, the target must roll DEX as a percentage to keep from falling.
The whip will loosen after one round.