The affix ṭhañ comes after the word nau and after bases having two syllables in the sense of 'he crosses therewith'.,
Karika
आकर्षात्पर्पादभेर्स्त्रादिभ्यः कुसीदसूत्राच्च ।
अवासथात्किशरादेः षितः षडेते ठगधिकारे ॥
This debars ठक् ॥ Thus नावा तरति = नाविकः 'a sailor'. So also after dissyllabic words: as, घटिकः, प्लविकः, बाहुकः ॥
The ष in ष्ठन् in the sutra is not part of the affix, but comes through sandhi rules. The feminine of बाहुकः being बाहुका ॥ In fact the indicatory ष् occurs in these sutras only 4.4.9, 4.4.10, 4.4.18, 4.4.53 and 4.4.74, in this book.,