tach / tack are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
tach: NA
tack: (noun) the heading or position of a vessel relative to the trim of its sails. (noun) a short nail with a sharp point and a large head. (noun) (nautical) the act of changing tack. (noun) sailing a zigzag course. (verb) fasten with tacks. (verb) turn into the wind.
tach: NA
tack: t·AE·k
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)