What if a sailboat is is flying a spinnaker DDW and you're on a starboard tack 90 degrees to them? I have always been a little fuzzy about that one? I assume I have right of way but...there's rules and there's courtesy.
Boat with Spinnaker is not flying their main sail
THERE IS NO RIGHT WAY.
But here is the line of thinking you should use.
What tack I am on? What tack is the other sailing vessel?
You are starboard. Them you cannot tell, if they were on port you would be stand-on.
Let's assume same tack - starboard. Windward gives way to leeward. You are leeward of them, they are windward of you. They are give-way, you are stand-on.
The problem occurs when you are on a port tack and a vessel is running wing-on-wing or with a spinnaker towards you. You cannot tell their tack. You are to keep clear. NOTE: no mention of stand-on or give-way.
This clearly spelled out in rule 12.
Rule 12 - Sailing Vessels
(a) When two sailing vessels are approaching one another, so as to involve risk of collision, one of them shall keep out of the way of the other as follows:
(i) when each has the wind on a different side, the vessel which has the wind on the port side shall keep out of the way of the other;
(ii) when both have the wind on the same side, the vessel which is to windward shall keep out of the way of the vessel which is to leeward;
(iii) if a vessel with the wind on the port side sees a vessel to windward and cannot determine with certainty whether the other vessel has the wind on the port or on the starboard side, she shall keep out of the way of the other.
(b) For the purposes of this Rule the windward side shall be deemed to be the side opposite that on which the mainsail is carried or, in the case of a square-rigged vessel, the side opposite to that on which the largest fore-and-aft sail is carried.
A sailing vessel on a starboard tack close-hauled is stand-on to all other sailing vessels, unless one can point higher.
BTW - it is foolish to sail with a spinnaker and no main.