The answer is . . . it depends.

There are a couple of forum threads out there (this one is more civil than the Sailing Anarchy one - https://www.sail-world.com/forum/forum_posts.asp?TID=13986&PN=1&title=is-weed-aways-an-obstruction)
Both threads hash over the topic extensively, but there's no definitive answer.

The SIs could specifically include kelp patches in the definition of obstruction.

You could hail for room to tack with the risk that the windward boat would protest you for breaking 20.1(a).

From your diagram, it would appear that B would want to tack to avoid the kelp patch anyway.

There is of course the matter of whether the kelp patch meets the definition of "obstruction" for a Laser. A kelp patch of sufficient size to be an obstruction wouldn't just slow a Laser, it would probably stop her dead.

Recall that in the previous rules, the definition of mark specifically excluded "an object attached accidentally to a mark". The current definition specifically includes "an object intentionally attached". I believe this continues the same logic, an object (like a kelp patty) that is accidentally or incidentally attached to the mark is not part of the mark.

If the kelp patty is not part of the mark then the last sentence of 20.1 doesn't apply and A should be entitled to hail for room to tack under 20.1. As Matt noted even if the hail is invalid B is still required to respond. When the first boat passes head to wind 18 would turn off.

If they both tack onto port (assuming overlapped) A would be inside and to windward so she'd be entitled to mark-room and her mark-room would include room to tack back onto starboard when she can fetch the mark. 

Once both boats are back on starboard (& presumably still overlapped) a new 18.2(a) mark-room entitlement would begin for whoever is inside. 

I believe mark-room would include room to avoid the kelp.

Thanks Matt. The Sailing Anarchy thread includes posts with the usual lack of civility, but I'm linking to it because it also contains several civil posts that helped me understand the situation better.

I think I've answered my own question by looking closely at the definition of obstruction:

An object that can be safely passed on only one side and an object, area or line so designated by the sailing instructions are also obstructions

*Pass* and *safely* are the key words here. Could boat A (which was me, btw) pass through the clump of kelp that was just a few feet wide? Yes, I passed through it. Was it safe for me to do so? Yes, no harm came to me or my boat.

The SA thread covers a different case where a fast-moving boat under spinnaker, in high winds, ran into a massive patch of kelp that caused them to decelerate quickly, causing crewmembers to get injured and cut up where 14 stitches were required. Could they pass through that kelp? No, they couldn't. Was it safe? No. In this case, kelp should be considered an obstruction.

So, size matters when we're talking about kelp as an obstruction. Big kelp patches you can't pass through are essentially no different than a sand bar or breakwater that a boat can't pass through and, therefore, should be treated as an obstruction. But small kelp patches that merely slow you down are not obstructions.

As much as I wish I could have called for room to tack at a clump of kelp, it doesn't seem like it would have been allowed under the rules.

Al, I don’t know that that part of the definition about safely passing on only one side refers to whether you can safely pass through the obstruction. I think that’s talking more about piers, shorelines, etc. You could certainly have passed the kelp patch on either side (although passing it to starboard would have prevented you rounding the mark).

The operative test, I think, is whether you could pass without changing course substantially if you’re sailing directly toward it within one hull length. If you’re able to safely pass through the patch without altering then probably not, but if you don’t think you can sail through it and you have to make a substantial course change to avoid then it’s an obstruction.

Given that a Laser is easily stopped and you don’t really know how much kelp is there until you’re in it, I think you’d have been safe in calling for room to tack.

Al,

This was a source of constant confusion and argument at the Santa Cruz YC, where sailing around kelp is a constant problem. It was finally resolved (for the most part) that kelp was NOT an "obstruction" under the rules. In your example, the existence of the mark has nothing to do with Rule 20. In my opinion, the key question is: "Is the kelp an obstruction?" Therefore, the competitor has no "right" to tack to avoid the kelp, given he could simply bear off and avoid it. Competitors who are racing often get it into their mind that they have some right to round marks. 

From the Definitions:
Obstruction

An object that a boat could not pass without changing course substantially, if she were sailing directly towards it and one of her hull lengths from it.  An object that can be safely passed on only one side and an object, area or line so designated by the sailing instructions are also obstructions. However, a Boat racing is not an obstruction to other boats unless they are required to keep clear of her or, if rule 22 applies, avoid her.  A vessel underway, including a boat racing, is never a continuing obstruction.

Rationale:
Kelp is not an object that might damage or endanger the boat or her crew. Catching kelp on an appendage simply slows a boat down. There are lots of things that can slow a competitor down and they do not qualify as "Obstruction". For example a pool of oil on the water, a tidal race, a lack of (or too much) wind in a certain area, etc... 

We've concluded that sailing through a piece of kelp is simply poor seamanship as it neither endangers nor damages the crew or boat.

Seems to me that the primary sentence of the rule says nothing whatsoever about damage or endangerment. 
"An object that a boat could not pass without changing course substantially, if she were sailing directly towards it and one of her hull lengths from it. "
Surely we need to go no further than that. The rest of the definition is about things that are also obstructions.
Is it an object?
Is it big enough that one needs to change course to pass it?
The logic chopping about whether you can pass through it is just that to my mind, but I submit that if you catch this lump of kelp on an appendage then by no dictionary definition can you be said to have passed it! 
To my mind another test would be "is it an object that one would change course substantially to pass in the absence of other boats". 

