The affix ṭac comes after the word nau when preceded by the word ardha in a tat-puruṣa samāsa ,
Thus अर्द्धं नावः = अर्द्धनावम् ॥ The compounding takes place by 2.2.2. The compound is of neuter gender, and does not take the gender of the term as required by 2.4.26: for gender is regulated by usage more than any grammatical rules (लोकाश्रयत्वं लिङ्गस्य) ॥,