The rules of priority on the sea are found in IRPCAS(Regulations for Preventing Collisions at Sea) which chose one “privileged” and a “non-privileged”. This simply means that even the privileged ship must do everything to avoid collision. Priority rules for regatta are found in the ISAF book. These essential documents are particularly soporific and can quickly become a real challenge facing the fight against sleep, or even an increased risk of falling down in a lethargic coma, Fortunately it is possible to summarize roughly.
1) Basic Rules
In practice, we must give priority to professional boats(fishermen, ferryboat, cargo boats…), which is quite logical. Between amateur sailors, sailing boats have priority over motor boats because motor boats are more easily maneuverable. A sailing vessel underway with just its engine is considered a motor boat. Between two yachts under sails which are about to cross, there are two basic rules:
.If they're both port or starboard tack(if they touch the wind on the same side), the one leeward of the other is privileged(the one that is farthest from the wind's source).
.If one is starboard and one on port tack, whoever is on starboard tack is privileged(the one where the wind is blowing from the right side of the boat).
After that in practice, if we meet a boat under spinnaker(therefore harder to manoeuvre)the common sense would give him priority even if we are starboard and him port tack, at least for cruising. Generally this is the less maneuvering boat that is privileged.
2)Regatta
No no against appearances it's not the noisiest crew who will be prioritary! For regatta it is a bit more complex(rules between parentheses are ISAF rules):
.I am prioritary when I am starboard tack(rule 10), downwind(rule 11), ahead(rule 12), on a steady heading(rule 13)faced with a boat that is changing tack, on a steady heading(rule 13) while the other boat is doing a penalty, going ahead(rule 20).
.I'm not prioritary when I am port tack(10), windward(11), behind(12), tacking(13), providing a penalty(20) or going backward(20).
When I am privileged I still have to keep an eye on collision risks, and avoid them(14) and leave space if I change my route(16) or if I have just taken priority(15). I also have to sail on the normal route(17).
When I have not priority I must obviously avoid collisions(14)and stay aside as soon as i'm not privileged. in spite of that I've got the right to have space when the privileged boat changes its way(16)or when I have just lost my priority, unless I have given priority(15).
At buoys(in the area of the 2 lengths); if I'm behind I stay away, if i'm ahead I do what I want including to get rid of the rule 16. If I pass the buoy outside I must stay away, from the inside I am allowed to have space to gibe on the normal way. However, there are exceptions, otherwise it would be too simple: if I take the buoy by inside being non-priority(port tack)with an outside boat and starboard tack, priority is shared during the mark's rolling up. If we tack in the two lengths area we don't become prioritary after that, and of course we can't make an another luffing up to bother him when I want to pass the buoy.
Obviously in case of unforeseen obstacle we are allowed to make an another boat tack or don't let a boat leeward of us(19).
Rule clarification 12(front / back)is no longer valid when the boat's bow which overtakes the other cut the stern's extension of the other, for example a non-priority boat can become privileged if he cuts the stern's extension of the overtaken boat leeward of him(then we change back to the rule 11).
But sometimes screaming “Water, Water” when are not not sure, on a misunderstanding it could work!