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Rule 3c says that the ball must be visible at all times. Presumably this means that any fully-enclosed tube would be a construction violation, unless it is transparent. However, if they mean "visible from outside of the device," this would impose a real limitation on track design and make loops much more risky, as there would likely be some points on many loops where the ball is not visible from outside the device. Personally, I am not inclined to be strict in the enforcement of this rule and am thinking of applying it only to the use of enclosed opaque tubing, but I'd be interested to hear any other thoughts on the matter.

"Must be visible" means that for each ball position, there's somewhere that the ES can put their head and see the ball. Doesn't have to be the same head position for all ball positions.

So no tubes, no tunnels, or wide supports that are larger than ball-size and create an effective tunnel.

I don't understand your comment about loops - if you look at any of the marble run toys, the track is u-shaped, in a loop the u-shape curves around in a circle, and the ball is always visible from a point level with the center of the loop circle and off to one side.

It seems like it should be quite easy to achieve a rule-conforming device.

Section 3.c says it "must be visible at all times". The loop rules specifically call out (3.k.ii) that the track cannot be enclosed at any point during the loop. The implication is that it _can_ be enclosed elsewhere, so long as it remains visible (and extrapolating from that it seems a transparent tube should be allowed).

Is the interpretation that the track cannot completely enclose the ball at any point in the device, meaning the ball cannot travel through what would be considered a tunnel? Our device was going to have a section of clear rubber tubing with a gap cut in the top. My question is does the gap need to be wider than the ball?

The FAQs have this rather unhelpful answer:

(Section: 3 / Paragraph: k / Sub-Paragraph: ii / Line: 1)
12/02/2022
Does the requirement that the track not be fully enclosed (Rule 3.k.ii) only apply to loops? Outside a loop, the ball could be visible (Rule 3.c) inside a fully enclosed track (e.g. in clear tubing).
The track is not allowed if it is enclosed, even if it is transparent.

Which makes things less clear.

Reading the rules as written, it is obvious that you can't have an enclosed tube (even a transparent one) in a loop, because the enclosed tube could support the ball at the peak of the loop, which is not the intent of the problem.

It is equally obvious that you can have a transparent tube anywhere else, because the "not fully enclosed" rule only appears in section 3k, which applies to loops and only loops.

I think that the FAQ just addressed the interpretation of "enclosed" in rule 3k, without actually engaging with the wording of the question as supplied, but this FAQ manages to reduce clarity.