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When you board an Ark, you cooperate with the Ark server to sign a pair of on-chain/off-chain transactions that produces a board VTXO representing your bitcoin balance within Ark. Once broadcast and confirmed, you can start making instant, low-cost bitcoin payments over Ark and Lightning.
Most users won’t need to boardYou won’t necessarily need to board to start using Ark. After installing an Ark wallet, you can begin receiving VTXOs immediately from friends, customers, or exchanges over Lightning or Ark. Boarding is expected to be reserved to businesses and power users.

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The boarding process

The boarding process follows these steps:
  1. Create transactions: The user constructs a pair of transactions with special spending conditions cooperatively with the Ark server: a funding transaction and an exit transaction.
  2. Pre-sign exit transaction: Both parties pre-sign the exit transaction, which spends from the funding transaction’s output. This ensures the user can always reclaim their bitcoin on-chain if needed, even without the server’s cooperation. Learn more about emergency exits.
  3. Broadcast: The user broadcasts the funding transaction on-chain, and stores the pre-signed exit transaction off-chain.
  4. Wait for confirmations: The user waits for the funding transaction to receive six confirmations.
  5. VTXO activation: After six confirmations, the user’s board VTXO becomes active and spendable within the Ark server. Their on-chain bitcoin has been moved “off-chain”.
This process happens outside of the server’s normal round schedule—users don’t need to wait for a round to initiate or complete their boarding.
Diagram showing how the board process relates to a board VTXO

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Atomic boarding guarantee

The boarding transaction is atomic—either both the on-chain funding and VTXO creation succeed together, or both fail together. There’s no intermediate state where the server could hold the user’s bitcoin without the user receiving a valid VTXO in return. The transaction structure itself achieves this atomicity: the funding transaction that spends the user’s on-chain bitcoin also creates the VTXO tree that grants the user emergency exit rights.

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Boarding fees

Users face two cost components when boarding:
  • On-chain funding fee: Users will pay standard bitcoin network transaction fees to broadcast the boarding transaction. These fees fluctuate based on blockspace demand and are paid to the bitcoin network, not the Ark server.
  • Future on-chain sweep fee: Each board VTXO will inevitably require the Ark server to make another on-chain transaction to sweep after expiry, and the Ark server will need to make sure this future cost is covered.
  • Ark server fees: The Ark server may charge additional fees to cover general operational costs.
Unlike refreshes, boarding doesn’t require the Ark server to deploy liquidity since users provide the bitcoin upfront through the funding transaction, therefore there are no liquidity costs. Second’s fee schedules are still in development.