If a player is steering a boat, a second passenger will only rotate client-side.
- Use the following command
- Run the /entitydata command on the armor stand and have a look at its Rotation, it should be [0.0f,0.0f]
- Get in the boat and turn only very slightly
- Run the /entitydata command on the armor stand and have a look at its Rotation
→ It is still [0.0f,0.0f]
The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.
This happens because the method net.minecraft.entity.item.EntityBoat.controlBoat() which modifies the net.minecraft.entity.item.EntityBoat.deltaRotation value is only called client-side. It might make more sense for not steering passengers to use the rotation of the boat.
Note: At some point the method net.minecraft.entity.item.EntityBoat.applyYawToEntity(Entity) corrects the value.