This chapter has 22 paragraphs on the details of clause 21 of Supplytime 2017, which deals with the owners’ right to put in a substitute for the original chartered vessel.
This right is subject to the charterers’ consent “which shall not be unreasonably withheld”, and the chapter then has 76 paragraphs discussing the effect of these words, and of wordings which require consent without “not to be unreasonably withheld”. This discussion sets about extracting from the caselaw (which is large) substantive principles as to when it is unreasonable to withhold consent that will apply not only in relation to Supplytime, but whenever “not to be unreasonably withheld” is used in any commercial contract.
The discussion also defends the project of looking for general principles about when refusal is unreasonable against dicta of Lord Briggs in Sequent v Hautford  UKSC 47, both by examining the actual reasoning in Sequent and by demonstrating that it is possible to find and state general principles which are useful.
The principles extracted from the caselaw are applied to Supplytime (in particular to when the charterers can refuse consent for the owners to take the vessel off to make an attempt at salvage, and to whether the charterers have to consent to a substitute vessel at a time when they are seeking to terminate the Supplytime charter). However, I believe that the discussion will also be useful generally.