I have been 'on the bank' for 25 years or so and found, on my return to the water, that many boatowners are now using the word 'cratch' to mean their deckboard or their front cockpit covering arrangements - a use with which I would strongly disagree.
Figure 1: Deckboard
My understanding, from the days when working boats were still to be found all over the system earning their keep as camping boats, is that the cratch is the whole structure at the fore end of the hold comprising deckboard, false cratch, top plank, side planks, staples, rings and strings usually covered in canvas. The deckboard's purpose was to prevent the cargo hold from flooding if (say) the bow wave overflowed the front deck due to the forward motion of the boat combined with the effect of a constriction in the channel or alternatively if a gate paddle were 'pulled' over-hastily in an uphill lock. The cratch structure further improved this protection.
In 2006 I was aware that John Jackson was bringing his Josher "Roach" up the Staffs & Worcs towards Crompton loaded with 19½ tonnes of solid fuel together with 2-3 other boats likewise loaded. Being tied at Crompton, I walked down 2 or 3 locks to set the 'road' for them. "Roach" was the last of the working boats to come through steered by John's wife. She brought it into the already-open bottom gates of the lock with quite some speed on. Anticipating what I would see I positioned myself so that I could observe the fore deck of the boat as it passed the bottom gates. Sure enough as it entered a bow wave quickly built up until it overflowed the top bends and swept across the fore deck to the deck beam where it spilled off behind the cants.
I have drawn some diagrams, as best I can using 'MS Paint', to illustrate the various components of a cratch and the way in which they were constructed.
Figure 2: Deckboard (rear)
The triangular-shaped deckboard would be made from tongued and grooved (but not V-jointed) boarding nailed to framing timbers. It would be positioned over the fore end deck beam and located by iron or steel posts to straps on the rear of that beam. Notched framing on the rear of the deckboard and on the front of the false cratch provided locations for side planks to support the cloths and decorative strings. The frame of the false cratch was 'birdsmouthed' over the gunnels and top plank to provide lateral location.
Figure 3: False Cratch
The deckboard was either painted, covered with canvas or fitted with a bulbous canvas-covered protrusion termed a 'bulk'. The clothed sides of the cratch would be decorated with a strip of scrubbed white fire hose over the forward end of the top cloth, fastened to the gunnel with brass clips. Strings from rings stapled to the gunnel would run over the topcloth through the ring on the opposite gunnel and back over the top plank to terminate in roundels trapped under the string by its own tension.
The only bulk that I have seen in recent years was, and probably still is, mounted on the deckboard of the Josher "England" which appears to be regularly moored 'on the line' outside the relatively new Kings Bromley marina on the Trent and Mersey above Fradley. It is a grand sight indeed to see this ostentacious traditional decoration on a working boat, albeit one which has been 'converted'!
Figure 4: Cratch (sideview)
For extra decoration strings would be taken from the rearmost rings, not across the top plank, but diagonally across the cloths and across the face of the deckboard, again terminating in roundels.
Figure 5: Cratch clothed & decorated
As far as I am aware the only positive location of the cratch was provided by the steel straps on the rear of the deckboard and the effect of the 'birdsmouthing' of the false cratch. The planks supporting the topcloth would not be nailed to either the deckboard or the false cratch. Instead a string, or series of strings, were fastened to rings stapled inside the structure to hold the components braced tightly against each other. This arrangement was favoured because, after unloading and with the fore end accordingly much higher in the water, the cratch may have had to be dismantled to pass through some bridge 'oles.
A more permanent fastening than the bracing strings (eg nails) would have been satisfactory on a regular route with no low bridges but with all components, except the top plank, firmly fastened together the structure would have been considerably more time-consuming to remove.
Planks over the gunnels beneath the cratch provided a convenient 'shelf' for storage of unused lines and other bits of rigging.