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In a campaign I am starting I want to make use of an encounter where the players could lose a device that will make it easier for them to pass a puzzle later on. The idea is the following:
In order to reach a set of caves the players must cross an alpine lake by boat. On their way, they find themselves in a storm. The storm will do a certain amount of damage each turn. The boat will also be attacked by some Merfolk. The idea is that when the players kill the Merfolk, the storm dissipates. If they fail to do so before the boat is destroyed, the Merfolk let them be, and the players must swim to shore. This means they will lose the heavy stone artifact.
I want the encounter to be such that the players have a reasonable chance of beating the Merfolk before the boat is destroyed, but there should also be a good chance of them not making it. In order to do so, I have to have an idea about the number of turns it will likely take, such that I have an idea as to how much HP the boat should have (or alternatively, how much damage die the storm should roll every round), for there to be a good possibility of both.
How would one get an idea about the number of rounds? Are there any tricks/methods that give you an idea of this?