The Quantity of Airbags Used for Ship Launching

The airbag bearing capacity should be adapted to the requirement of the weight of ship launching to guarantee safety during launching. The launching calculation must be conducted before the ship launching using airbags, so as to calculate the requirement of total airbag bearing capacity in accordance with the weight of ship launching. Then the number of airbags will be decided and the type code of airbag will be selected according to the area of the ship’s bottom.

The Type of Airbags Suggested

Reference to select the type code of airbags in accordance with ship launching weight.

The Quantity of Airbags Used for Ship Launching

According to the weight of the ship being launched, the quantity of the airbags needed for this operation shall be calculated in accordance with Formula (1).

N = K₁ x (Q_g/C_b x R x L_d) ——— (1)

where
N is the quantity of airbags used for ship launching;
K₁ is a coefficient, in general, K1≥1.2；
Q is the weight of the ship (ton);
L is the contact length between the ship bottom and the body of the airbag at the midship section (m)
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 airbags (kN/m);
L_d is the contact length between the ship bottom and the body of the airbag at the midship section (m)

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

For class ships that are more than 5 000 tons in weight, or more than 150 m in length (LOA)., prior to launching, calculations shall be made for transferring from docking blocks. The product of the bearing load of the airbag times the distance between the airbag and the ship’s longitudinal center of gravity shall be less than 1 % of the product of the ship’s launching weight times the distance between the perpendiculars.

The Quantity of Airbags Used for Ship Launching

The center-to-center distance between two neighboring airbags shall be less than or equal to that found in Formula (2) and equal to or greater than that found in Formula (3).

L/(N – 1) ≤ 6k ——— (2)
L/(N – 1) ≥ πD/2 + 0,3 ——— (3)

where
L is the actual length of the ship bottom that can make contact with the airbags (m);
N is the number of airbags used for ship launching;
k is a coefficient, K=1 for steel ships, k=0.8 for wooden, aluminum, and glass-fiber-reinforced ships;
D is the nominal diameter of airbags (m)

In general, the long axes of the airbags shall be arranged perpendicular to the direction of the ship’s
movement. When it is necessary to move a ship in a curved manner, the long axes of the airbags shall be
arranged perpendicular to the direction of the tangent line to the curve.