The rules do NOT prohibit it. If you want to prohibit a change of crew during a series (including the sole crew of a single-handed boat) then you have to say so in the NoR.

See for example the NoR for the 2024 ILCA 7 Men's World Championship at https://2024ilca7men.ilca-worlds.org/wp-content/uploads/sites/19/2023/12/NOTICE-OF-RACE-ILCA-7-Men-2024-Amend-3.pdf which states:

1.3 ILCA class rule 7(a) is changed as follows: “Only one person (the registered competitor) shall be on board while racing.”

and

4.4 Crew substitution of the registered competitor is not permitted.

One additional thing to check to resolve this. Many boats are governed by Class Rules. Those Rules are rules for the Regatta if included in the Notice of Race (preferable) or Sailing Instructions as rules. This almost always limits single-handed boats to one person aboard at the time. However, it may not place a limitation on the substitution of crew during a regatta. Obviously, one would be checking the Class Rules for issues like this prior to making those Class Rules into rules.

I agree, the rules don't prohibit it.  The rules are that it is the boat that is entered in the regatta and not the competitor.

We had this same discussion for RC sailing.  Some development classes allow you to change boats (just once!) but not skippers, i.e. it is the competitor that is entered in the regatta and not the boat.  For others it is the boat and not the competitor that is entered,  And for some it is both the competitor and boat.

If you want something other than what the rules define then it needs to be defined in the class rules or the NoR and entry form.

Graham is correct here, rules don’t prohibit. Also, don’t assume class rules can solve this issue. Graham’s pointed out the wording in the ILCA Worlds to specifically cover this case. The reason is the class rules cannot in fact modify the rules to do this. This is why the Worlds had to specifically call this out in their NOR.

The Class Rules are always in effect:  see Definition of Rule and Rules(s) may not be changed or written out (see Rule 86).

They should not be invoked in the NOR or SIs as that is redundant.

Yes, class rules are always in effect, though you do need to call out in your NOR what class(es) are racing. For example, a one design boat (say a J/24) racing under a handicap system uses that handicapping system's class rules.

In any case, that wasn't the issue here. Class rules may only change RRS 42, 49, 51, 52, 53, 54 and 55, they cannot change any other rule. That means they can't change any of the rules (say 75) to change a "boat" to a "competitor". The ILCA Worlds took an interesting approach here in the NOR. ILCA class rule 7a) describes how many people can sail on a boat (it can be two!), and requirements around that. The NOR changes that to one person can be board, and to restrict "crew substitutions". It's a neat way to get around the restrictions that class rules have with that wording.

Tom re: "...a one design boat (say a J/24) racing under a handicap system uses that handicapping system's class rules. ....In any case, that wasn't the issue here. Class rules may only change RRS 42, 49, 51, 52, 53, 54 and 55, they cannot change any other rule. That means they can't change any of the rules (say 75) to change a "boat" to a "competitor" 

FWIW .. we had a thread (below) which dove into the limits of CR's and (possibly) unintended consequences when handicap systems put in their language that OD boats 'shall comply with their OD CR's' when being rated based on their OD configuration.  (It's a long thread ... and the discussion turns to the above issue about 1/2 through)