This is the usual usa problem of having extra restrictions on the rules. Some of the conflict of interes rules had to be changed to get over us draconion measures.
You should have decclared you saw the incident but would not be giving testamony. 
Clearly you could not mention what you saw to your fellow committee members. 
It seems an excellent decision was reached from the facts, and you should be proud of your service to yachting. 
It is so hard though at events when so little resource is given to the protest committee. 

I hate this prescription.  It used to be we could give testimony.  The judges manual instructed us to get up and sit on the other side of the table and act like any witness.  To me, the "when practicable" is an important phrase.  Given the pool of judges, you really needed to be on the panel.  What if neither side has any witnesses and the guilty party starts telling a tale different from what you saw?  You've declared that you saw the incident so the other judges know, and then in deliberation you say, "well, I believe party A."  How is that any different from you telling the panel what you saw *without being cross examined*?  In my opinion, in this case you should testify and it isn't practicable to exclude you from the protest committee.

I still believe you should declare you saw the incident.
It is for the party to decide if you should be called as a witness or not, at of course there risk as they do not know what you saw. 
The definition of conflict of interest means this should not be an obsticle. 
This leaves "when practical" so perhaps ask the two lay members if they want to hear it alone, and if /when the response is no, continue. 
All boxes seem ticked. 

I don't think the US prescriptions really change the central issue, is it best practice for someone who was a witness to an incident sit on the jury?  Ideally, no, which is all the US prescription really says.  It forces a jury to confront that possibility.  But from a practical standpoint, if we are going to have judges on the water to address rule 42, then this is always a distinct possibility, i.e. it is not "practicable".  Also, the situation you were in is not atypical, where the lack of experienced judges prevents you from recusing yourself.  All reasons for you to remain on the jury.

I think I would have handled this differently.  I would have disclosed that I saw the incident and, if the parties did not want to call me, then I would have called myself.  The purpose would be so that the parties knew what I saw and could question me.  Then they would know how what I saw fit in with the other testimony and my input into the decision.  They could then decide if they thought I might have a significant bias which would be a reason for an appeal or reopening with a new judge in my place at a later time.

I think the telling statement you made in your OP was "our committee was able to find facts that agreed with what I saw".  What if it wasn't?  You would not have known that until hearing the testimony, past the point when you are supposed to tell the parties that you saw the incident.  You would also not be sure of how what you saw influenced your assembly of the facts.

Thank you all for your thoughts.

John says: "I think the telling statement you made in your OP was 'our committee was able to find facts that agreed with what I saw'.  What if it wasn't? "  And Ric says "What if neither side has any witnesses and the guilty party starts telling a tale different from what you saw?"  Right!  That's really what got me going here.  Realizing that it might not have worked out.

Sometime afterward I re-read much of Part 5 including 63.4(d) and realized my error.  [Aside: There's lots to Part 5.  In the US rule book it's 10 pages even before misconduct, while Part 2 that we want to concentrate on is only 6 pages.]  

I should have declared to the parties that I saw the incident, of course.  That's a rule, and you all agree on that point.  I think if it's to some degree in conflict with the US Prescription, that's where there's some leeway to use judgement.  There's none in rule 63.4(d).

Every experience provides an opportunity to learn something.   One thing:  If I have to run the show on my own I should resist the pressure of time and go as slow as I need to, to make sure I go through everything in Appendix M.

Thanks again!
-V

OK .. let's stir the pot a bit on this one.

Let's say that a US Sailing Judge is doing OTW 42 and witnesses an incident, and like the OP, this judge is the only experienced and certified judge on a 3 person panel.  This judge makes the assessment that to ensure a proper and well run hearing, they need to stay on the panel.

The hearing opens and this judge declares, "I was on the judge boat and I witnessed this incident.  Though US Sailing Rx's state that when practicable a person who witnessed an incident should not sit on a panel, I have determined that since I am the only US Sailing Certified Judge on the panel, it is not practicable for me to step off the panel.  I have decided that I will not give testimony."

Scenario #1:  One of the parties to the hearing objects and demands that the hearing only be held by the 2 uncertified panel members.

