Craft the island sharts - server/client communication

Hi, this is my first Devlog at itchi.io :)

several months ago  I start to work on Shardcraft. Front end is made with unity and the back end server is written in     node.js. Shardcraft should be in the end a MMO-RTS, so server communication is very important and it is also important that this communication is as efficient as possible. Till now any movement is calculated on the server and just shared with the clients.

For catching the island shards I use Unitys physic engine (catching a shard is creating a Rigidbody  on the shard  and it is connected with the  ship by a physical simulated chain. Catching shards is very important, because this is the way how the player could growing his island.

At the client it looks nice for my first Unity physic engine experiment ;) But I'm not sure how to  share the physical simulated position of the shard.

• I could just share the start condition, the rest is done by the client physics engine - but this is an chaotic system, so I need to synchronize it.
• I just run the physic on the "master" client, which has catch the shard and share the position of the shard  by the server -> lot of traffic
• I could share the start condition and do an update when an other user client is next to this scene and then the other client should run his own physic simulation and I just share all view seconds an update of the position 'master' client  -> server -> 'slave' client => less traffic but most complex 

At the attached files you could see at one picture a ship connected to an island shard and at the other picture a ship without a shard.