Demurrage and detention both involve delay, but they are not interchangeable. One is calculated, the other must be proved, and the difference decides who ends up paying.
Demurrage is the agreed rate payable when laytime is exceeded during cargo operations. The charterparty fixes the relevant time allowance and the price of exceeding it.
The basic calculation is contractual: identify the allowed laytime, establish when it started and stopped, and apply the agreed demurrage rate. The owner does not have to prove the extent of its loss.
Detention is different. It is a claim for damages arising from a separate breach that caused the vessel to wait.
There is no automatic entitlement to detention merely because the vessel was delayed. The claimant must establish the contractual obligation that was breached, how that breach caused the delay, and the resulting loss.
Demurrage is calculated. Detention must be proved.
In The Timna, the charterer failed to nominate a discharge port. The vessel was detained as a result, and damages for detention were recoverable.
Nolisement illustrates the other side of the distinction. Once cargo operations had been completed, unused laytime could not be treated as additional time for producing documents. Laytime belonged to the cargo operations regime, not to every subsequent source of delay.
A charterparty and a sale contract are separate contracts and may allocate the consequences of delay differently.
A clause requiring payment of demurrage as per charterparty does not automatically pass through every claim the owner may have against the charterer. In particular, it does not necessarily make the buyer liable for detention or for losses arising from a different breach.
If detention is claimed under a sale contract, ask what obligation was breached, why that breach made the vessel wait, where the contract says that this loss is payable by the buyer or seller, and how the amount has been calculated and proved.
Without the chain of breach, causation and amount, detention is only a label on an invoice.
If delay risk is intended to pass through the contractual chain, the charterparty and the sale contract should mirror each other as closely as possible.
Define which delays are covered, who bears them, how liability is calculated and what evidence is required. Check the time limits separately: a 20 or 30 day deadline for a demurrage claim does not automatically govern a detention claim.
Demurrage is the agreed price of exceeding laytime. Detention is damages for a separate breach, and it must be proved. Before paying either, establish which contract the claim arises under and why it should land on you.