Final answer:
To avoid cracking, the metal must be cooled at a rate not exceeding 11 e3 degrees per second by using methods that provide regulated cooling, such as controlled water baths or gradual air-cooling, capitalizing on water's high specific heat capacity.
Step-by-step explanation:
If a hot piece of metal is cooled too rapidly, it risks cracking due to thermal stresses caused by differential contraction. The challenge when cooling metal objects, like the hull mentioned in the initial question, is to reduce their temperature at a rate that doesn't induce structural weakness or failure.
The set of possible cooling directions refers to the approaches or methods used to decrease the temperature. To cool the metal at a safe rate and avoid cracking, one may proceed with cooling methods that control temperature reduction, such as providing a regulated environment or using a medium that absorbs heat gently from the metal. The medium used for cooling should have sufficient specific heat capacity and thermal conductivity to draw heat away at the required slow rate.
For instance, submerging the metal in water with a controlled temperature descent or using a gradual air-cooling process can dissipate heat efficiently without exceeding the 11 e3 degrees per second limitation. Using water is effective since water's high specific heat capacity allows it to absorb a large amount of heat with only a small change in temperature, reducing the risk of inducing thermal stress in the metal.