Many shipyards feel it difficult to choose marine airbags. Especially some shipyards that have just started to use airbags. In fact, the selection of ship launching airbags is not complicated, just simply consider the following aspects.

## How to select the air bags diameter?

The common air bags diameter include 0.8m, 1.0m, 1.2m, 1.5m, 1.8m, and 2.0m. Choosing the right airbag diameter can not only launch/landing safely, but also save costs. Normally, please remember two principles.

- Airbag diameter should be bigger than heights of keel blocks under the ships.
- If smaller diameter airbags are enough for the project, no need to select bigger airbags.

If the blocks are less than 1m high, can select 1.2 or 1.5m diameter airbags. If the blocks are about 1.2~1.5m high, can select 1.8m or 2m diameter airbags.

## How to select the air bags length?

The common airbag length is from 5m to 25m. To select the suitable airbags length, have to consider how to arrange airbags when using the end launching type. There are three ways to arrange air bags. They are (1) linear arrangement, (2) staggered arrangement, and (3) two-lines arrangement. As for what length airbags to use, it will depend on the ship’s width and the arrangement way.

- When the ship’s width is not great than 25m and select the linear arrangement, the length of airbags should be longer than vessel width.
- When the ship’s width is greater than 25m and select the staggered arrangement, can divide the ship’s width by 2 and plus 1-2 meters to decide airbag length.
- When the ship’s width is greater than the combined effective length of two air bags (25m/each airbags), or for special ship such as catamaran HSC or split hopper barge, the two lines arrangement shall be selected.

## How to select the numbers of airbag layers?

The common airbag layers is from 4 layers to 12 layers. The most common is 6 layers air bags that are suitable for most of ship launching/docking projects.

The user can determine the lifting force of each airbag according to the weight of the ship and the number of airbags required. In order to calculate the appropriate number of airbag layers. However, in actual operation, we often calculate the number of airbags required according to the jacking force of the commonly used airbag layers (such as 6 layers, 7 layers, and 8 layers). The two are interdependent and mutually check calculations.

When the bottom area of the ship or structure to be moved or launched is limited, consider using more layers of high-pressure airbags. As the number of layers increases, the performance and working pressure of the airbag will increase, but not proportionally. The balance of payments should be considered.

Increasing the number of airbags is better than buying airbags with more layers. Using airbags correctly is more effective than buying airbags with more layers.

## How to select the air bags quantity?

How to decide airbags quantities for a ship launching or hauling project is not difficult. According to the weight of the ship being launched, the quantity of the air bags needed for this operation must be calculated in accordance with Formula (1):

where

N is the quantity of air bags used for ship launching.

K_{1} is a coefficient, in general, K_{1} ≥ 1,2;

Q is the weight of the ship (ton);

g is acceleration of gravity (m/s2), g = 9,8;

C_{b} is the block coefficient of the ship being launched;

R is the allowable unit bearing capacity of the air bags (kN/m);

L_{d} is the contact length between the bottom of the ship and the body of the air bag at the midship section (m).

For ship shifting, 2 to 4 additional air bags shall be made ready and available.

The centre to centre distance between two neighbouring air bags should be less than or equal to that found in Formula (2) and equal to or be greater than that found in Formula (3).

where

L is the actual length of the ship bottom that can make contact with the air bags (m);

N is the quantity of air bags used for ship launching;

k is a coefficient, k = 1 for steel ships, k = 0,8 for wooden, aluminium and glass-fibre-reinforced ships;

D is the nominal diameter of air bags (m).

Different types of vessel may need to adjust the airbag quantity according to practical conditions. It is better to consult with professional marine engineers.