Slider

From KayakWiki

Method of tightening non-bungee, non-elastic deck lines on a Greenland kayak.

http://www.zahnisers.com/zcrb/dpr2001/briannystromsof1.jpg

This white kayak is from Bryan Nystrom. It clearly shows sliders on the lines.

Here is how he describes them:

All you need is a pair of straps/cords 4" apart, running across the aft deck. Install sliders on them for tightening (4" slider with 2.5" hole spacing). This allows you to set up an outrigger quickly, with a paddle float or just the paddle. I used this setup fore and aft of the cockpit, as I use a fore outrigger for resting at times.

To loosen the slider/deck straps, move the sliders toward center. To tighten, move the sliders toward the gunwales.

A huge benefit of sliders/non-elastic deck lines is that they will retain a paddle in a position parallel to the length of the kayak (paddle park) or nearly perpendicular to the kayak, for use as an outrigger. Often for getting into a low volume Greenland kayak the paddle is placed under the decklines angled about 45 degrees past perpendicular. Stiff decklines also act as a handle to enable you to lower yourself smoothly into the cockpit (when entering from a rocky bluff or high dock) and to help pull yourself inside. Bungies hold things relatively well perpendicular to their length, and very poorly near parallel to their length.