You are correct, both boats were on a beat (the shoal ran parallel to the port layline). The shoal was shallower than what the Red boat's draft requirements so it was a continuing obstruction (rule 18 does apply). Red had two avenue's available to them to avoid the blue boat - Duck them or slow down. They could not tack due to the water depth. Since they could not tack because of an obstruction, does that mean that rule 14 does not apply?