Dr. Sudoku Prescribes #11 – Nurikabe

Nurikabe Time by Thomas Snyder


or solve online (using our beta test of Penpa-Edit tools)

Theme: Nurikabe Time; don’t wait for the 13th hour to figure out where all the clock islands go.

Rules: Standard Nurikabe rules.

Answer String: Enter the length in cells of each of the black segments (the unnumbered, connected “ocean”) from left to right for the marked rows, starting at the top. Separate each row’s entry from the next with a comma.

Time Standard: Nurikabe Master = 3:00, Expert = 9:00, Novice = 30:00

Solution: PDF and solving video; advice on solving this puzzle has also been posted in “Ask Dr. Sudoku #2“.

  • nehsb says:


    Really nice connectivity logic. Got stuck for a bit after the initial deductions before I saw it. I think this is my favorite plain nurikabe I’ve done.

  • Tom Collyer says:

    A nice clean solve, thanks! The theme is similar to something I did a while ago, if anyone is interested:


    • Jack Bross says:

      I remembered that puzzle when I saw this one, Detuned.

      And very nice logic on this one — not that hard for a Friday, but very fun to solve.

      • Avatar photo drsudoku says:

        This is my fourth clock puzzle, but definitely my first clock Nurikabe. I certainly aimed for only needing 12 numbers but that I ended up at 1-13 seems a good new twist on the theme.

  • Wow! Very nice. Loved the 9-12 islands.

  • hagriddler says:

    Maybe I overlooked something, just wondering is it possible to solve the bottom right without logic like “suppose cell X is black -> then bla bla bla -> contradiction -> so cell X must be white” ? I used that 5 times.

    • Zotmeister says:

      I’m assuming you had only the 5, 6, and 13 islands left when it got ugly. The 5 can be completed with a little foresight: R11C12 can only be white if the 13 runs all the way along the edge to reach it. Walling it off with black after doing that, however, would make an elbow, forcing R13C10 to be white, which the 5 would immediately have to expand into and be overloaded. This means R11C12 must be black, which makes R11C11 white, finishing the 5.

      The simplest logic to finish the 6/13 separation is a little math. A count of the remaining cells shows exactly five more cells must be black. Those black cells must be enough to separate the 6 and 13 islands. There’s exactly one way to do this using only five: For a rectangle of a given area, the closer to a square the shape, the smaller the perimeter; the “boxiest” that 6 can get is 3×2, which would take five black cells to surround. Since this is the mathematical minimum, yet it takes every cell we have left, that must be the unique solution. – ZM

  • skynet says:

    38 mins 24s….I feel you can increase the novice times a bit motris…feels very low to know that i dont even come under any category…you were giving 50mins time limit for the sudokus…sudoku is the single most universal puzzle that even a starter should complete within 30 mins…but puzzles like these are not that universally catchy….i felt very bad that after sweating out for 35+ mins i am not even a beginner….novice times for this puzzle should be 1 hr atleast …..

    • Avatar photo drsudoku says:

      I set one time rating (the fast one) and then use scalar multiples for the rest (3x and 10x). So they are arbitrary but unlikely to change. If you are just getting started in a puzzle style, you should not feel any pressure to meet any of these solving times. As you solve 10+ Nurikabe, I hope you then start to see improvement and can beat these standards.

  • Ravi says:

    Solved a difficult Nurikabe after long time. Spent 2 min to figure out the island distribution. Once done it was just smooth solve. Just over expert 11:00

  • Minfang Lin says:

    Some cells must be shaded, and only one club can reach there.

  • muhorka says:

    I think, that this nurikabe is one of my most favorite nurikabe that I ever seen! After a good start, i stuck for a while, but then done really quickly (3:19 total).

  • 2.37. This one went really well for me. Could just draw the lines instead of shading it out for a lot of it, which saved time. I think Nurikabe fits in well with the clock theme, the top interactions were really nice.

  • chaotic_iak says:


    Shoot; it was like 30 seconds to Master (2:30) when I got stuck in the ending, and I just bruteforced after I don’t exactly know where to go. I got lucky in my first guess that the 5 goes right, but after realizing, 5 goes down leads an immediate contradiction as the 13 is forced and a 2×2 ocean is formed. Great puzzle!

    After I solved this, I read the Ask Dr. Sudoku post. I’ve read it that it’s about this, but I certainly have forgotten many details. Even more different approaches it seems…maybe.

  • Carl W says:

    Lovely deductions to fill out all that blank space. Thanks!

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