When the upstream end (stern) of the boat goes under water, it fills the entire boat up within seconds.

The stern gets underwater when the weight in the back, current, and angle of the rope add up to more downward force than there is floatation force pushing upwards.

The shorter your anchor rope, the more the force pulls straight down, as the "downward force" is coming directly at the angle of the anchor rope.

The faster the current, the more force that is being exerted down the anchor rope angle.

The more weight in the back of the boat, the less force that is required to pull the stern under.

Can't really math it out, per se, as there are a lot of variables within those three main ones...but the moral is to not overweight your boat, don't anchor in heavy water, and more anchor rope will lead to less downward force on your boat.

Your anchor does not have to be stuck for any of this to happen, but if it is stuck you are in a much worse position...you can actually pull yourself underwater by pulling up on the anchor rope...anchor doesn't move, but boat gets pulled down.

Fish on...

Todd