Logically, the weight of the metal armor is not the reason one's running speed decreases while wearing it. If weight were the cause, one's speed would also decrease if one were carrying it.

The speed decrease must be a simulation of either the increased bulk or lack of flexibility of metal armor compared to troll, leather, and rag armors.

Metal armor has no effect on the speed with which a viking can swing an axe, pickaxe, or a melee weapon, or block with a shield, or craft items. It only effects a viking's running speed.
Since a viking cannot run and use a ship's tiller at the same time, the metal armor speed penalty cannot apply to ship speed.