There's loads of stuff written about the 'roses and castles' painting of narrowboats for anyone interested in the tradition. I recently found an old copy of 'The Water Gipsies' by A.P Herbert, written in 1930, which gives a romantic account of the decoration of a working boat and butty. He mentions 3 designs: roses, castles and hearts, saying that they represented beauty, worldly honour and love, rsepectively.
He suggested that the castles on different boats often bore some ressemblance to actual castles or stately homes in England, which doesn't seem to be the case today, judging from the ones I've seen, which tend to look more like the castle that little fairy flies up from at the start of Disney films!
On Canal Junction Tony Lewery explains that there was a much richer variety of designs than just the 'roses and castles' to which we've reduced canal art - "There certainly are roses of a sort in profusion, but there are also daisy-cum-marigold shaped flowers, dahlia/ chrysanthemums, pansy/primroses and other strange floral >hybrids invented by the brush of the painter working at speed. Castles or big country houses predominate in the picture panels, but there are also churches, cottages, lighthouses and portraits of dogs, horses and the sailor's head from Players cigarette advertisements"
As well as having similarities with folk art across Europe and beyond, I think much of the style and content is inspired by what people with little money or literacy had to hand - playing cards, printed cloth, cheap Victorian patterns on things like these tea-trays.
It's funny that some people who are quite vocal about preserving the 'tradition of roses and castles' may actually be strangling that tradition by reducing it to an extremely narrow set of 'rules' whereby the castles all look the same and there is a set list of ingredients to make up the whole. Courses teaching this may be helping stultify this art form that I reckon was once much more quirky, fresh, individual and inventive.
We can see this happening in another art form - graffiti. There's wonderful self-expression, often influenced by popular culture, comics, TV, (the playing cards and tea-trays of the past) but when you try to confine it, teach it, preserve it, it gets reduced to a set style with boring results.
I do like seeing traditional canal art on boats and it's wonderful to preserve it but if we really want to keep canal art alive, we should maybe try to include our own intersts and influences as our boater predecessors did. I like seeing how some boaters have portraits of their dogs or canal wildlife on their doors and others go mad and paint their whole boats with wonderfully imaginative designs. A couple I met a year ago are a good example. He had painted a lovely card for his partner and she liked it so much, they painted the whole of their boat with that design! It even has some flowers and hearts too I think, that should keep the purists happy :-)