Jim, perhaps the root of the problem is that the offending object, "kelp", comes in various sizes. These vary from a 5" long bit that might stick to the rudder or centerboard of the Laser in the example, to a field of it thick enough that heavy birds walk on it. With out knowing the weight, size, and strength of the "kelp" being discussed one is left concluding that there is no obvious way to answer the question.

While I think a pad of kelp large enough to bring the boat to a halt would be an "obstruction" under the rule, that wasn't what Al described in the original post where he said: "For those not familiar, the kelp is a significant issue, since it thick enough to cause a boat to lose about 10 boatlengths relative to the competition if it needs to be cleared from a Laser rudder while downwind, due to a temporary inability to surf downwind."

What you're suggesting is that any object that slows a boat down is an "obstruction" under the rule. 

Jim, you raise an excellent point: if the kelp catches on your appendage (daggerboard/keel/rudder), then it's traveling with you and you haven't passed it. When you catch kelp, you can sail around for hours and it's still there, traveling along with you.

Then there's the second test: would I have had to change course substantially to avoid a ~three foot wide chunk of kelp attached to a mark? The answer is no: I could have borne away 10 degrees to avoid the kelp. I would have also missed the windward mark, but we know that Rule 18 doesn't give me the right to tack onto port to round a windward mark.

So this gets back to the point that, with kelp beds, size matters. A small amount of kelp requiring an insubstantial amount of kelp doesn't invoke Rule 20. A large amount of kelp that requires a substantial change of course, when one boatlength away, does invoke Rule 20.

It comes down to, What counts as a "substantial" course change? There's this thread from 2014 that references US Sailng Appeal 15. Looking at that diagram, Boat L, approximately 25 feet long, when one boatlength from the 50 foot police lanch, would have had to alter course ~60 degrees from her current course to pass to leeward of the launch, and 90 degrees (i.e., tack) to pass to windward. This is a venerable appeal, since it went into effect in 1959, 64 years ago.

(Could less than 60 degrees count as "substantial"? Maybe. But we have one well-established case showing that 60 degrees means substantial.)

Now it's a simple matter of math, specifically trigonometry: 
 

So, when sailing a Laser, any patch of kelp 12.5 feet or longer *from your current path* allows you to call room, per Appeal 15 and the definition of obstruction.

More generally, any patch of kelp or anything else that can hook your appendage (i.e., you don't pass it because it stays with you) that is 0.9 boatlengths from your straight line course allows you to call room.

Maybe that's not the spirit of Rule 20, which seems to be about ensuring safety, but that's where the wording of the definition of obstruction, math, and Appeal 15 all lead us. Maybe the definition of obstruction needs to change, but this is the rule that we have today.

Beau, I agree with your post: size matters. The patch I encountered merely slowed me down and didn't cause a safety issue, unlike, say, the San Francisco seawall that we are both familiar with. But if I've learned anything from talking with Dave Perry at rules clinics, it's to follow the wording of the rules, and that's what I've done above. Again, the patch I encountered did not extend 7 to leeward of my path, so I could NOT have called for room at it using the reasoning above.

> What you're suggesting is that any object that slows a boat down is an "obstruction" under the rule. 
That doesn't seem unreasonable to me. 
The rule as written has two conditions. It must be an object, and it must be over a certain size. To say a sufficiently large object is not an obstruction because one might ram it without damage would seem to defy the literal wording of the definition.

Muddying the water, perhaps, are the words in RRS20, where a boat may only hail if she "will soon need to make a substantial course change to avoid it safely," I can certainly see people arguing that it is safe to run straight over the kelp patch.That seems perverse to me, because while it might be safe, no avoiding has taken place. It also seems to me that one doesn't know whether it is safe to ram the kelp patch until the deed: if the kelp has gathered round something awash and solid then you only find out when you hit it, which doesn't feel like good seamanship to me. 

Can we work through the rules and definitions.

Is kelp an object - Yes.
Is the course for a boat to finish as soon as possible a course that avoids kelp -yes. 
Is it seamanlike to sail across or through kelp - no

Can we agree then that any kelp that is big enough or visible enough to be seen early enough for a boat to take an avoiding action is 'an object that a boat could not pass  without changing course substantially: in other words it is an obstruction. How big depends on the boats being sailed and the conditions.

In this case is the kelp intentionally attached to the mark - no. So the kelp is not part of the mark. Therefore RRS 19 or 20 apply.

Under RRS 20 if A decides to tack she must hail fro room to tack then give B time to respond.Under RRS 20.

Yes, indeed. The local interpretation @Beau Vrolyk tells us about appears to be using a dictionary definition of the word obstruction rather than the current RRS one. That may well have been valid before the RRS definition acquired its current form, but I suggest it's incorrect now. Perhaps an appeal would be desirable to gain clarity since it would seem to have safety implications. 

Don't even bring rule 20 into this, what would happen?  A would simply luff head to wind and hold briefly.  Not wanting to just sit there HTW, B would tack away or keep clear by luffing slightly and crossing ahead of A and then A can tack.  No need to discuss whether it is an obstruction, how big it has to be to become an obstruction, whether it is seamanlike to sail through it.

IMHO, one more reason rule 20 should be deleted as unnecessary..