protesting as a witness

A boat seeing an incident between two other boats most certainly can protest one or both of them. This is specifically contemplated in rule 60.

60. RIGHT TO PROTEST; RIGHT TO REQUEST REDRESS OR RULE 69 ACTION
60.1.
A boat may
protest another boat, but not for an alleged breach of a rule of Part 2 or rule 31 unless she was involved in or saw the incident ...

The representative of the boat that saw the incident, and is protesting needs to have been on board at the time of the incident, but that won't usually be a problem.

A protestee that does not come to a properly notified protest hearing risks the protest being decided on the evidence of the protesting boat alone, and obviously forfeits the chance to explain or defend her position.

There's no obvious reason why a protest committee would not believe A's evidence about B hailing them to cross in the absence of B.

Even if C gave evidence that she did not hear the hail that, on its own is not evidence that it was not made

I agree with Mozzy. *Very roughly* speaking Port can sail what course she likes provided she keeps clear of Starboard. Starboard can sail whatever course she likes so long as she gives port room to keep clear of her. If there was no collision and no crash tack by starboard then it would seem that both boats fulfilled their obligations under the rules.

Originally posted by Mozzy What rule would C protest A under? There was no collision, so not 14. And I don't think that B changing course is evidence of rule 10 being broken as C's opinion on why B changed course isn't a fact. B maybe altering course to avoid a collision, in which case it's an infringement. But they might also be changing course for tactical reasons. Just because RoW boat changes course, it does not automatically mean a rule has been broken.

It's a straight up rule 10.

B changing course may not be CONCLUSIVE evidence of breaking rule 10, but it is evidence nevertheless.

Case 50 https://www.racingrulesofsailing.org/cases/1745?page=5, Decision, Second paragraph, illustrates that a conclusion that a boat did not keep clear can properly be based on the positions and manoeuvres of boats alone.

A protest committee is not bound by the rules of evidence. It's perfectly entitled to take account of an opinion expressed by a party or a witness. FWIW I'd be inclined to qualify a party as an expert.

So unless B turns up and states that the course change was to avoid a collision then I don't think C has much hope of winning the protest. Even if neither B nor A turn up, I still don't know how C would win the protest as they can't offer any facts on why B changed course.

See above. The protest committee does not need to get evidence of state of mind. I don't want to talk about 'presumptions' and 'defences', but in the case of a 'wave through', or a 'duck to keep going', I think the port tacker needs to bring the evidence of that, as I said, her own evidence of a hail, if uncontradicted should probably be accepted.

C's biggest problem will be with the quality, completeness and detail of her evidence. In Match Racing, you need two perfectly positioned Umpire Boats to call close crosses reliably.

C is going to have to give good solid evidence about:

Where was C? How far away from A and B? At what angle?

How good was C's observation? Continuous?

What were the relative positions and speeds of A and B?

How far apart were A and B when B changed course?

How far apart were A and B at closest?

How did C judge distances and angles?

What tack was C on and why? Was Starboard tack favoured? Was Port tack favoured?