How Calc approaches launching

General Launching Particulars

The draft and trim are calculated for each interval of travel, allowing for camber and the position of the ship relative to the ground ways. The draft and trim are then used to interrogate the geometry model to determine the hydrostatic particulars. The buoyancy, moments of buoyancy, moments of weight and pressure are computed allowing for stern lift. The process is repeated for each interval of travel until either the buoyancy is greater than the specified launch weight or the limit of the number of increments is reached.

When the moment of buoyancy about the fore-poppet exceeds the moment of weight about the fore-poppet, the ship is then balanced about the fore-poppet to give the draft at the AP.

Once the stern has lifted the pressures are no longer given and the program continues until the ship floats off or passes the way ends and drops off, that means, suddenly becomes unsupported. The free float drafts at the AP and FP are then calculated.

After the three tables of launching particulars have been produced, a series of parabolic interpolations are performed on the various curves to determine the stern lift characteristics, anti-tipping moments and any drop-off as follows:

• Stern lift travel.

• Loads on the fore-poppet at stern lift.

• Drafts at stern lift.

• Maximum tipping or minimum anti-tipping moment and corresponding travel. Maximum pressure on way ends at corresponding travel.

• Free float drafts.

• Drop-off at fore poppet.

The output is also presented in graphical format.

Dynamics of Launching

Using the components of weight and buoyancy with the specified frictional coefficients (water, grease and chain drag) in the equation of motion describing the vessel's progress down the ways, the speed at each interval of travel is determined. McNeil's energy method, together with the total distance travelled before stopping (that means, McNeil's general energy equation) is used to solve the equation of motion. The difference in speed between each interval of travel determines the acceleration and hence the time taken can be derived. The program terminates the calculation when the speed is zero. This is estimated by a parabolic fit through three values of velocity.

A table and graph is produced detailing the velocity, elapsed time and acceleration as the vessel progresses down the ways. The calculated and available travel before coming to rest and the maximum velocity attained during launch are also reported.

The pressure required to prevent the ship sticking is calculated using an empirical formula. First the static pressure is calculated as follows:

The pressure required to prevent the ship sticking is calculated using an empirical formula. First the static pressure is calculated as follows:

Static pressure = weight / (width of sliding ways length of sliding ways)

Where the length and width of the sliding ways relates to the area in contact. The minimum allowable pressure to ensure the ship does not stick is calculated using the following formula (for metric units):

Minimum allowable pressure = 0.3625D4 - 9.683D3 + 96.508D2 - 425.67D + 711.45

where D = 100 x tangent of declivity of ways at start, measured at midships (this is calculated by Calc from the slipway information supplied). If the static pressure is less than the minimum required pressure, a message is printed that states that the ship is likely to stick.