Blowing ballasttanksquestion

Shirkon · 07-27-07, 07:56 PM

Being an ex-bubblehead I can answer this one. First, as someone else stated the MBT's do have permanently open flood grates on the bottom of the tanks. In the top of the tanks are placed valves (MBT vents) which when opened allow the air in the tanks to escape while water enters through the open flood grates. Once submerged the MBT vents are shut so that in an emergency air can be blown into the tanks immediately. When you are going to surface, the MBT vents on the top of the tanks are checked SHUT and then once the ship is forced high enough so that the top of the Air Induction/Snorkel Mast is out of the water the Low Pressure Blower is started forcing a large volume of air at low pressure though air lines into the top of the MBT's which forces the water back out through the open flood grates. It is normal when doing the LP Blow that air will spill out of the flood grates which is about the only real way you can tell that the MBT's are actually empty. It is the diving planes and the propellers that force the boat high enough to start the blower. Only in an Emergency Surface would High Pressure air from air flasks (normally at 4500 psi) be emptied into the MBT's since refilling those flask is a long time consuming process for the equipment available on the WW II boats.

To get an idea of how the MBT's work, take a styrofoam cup and cut a small hole on the bottom, turn it upside down and place one finger over the hole, then lower it into a sink full of water. The air trapped in the cup cannot escape until you lift your finger off the hole and then the air in the cup will bubble out and water will enter the cup from below.

07-27-07, 07:56 PM	#6
Shirkon Swabbie Join Date: Jul 2007 Posts: 7 Downloads: 54 Uploads: 0	Being an ex-bubblehead I can answer this one. First, as someone else stated the MBT's do have permanently open flood grates on the bottom of the tanks. In the top of the tanks are placed valves (MBT vents) which when opened allow the air in the tanks to escape while water enters through the open flood grates. Once submerged the MBT vents are shut so that in an emergency air can be blown into the tanks immediately. When you are going to surface, the MBT vents on the top of the tanks are checked SHUT and then once the ship is forced high enough so that the top of the Air Induction/Snorkel Mast is out of the water the Low Pressure Blower is started forcing a large volume of air at low pressure though air lines into the top of the MBT's which forces the water back out through the open flood grates. It is normal when doing the LP Blow that air will spill out of the flood grates which is about the only real way you can tell that the MBT's are actually empty. It is the diving planes and the propellers that force the boat high enough to start the blower. Only in an Emergency Surface would High Pressure air from air flasks (normally at 4500 psi) be emptied into the MBT's since refilling those flask is a long time consuming process for the equipment available on the WW II boats. To get an idea of how the MBT's work, take a styrofoam cup and cut a small hole on the bottom, turn it upside down and place one finger over the hole, then lower it into a sink full of water. The air trapped in the cup cannot escape until you lift your finger off the hole and then the air in the cup will bubble out and water will enter the cup from below.