Copyright laws vary from place to place, but in general anything that has been created of an artistic nature (literary works, drawings, scuptures etc) is covered by copyright . In the case of a set of plans that would mean that the drawings and accompanying instructions were covered by copyright, so you can’t just start copying them and selling them at the local market. Well, you can, but you can also then get sued by the owner of the copyright, so better not.
However…the copyright does not prevent you using those plans for the purpose for which they were sold eg building the model. It also does not prevent you selling the model you built, and if you should happen to build more than one, well, you can sell those too. There is a sense in which the designer has invited people to do this by publishing the plans.
If the designer wanted to control the production of parts to match the plans that he has published, he (or she) would have to sell them with an attached set of conditions, eg a contract, limiting say the number of copies you can make. The boat designers do this because they are not going to make bulk sales of most plans, and so they have to charge enought to pay their costs on the ones they do sell.
It is actually quite hard to get protection of the technical aspects of a design, copyright is intended for artistic works and does not cover fact, apart from some extension in the area of databases in some countries. Patents cover technical features, but in theory only if they are novel and non obvious. In the case of a scale model, many of the bits have to much the way they are for the model to look like the prototype…good luck claiming copyright protection for something that in the artistic sense is a copy of the full size work .
I think to some degree in our hobby there is a degree of gentlemanly behaviour going on…if a design was originated by one supplier then other suppliers leave it to them to supply the bits. If it was published in Model Engineer with no supplier connection then there is really not much to stop anyone selling castings to suit it, in much the same way that people can sell pattern parts to suit cars they don’t actually make. But of course a supplier in our hobby is likely to do better by offering something that nobody else has than by doing a “me too” of the same thing.
But of course, I am not a lawyer, and the above does not constitute legal advice…
I don’t know if anyone still has any of the Clarkson designs/parts still available. But if you get hold of a set of plans, I don’t think you need to worry about making some patterns and getting them cast.
regards
John