Scenario #2:  Nobody objects to the judge staying on the panel, but during the hearing one of the parties calls the judge as a witness.  This 2nd one is interesting.  Can a Race Official for an event refuse to give testimony?  Can a PRO or RC member if called?

1. they  cannot object is is a panel decision. If they object on the procedure they can appeal ( I believe this may be a two stage process in the USA) The original facts would remain unless a re hearing was ordered.  Then we get an experienced panel with the original juror as a witness. I doubt the result would change.
2. Why I had my second post. Anyone can refuse to give evidence but it looks bad. I think evidence should be given (but not disclosed to the party beforehand) they take what they get.
I have been called myself before a full jury when I was not expecting it, it is terrifying. I did remain part of the Jury here. on other occasions, when we have had the numbers I have not formed part of the Jury but just attended as a witness..
If you give evidence you can be questioned, this can be good for all and test your own assumptions which may change your own perception of what you saw.  We often only see part of an incident.

Angelo in scenario 1 you imply that the member of the protest committee who saw the incident might be excluded.

Rule 63.4(d) says "a member of the protest committee who saw the incident shall, as soon as reasonably possible, declare this fact to the parties attending the hearing."

Rule 63.3(b) says "A party to the hearing who believes a protest committee member has a conflict of interest shall object as soon as possible." Either party can object to the member of the protest committee who saw the incident. However, they should only be excluded if they have a conflict of interest. Seeing the incident does not in and of itself present a conflict of interest.

During the hearing either party can call the protest committee member who saw the incident. They usually don't.

I've been thinking a bit about under what circumstances a Race Official can refuse to give testimony (without expecting any repercussions).  Here are some of my thoughts (definitely a "work in progress") ..

When acting in their "official" capacity, my sense is that they are no longer just some individual, but rather are the embodiment of their office with official duties.  Those duties include being accountable to the sport and the event for their official acts.

In the OP, the OTW judge was out there to look for Rule 42 violations.  That was their "official" duty.  While doing this, they witnessed a Part 2 rule breach.  Looking for Part 2 breaches was not their official duty while on the water.  Being outside those duties, one might say that they witnessed the Part 2 foul as an individual, not a "race official".

However, if that same judge was called to give testimony about one of their Rule 42 calls, in that case they are being asked about their official acts.  If they refused to provide testimony .. this might be grounds to report this to their accrediting MNA or body as a Race Official must be open to accountability for their official acts.

OTHER NON-TESTIMONY EVIDENCE (audio, video, pictures)

OK .. so what about evidence like recordings and pictures taken by a Race Official?  I think I come to the same conclusions.

For instance, if a PRO is recording starts and finishes with video and audio, and that PRO is asked by a party to produce those during a hearing, I do not believe they can refuse to do so.  Those products were generated while acting in the RO's official capacity.  Though they might exist on personal electronic devices, they are being recorded as an official act of the Race Official, and therefore the content belongs to the event as a whole and not the individual.  Refusing to provide them, or purposely destroying them in the face of a known need of them .. again I think that might be something that might be reported to the accrediting MNA.

On the other hand, if that same RO was taking a selfie with their RC pals and they catch something in the background, that image is a personal image, not generated as an official act.

Anyway ... those are some of my thoughts.  I put them out there as a catalyst for discussion.

PS:  In all the above instances, there is no way to "compel" the RO to give evidence.  The only recourse, I believe, is a report of those facts to the RO's accrediting MNA.

I have faced this situation several times this spring in College umpired team race events and recommended to the OA that the prescription be written out.  As we are on the water umpires we make the calls on the water but in some instances where an advantage has been gained or there is a 14 violation requiring a hearing for damage we have to have a hearing. Most of the time we have had only 1 or 2 certified officials at the events so it is up to them to hold the hearing.  It seems to me it would be foolish to exclude the Umpire’s testimony as they are there on the water to make the calls sometimes there is only 1 or 2 items needed to make a decision.  Is it prudent for the certified individual to not be on the panel?  Is it prudent for the certified individual not give testimony. The same might apply for fully umpired fleet racing. Should the prescription be changed to exclude umpired events?