From what I have read online the first season was shown 5 times a week from Sept 1985 to Dec 1985. The airing of Tiger Sharks & Silverhawks occurring during the Thundercat hiatus in 86-87.
Hi, despite rumours to the contrary, ThunderCats began production in early 1984. It began airing in September 1985 and had an unbroken 13 week run (Monday to Friday). In late 1984, largely because Rankin/Bass were keen to exploit the action/adventure genre, they began to develop SilverHawks. This was well before the success of ThunderCats had yet been established and it was merely a way of getting another commission for the company. SilverHawks ran for a 13 week run in the fall season of 1986, when only five new episodes of ThunderCats were aired (the five part 'ThunderCats HO!) serial. The following year, ThunderCats resumed airing 20 new episodes per year (with the remainder of the ‘season’ being filled with repeats). This meant the second series (which were always commissioned as a second season of 65 shows) aired as follows:
5 new episodes in 1986
20 new episodes in 1987
20 new episodes in 1988
20 new episodes in 1989
The Comic Strip (of which TigerSharks was a part) aired for a 13 week uninterrupted run in fall 1987. And, yes, in part the rationale for splitting ThunderCats up over that period was due to the huge production work involved in producing three shows.
I think the last episodes were a turn down for many fans. Since they learnt about the reforming of the planet and made all those trips there, the show turned kind of childish, and then getting rid of the main scenario the fans grew to love and get interested in, and also two of the main characters, caused ratings to drop (leaving Third Earth, and Pumyra and Tygra with it was a mistake IMO). Then producers thought of these 2 shows to prepare the TCs way back, but it didn’t work.