—THE CORRECT QUESTION TO ASK AND ANSWER—
The question is: “whether the defect causing unseaworthiness, was of so latent a nature that due diligence could not discover it?”
This duty (to exercise due dilligence) is non delegable and even if Shipowners appointed reputable surveyors (ie an agent), Shipowners would still be deem “at fault” if the Surveyors did not do a good job in detecting the defect.
For such vessel “latent defect” (it is submitted this is akin to H/HV 4(2)(p) defence for cargo claims) argument to be successfully argued, Shipowners have to prove that due diligence checks would not discovered the defects.
Cargo interests (attempting to establish claims against shipowners) would be keen to prove that the performance of the “what if” checks would have detected the defect. Cargo interests would also be keen to prove as many “what if” checks should have been deployed by a reasonably due diligent shipowner.
It is for the Shipowners to answer the question (of whether due diligence have been exercised) and prove that even if the “what if” checks have been performed, the defect would not have been detected. If this is not shown by Shipowners, due diligence is then not proven to have been exercised by them (or their agents or servants).
—POINTERS FOR SHIPOWNERS—
Even if your employees (ie servant) or surveyor (ie agent) failed to do certain steps while “checking” the vessel, focus on building up the latent defect defence (ie the due diligence defence) by building up expert evidence that EVEN IF those “what if” steps were taken, the defect is so latent that you will be “forgiven”.
NB: it is altogether another issue of what the “what if” steps could be, but the standard is NOT of a meticulous man, but of one who is reasonably prudent.
Thus, the extent of checks would depend on whether there were reasons to do additional checks. It is a matter for a technical expert to comment on but if the additional steps could have been taken without considerable costs and time, then it is better to err on the safe side and conduct the additional but not that troublesome steps.