Unpinning Strategies and the Z-Rig
With plenty of ropes and lots of people, it's possible to apply force in various directions and drag a boat out of a difficult pinning spot. But you don't always have lots of people - you may be a minimal group at the end of a boat-chase, others may be busy chasing down other kit or managing a casualty, the retrieval place may not have room for many folk, or you may simply want to keep as many people as possible away from the place where the boat is stuck (perhaps so they can manage the rope that is protecting the person doing the unpinning).
There are a variety of ways to increase the force you can apply to a stuck boat, depending on how easy it is to reach the boat. If it is fairly accessible, so you can clip things to it readily, the quickest approach is to clip a karabiner and pulley to it, thread your throwline, tie one end to a belay and pull on the other at a variety of angles. It's quick, uses a minimum of kit and doubles the force you can pull with whilst making it fairly easy to vary the direction from which it is applied.
If the boat is less accessible from the shore, but you can get someone to paddle to it, then attach the end of a line to it. If the person on the scene can move the boat, you may be simply able to retrieve it with the line after it is loosened. More likely the person who got to the boat has more on his plate and you need to do the rest from the shore. You may have had to get someone out to the boat on a line, and need to retrieve him safely before getting on with freeing the boat... The full pin kit enables a small group or even a single person to apply three times the force they can pull with whilst staying safely away from the bank.
Establish a belay with the sling, bring the line from the boat to a pulley, and clip it to the belay. Now take the slack end, and add another pulley, clipped to a prussik tied round the line to the boat. Push the prussik as far towards the boat as is safe. Pulling the loose end now has a three-to-one advantage. This is the basic Z-rig. If that's enough to free the boat, you are in business. But sometimes if you are short of bank space, need to apply a bit more force and are short of personnel, a bit more kit will help.
Playing Z-rigs at home is better than learning under stress in the cold.
The first thing to note is that if the prussik fails (it's made of thinner rope than the throwline, but is taking two thirds of the load being applied at the boat) the energy stored by the stretch in the line is going to be transferred to the krab and pulley which will come flying back towards the belay (ie. you) at high speed. Aside of whether that is enough to cause injury, it is certainly going to cause grief and perhaps cause you to slip and fall. Running the pulling end of the rope through a third pulley at the belay gives you the chance to stand somewhere out of the line of flying hardware and may give you more or safer space to pull from.
For every metre that the boat moves, you will pull three metres of rope, but the pulley and prussik will also move one metre towards the belay. If the boat is further from the bank than the bank is from the belay (pretty likely in a gorge, for example) then potentially the pulley is going to arrive at the belay before the boat comes within reach. In fact, it's worse than that since the rope you are using is quite stretchy and you may find that the prussik reaches the belay just in tensioning up the line as it stretches. At this point you need to get the tension off the prussik without letting it off the line to the boat, so you can slide the prussik back out as far as possible towards the boat again. To do this, attach a second prussik to the belay, and to the line going to the boat. Keep sliding this forward as the boat is pulled in, until you need to move the first prussik. At this point tighten the second prussik and allow it to take the load, so you can slacken the pulling line. You now have the slack to slide the first prussik forward and start hauling again. Repeat as required.
That's actually a lot simpler than it all sounds in a wordy description, and is much easier to see in a picture. Easier again in a video, but that will take a bit more setting up...
The Z-rig at the belay. Easiest if one person operates the prussik whilst everyone else pulls.
If you are short on kit, the Z-rig can be set up using just karabiners without pulleys, but there is a lot more friction so the force you need to exert is more than a third of what reaches the boat. If you are short of pulleys, always use them first at the points where the rope moves furthest - ie. at the belay, then at the prussik, and last at the pulling end to redirect the rope for safety. If you don't have any prussik slings, you can get the 3:1 advantage by tying a knot as close to the boat as possible and clipping that with the krab where the first prussik would normally go. But if the boat hasn't come free by the time the knot comes up to the belay, you have nowhere else to go...
Absolute minimal Z-rig - works if boat is not too far from shore.
If you have more people, and can get a second line to the boat, your main hauling rig can pull the boat towards where you want to retrieve it, whilst a second line can pull it to reduce friction with what ever it is caught on - maybe an upwards pull to get it out of current, or a pull away from the rock it is pressed against. If you can clip a line to one rescue point and then pass it under the boat, you may be able to pull the boat up so water starts to empty from the cockpit. Every situation is different, so take a moment to work out what is going to work best in each case.
Like all other rescue techniques, just reading about it is not enough - you have to practice. Whether in the garden at home, in the beer garden at the pub of your choice for a club social evening, or in carefully set-up situations on a real river, ensure you are safe and suitably dressed for the location. I practise this quite often at home - usually to get the ride-on mower out of soft spots and ditches - with stronger ropes the technique works on cars too!