Using marine airbags to haul out vessels can overcome the limitations of instruments and slipways and make haul-out work easy. The haul capacity of this system is decided by the pull winch and slope angle of slipway or ramp so it is easy to get improved. Generally speaking up to 4,000 tons vessel can safely be hauled out using airbags.
The basic principle of airbags ship launching or haul-out is changing the force of sliding friction into rolling friction, just like ancient Egyptian using logs to transport heavies. The rolling friction coefficient of airbags and ground is decided by the ground situation and airbags’ bearing forces. People can use 0.006-0.01 as the estimate figure if the ramp is concrete.
The essential instruments of airbag haul-out system include winches, rigging and marine airbags. To complete a successful haul-out project the following conditions should be satisfied.
1.Enough pulling capacity of winch. Normally the maximum pull force should be 1.5-2 times of the calculated result.
2.Appropriated ramp. It is easy to understand the angle of slope decides the needed pull force. The bearing capacity of ramp must be higher than the maximum inner pressure of airbags. For big vessel, the professional designed concrete slipway is needed. Generally airbag haul-out slipway gets one underwater part which has bigger slope and another shore part with smaller slope.
3.Suitable airbags. Airbags with 1 meter diameter are most often chosen since working heights of airbags for haul out usage are about 30 centimeters. And for bow lifting, shorter airbags are convenient to use.
4.Good water level. When the bow is lifted by inflated airbags, the stern should not contact the seabed unless it will be difficult to pull the ship out of water.
5.Clearance of shucks and anything else under vessel’s bottom that will damage airbags or baffle the rolling of airbags.
Vessel haul-out procedure
1. Fasten the vessel with guy ropes or cables attached firmly with the winch. Generally ship will need to be controlled by 4 ropes from 4 directions to ensure its precise landing spot.
2. Select a reasonable position near the bottom of bow to insert marine airbags which will act as the fulcrum for winch to move the ship. If it is difficult to insert airbags at the bottom of bow, people may insert them from one side to another side. For heavy ship the quantity of bow lifting airbags should be added until they can lift the bow. For V shape bow vessel the bow lifting marine airbags should be inserted at the position where they can have bigger contact area with the bottom of vessel. Also, people may place airbags at appropriate positions at low tide and position the vessel upon marine airbags at high tide, and then inflate airbags to lift the vessel. People should make the vessel haul-out plan according to the practical situation.
3. After the bow lifted, start the winch to pull ship towards lands and insert more marine airbags to the space between ship bottom and land until the full ship is pull upon the slipway.
4. Pump marine airbags further to lift ship bottom higher, then insert the wooden stands and withdraw marine airbags.
5. There are some artifices to overcome difficulties during the haul out process. To lift the bow by airbags at the beginning people can build sand hillocks to make airbags get more contact areas with the bow. And wedges may be used to help insert airbags. When the ship is tilted athwart ships, people may adjust airbags pressures to balance the ship. If the ship’s movement is deflected from center line, people can adjust the new inserted airbags’ directions to make ship back to center line.