Plank puzzles

Your challenge is to find a route across a crocodile infested swamp using just a handful of rather short planks. Fortunately the planks are light enough to move around, and the swamp is full of old tree-stumps which will support the planks to form temporary bridges. So by careful planning, and re-use of planks, you might just find a route. Needless to say you can only move planks you can physically reach, so try not to leave any too far behind.

Update August 2007: Plank puzzles is also available as a mechanical puzzle called River Crossing produced by ThinkFun. Four editions are now available. For an introduction to the whole range click here.

page index...
Rules & introduction

Two of five
Route 66
Hex swamps
Fiendish No.15
The Centrifugal Force

Puzzles by SwampBeast
The solvers list

Example plank puzzle

play this puzzle interactively below

Acknowledgements

New Sept 2003: If you find plank-puzzles inexplicably tricky, you are not alone. Plank puzzles have been proven 'hard' to solve. For more info see The Complexity of Sliding Block Puzzles (and Plank Puzzles, too) by Bob Hearn of MIT.


Rules and introduction

The best way to understand the plank puzzle concept is to imagine yourself stuck in a real swamp, with tree-stumps too far apart to jump-between, and planks too heavy to move more than one at a time. The remaining rules get a bit fiddley, but here they all are:

  • You can pick up and carry a plank provided you have access to one end.
  • You can only pick-up and carry one plank at a time.
  • Planks must be supported both ends, and must be an exact fit.
  • You cannot jump between stumps or planks.
  • You cannot move a plank if you are already carrying one.
  • Planks cannot be used diagonally, only north-south or east-west.
  • Planks cannot support other planks, and cannot be stacked.
  • Planks cannot bridge intermediate stumps or planks.

Try the following warm-up puzzles and see the rules in action:

Sorry, this is a Java applet.
 

Controls
Click on Restart to restart.
Click on Undo/Redo to revisit previous moves.
Click on Replay to restart (with redo active).

Movement
Click on the play area to pick up or put down a plank. Accessible stumps will be automatically highlighted for you.


Two of Five

This applet features two of the original five puzzles which existed when the plank puzzles first went on-line in July 2000. These puzzles are now entitled Deep-end and Smash-hit.

Deep-end (as in in-at-the...) is a firm favourite. It illustrates many of the best plank-puzzle features, in a tidy small package. If you're new to plank puzzles, or only have time to try one, this is the one for you. Smash-hit was my featured challenge for a long time and is still pretty hard to beat.

 

Sorry, this is a Java applet.


Route 66

Sorry, this is a Java applet.  

Route-66 has the longest solution of all plank-puzzles to date. It takes at least 66 moves (plank relocations) to solve this one, that's over double any of its predecessors.

A little bit of luck helped with the design of Route-66 since the original layout had a planned solution of mere 48, but a short-cut came to light which couldn't be blocked without breaking the true-path. It was sometime before I noticed there was a unplanned longer solution that was still intact. For an interesting view of Route-66 see its span diagram.


Hex swamps

The hexagon based swamp is a variation of the plank puzzle concept, introduced by Graham. This one is called Four-by-four, by virtue of being one of the first to feature four planks (lengths 1, 2, 3 and 4).

Four-by-four illustrates a few features unique to hex-swamps. For instance, note the two approaches to the exit. Which one is the right one to aim for? Later hex-swamps (see Henleymob) all play on the word Twist, thanks to their unique ability to almost spin you on one spot. A feature I felt a strong urge to exploit, once discovered.

 

Sorry, this is a Java applet.


Related pages


concept & maze designs – © Andrea Gilbert 2000-01
applet (& hex-puzzle concept) – © Graham Rogers 2000-01