Sea trials are performed according to the contract between the yard and the owner but they are also performed as a part of the classification process. Representatives from the classification society (Lloyd's, ABS, Bureau Veritas...) will therefore be present. Many tests will concern safety equipment and compliance with different regulations as MCA, SOLAS and MARPOL (pollution prevention).
Because of the complexity and large number of subcontractors engaged in the project you will also find many participants at the sea trials coming from companies other than the yard. The engine supplier will have its people onboard at sea trials as well as the supplier of the IBS (integrated bridge systems), navigation equipment, fire protection systems, alarm, monitoring and control systems and so on.
Of course there are hundreds of tests to be performed on the new yacht. Many of the tests concern the onboard systems that now will be working together for the first time. I will not even try to enumerate these tests but just put the finger on three spectacular tests.
Top speed is of course contractual - it is therefore an important part of the sea trials. Normally it should be reached with a margin of one knot or so. In the case of more complex propulsion packages (gas turbines, waterjets, pods...) sea trials could reveal additional problems though. Therefore it is not unsual that the final sea trials will take place at a later date when additional work has been performed by the yard as a result of the first sea trials.
Another test is the "crash stop test" meaning that the ship will be making an emergency stop from full speed ahead by means of reversing the propellers (or by setting variable pitch propellers to reverse). The distance to full stop is part of the contract.
The manoeuvrability tests are also important - they test rudders, stearing gear, propellers, thrusters etc. A contractual test is the full turn at full speed where the turning radius is measured in order to verify compliance with the contract.
The controllability of the ship is furthermore verified at the "zig-zag tests".
As you will understand these tests call for a lot of free space and this means going "off the beaten track". Some parts of the North Sea are today quite crowded making them less suitable for sea trials.
If you want to know more about the theories behind the testing of ships there is an interesting article here:
Modified by Andy Lindy at 9:32 AM 7/9/2004