From the first sign it may appear that, if damages for detention are calculated at the demurrage rate for the period of delay, then there is little difference between demurrage and damages for detention.
In Harris and Dixon v Marcus Jacobs & Co. (1885) 15 Q.B.D. 247 Sir William Brett, M.R. said at p. 250:
In order to determine this case one must see whether the charterers would be liable if they were in the position of these defendants. That would depend on the construction of the charterparty, and in whose favour the word "ready," which is an express word, was inserted there before the words "quay berth as ordered by charterers." It seems to me that the stipulation that the vessel should proceed "to such quay berth as ordered by the charterers," was one which would be in favour of the charterers, and that when the word "ready" was inserted before "quay berth," that must be in favour of the shipowners, in order that the ship should not be kept waiting until a quay berth was ready. The meaning of the charterparty is, I think, that the charterers undertake to order the ship to go to such dock and to such quay berth there as they may wish, all of which is for their benefit, but with this stipulation in favour of the shipowners, that it shall be to a quay berth which is ready. That being so the charterers would be bound to name a quay berth which was ready, and there was a default on their part in the present case as the quay berth was not ready for the vessel.
Here, I think that another meaning can reasonably be attributed to the words "on her arrival", and I have little doubt but that that meaning reflects the true intention of the parties, and, indeed, the true meaning of the words in that context. The parties, in using the words "on her arrival", did not have in mind, or at least did not have solely and exclusively in mind, the technical meaning of "arrival" in respect of an "arrived vessel" in a port charter-party: they had in mind her physical arrival at the point, wherever it might be, whether within or outside the fiscal or commercial limits of the port, where the indication or nomination of a particular loading place would become relevant if the vessel were to be able to proceed without being held up. At that point the charterers had to nominate a reachable place, which involves that it was the charterers' responsibility to ensure that there was at that point of time a berth which the vessel, proceeding normally, would be able to reach and occupy. The time of her arrival, within Clause 6, had come when the vessel had gone as far as she could go, whether to the verge of or within the port, in the absence of a nomination by the charterers of a place, which she could reach without being held up, where she could load.
PEr Lord Roskill in Nereide SpA di Navigazione v Bulk Oil International Ltd (The Laura Prima)  3 All ER 737,  1 Lloyd’s Rep 1
The berth is required to have two characteristics: it has to be safe and it has also to be reachable on arrival.