You have two objects within three boat lengths of each other. The sunken marine hazard and the permanent red channel mark.

We all seem to agree that the sunken marine hazard is an obstruction and it is not a mark or another boat overlapped with each of them. So rule19 and rule 20 apply between two boats when they are at the sunken marine hazard.

The permanent red channel mark is an obstruction and a mark. You can sail around the mark without leaving the zone. Does this make it a starting mark surrounded by navigable water?
 

 

If we remove the permanent red channel mark and just consider the sunken marine hazard.  Why does Yellow not have to tack?

Yellow is on starboard tack. Blue is overlapped to leeward and will need to make a substantial course change to avoid the sunken marine hazard safely. Blue hails for 'room to tack' under rule 20.1(a). Yellow does not respond either by tacking as soon as possible, or by immediately replying ‘You tack’ and then giving the hailing boat room to tack and avoid her.

Yellow failed to tack as soon as possible after Blue hailed "Room to tack", as required by RRS 20.2(b) and (c).

Yellow DSQ

Welcome Charlotte!  Happy to have your contribution. 

Ang

As a retired IJ and IU my take is that Blue, being a fast reaching multi-hull, is on the cusp of breaking rule 17 at position 6.

I think that wording is quite workable, if that's what you want to do.

Rule 89.2 provides the option to change the NoR provided adequate notice is given.

As brought out by John d Farris, any change needs to be communicated to all stakeholders.