I hope that I have worked out this question enough for not being opinion based. I am not asking whether UML is good or bad, because I have no doubt that it is useful for those who work with it professionally.
I would rather like to know if there is kind of a critical mass (measured in, say, hours of coding per week) for UML tools to be useful for a lone hobby programmer.
Of course this assumes that my lack of success with UML is mainly due to lack of practice. If maintaining practice means spending 90% of my spare time with UML, without getting much working code, that's what I would consider an example of being "below critical mass".
By hobby programmer I mean: the typical source code size of my projects is below 1 MB. Sometimes I draw inheritance trees by hand, but that's it. I have got 35 years of programming experience and so just writing down code without an explicit design process (except for the usual OOP design principles and patterns of course) is my natural approach to a project and works quite well.
I have had some superficial contact with UML tools (Rhapsody, Enterprise Architect) in my job in the past, which is not primarily software development. For example somebody asked me once to model a mechanical concept in SYSML.
I know most of the language constructs and presumably also what they mean, but to be honest I don't really know how to use them for my benefit. It just takes so much time that doesn't feel productive. And what's more there is no direct feedback compared to coding some lines and just test them. Also when trying to use UML I have found myself in a situation several times, where the problem seems to become more complicated and over-designed instead of getting clearer. It is as though using the abstract language keeps me from seeing what's really necessary and relevant practically.
On the other hand I sometimes feel that I might be missing something by not using UML.