A finned fish is a crippled fish, a lock on crossing positions hindered by the presence of one or more extra candidates which, going along with the fishing motif, sudoku people call a fins. The fin spoils the would-be fish’s grip on crossing line positions, but the imperfect fish can still eliminate any victim that “sees” the fin. How about a simple case that shows how such a thing can happen?
In this example from KDST 555, meaning KrazyDad SuperTough vol. 5, book 5, puzzle 5, the would-be 6-wing is spoiled by the extra fin 6r5c5. If the fin could be wished away, the perfected 6-wing would eliminate 6r3c3 and 6r6c3. But 6r3c3 sees that fin, via 6-chain. If 6r3c3 is true, it will erase the fin, allowing the fish to prove it false. It’s a case of “can’t win for losing”.
Potential victim 6r6c3 escapes the fish because it cannot see the fin. If true, it would confirm the fin. That is often the easiest way to show that it cannot see the darn thing.
A much more common way for a finned fish victim see the fin is to be in the same box with it. When the victim sees the fin by inference chain, the fish is called a kraken fish. I call such victims krakens, and the task of testing victims for kraken escape, I call kraken analysis.
Have you figured out why the 1’s are eliminated? If not, you are admonished to keep your thinking cap on.
A finned fish from the KrazyDad SuperTough v.5, b.9, #5 has more than one fin. In these cases, seeing one fin is not enough. To be eliminated a victim of the perfect fish must see all of the fins.
When we identify fins and look for finned fish victims in the fin box, that comes with the possibility of kraken victims as well. Here krakens are easy to dismiss because the fins are both visible only from c3.
More typical is this finned jellyfish from World’s Hardest Sudoku, Very Hard 65. Identifying 9r6c7 as a fin gives us the immediate victim 9r5c9, but also suggests a search for kraken victims as well. Where are the possible kraken victims?
Once you have found them, try your hand at proving whether or not any of them see the fin. In other words, do the kraken analysis of this finned fish.
To check your work, see the review post of 6/5/18 on World’s Very Hard 65.
Sashimi Finned Fish
There is a type of fish that violates normal rules and gets away with it. When scanning lines for a swordfish or jellyfish, you fail to come up with the third or fourth line that matches the crossing positions. But there is a line matching one position. Consider the remaining X candidates on that line to be fins, and treat it as a finned fish. If you have a victim seeing these fins, you have a sashimi fish. The seeing can be by fin box or by inference chain.
It isn’t getting something for nothing. The Sashimi is by definition a finned fish. Eliminations depend on seeing the fins, just like any finned fish. With multiple fins in the single match line, that is not easy and is often impossible.
One thing you should know, however. Sysudokies may look for Sashimi swords and jellies, but not for Sashimi X-wings with a single extra X-candidate in the added line. An early post, of 4/17/12, pointed out that such a Sashimi X-wing is also an X-chain skyscraper! The skyscraper is easier to spot, and accounts for more.
Here is another look at the two skyscrapers of KrazyDad SuperTough v.5, b.6, #5 that were featured on the X-chain page. This time, each of the three eliminations is interpreted as a separate Sashimi X-wing. The 3 valued skyscraper accounts for the red and green Sashimi victims.
As an example of a order Sashimi, here is the blank tallied X-panel of a Sashimi swordfish from Andrew Stuart’s The Logic of Sudoku. The X’s and ?’s both represent X-candidates, and there are two fins. There is a fin box, and two victims found themselves in it.
Are there any krakens? You can do your own kraken analysis to find out, but I wouldn’t bet against Andrew on this one. Try that, and then look below to see how you can document an involved kraken analysis.
Beyond the two fish pages in the Guide, there are two extremely advanced fishing topics to consider, but they will be deferred until more practical sections of the Guide are complete. After this level of fishing, you on Coloring, AIC chains, and Pattern Analysis are completed. These fish topics are Franken or Mutant Fish, in which boxes enter the competition for line placements, and a Sysudoku scratchpad algorithm for unit and fish subsets and ALS. But you don’t have to wait. A series of posts on these topics are available, and are listed on the Titles page under Fishing.
The South box possibles confirm the fins, and the other possibles confirm the fins by removing Xr2c3.