There is no direct connection between horsepower and pull. It depends on the propeller.

If you can imagine a "propeller" with the blades all aligned with the shaft (as if they were feathered), then turning the shaft would give no pull, irrespective of how fast it turned. But it would take a lot of power to turn it fast.

Now turn the blades to give them a slight twist, and you will get some pull. Turn them further, and the pull will increase. But as you continue to increase the twist the pull will reach a maximum, then start to decrease again until when the blades are roughly at rightangles to the shaft there will again be no pull.

But the angle giving the maximum pull will vary with the speed of the boat. A very fast boat will need a large pitch propeller, i.e. will need the blades at only a small angle to the shaft. A slow boat will need a low pitch propeller, one with the blades at a large angle to the shaft. Hence there is no direct connection between the engine horsepower and the bollard pull.