Practically the advice for anchors used on the inland waterways is to pick the heaviest you can deploy. If you attach 10 to 15 feet of chain to the anchor, you will need to be able to lift not only the anchor, but the first few feet of chain that is attached to it.
Most experienced boaters will carry an anchor of 20 to 25kg plus a length of 10 to 15 feet of appropriately heavy chain. Remember the chain is an integral part of the tackle and the time to realise you should have got the next size up is not in an emergency!
There are figures, aimed at sea going vessels, that suggest the amount of rope to attach to the chain. These figures are related to worse case scenarios of mooring
in adverse weather conditions, but it’s better to be safe than sorry. The rule of thumb is 5 to 8 times the depth of the water plus the vertical distance from the surface of the water to where the rope hangs over the boat. In a 6ft deep river on a standard narrowboat, that would be around 40 feet of scope.
To be on the safe side, a 20kg anchor should be attached to 5m of chain attached to 50ft of rope and would be ideal for an average 56' narrowboat.
Splice loops into rope for attaching shackles. A knot in a rope is a potential failure point and splices spread the load. Use good quality shackles rated for the breaking strain of the rode.
Recovery is another matter. On a widebeam canal boat there may be space for an electric or hand operated windlass. For many narrow-boaters that is not an option considering the amount of space we have available. The practicality is that if you had to deploy the anchor in an emergency, the last thing you'd be concerned with is the recovery of the equipment that was designed to arrest your progress over a weir.
Oh, and once you've installed your nice shiney tackle on the bow of the boat, remember to shackle it to a secure mounting point!