Drilling under high heads of water creates different parameters concerning pressure requirements. When a hammer is operating under a head of water sufficient pressure is needed to unload the hole. This is often referred to as "peak unloading pressure" and is calculated by the depth of the water in the hole.

One foot of water in the hole is equal to .434 psi (.03 bar). Therefore, if there is 100 feet (30.5 m) of water in the hole there will be 43.4 psi (3 bar) of back pressure.

Once the head of water is broken, the pressure will drop to the drill operating pressure. If the influx of water into the hole is great, the drill operating pressure will increase and the drill performance will be reduced. In some applications it may be necessary to utilize a high pressure booster compressor to continue drilling.