The reason they sunk on d day was because they were swamped by the waves.
They were launched too early becasue the boat captain lost his bottle,so were too far from shore.Then the crews through no fault of there own followed orders and instead of going with the tidle flow and landing up the beach they kept heading for the point were they were orderd to land.
In doing so the crews headed across the waves and got swamped,the poor buggers drowned in there tanks when if they had gone with the tide they would have landed further up the beach.
The takns worked ok on the other beaches with very little losses to sinking