Why is navigable correct?

Question

I corrected someone using "navigatable" when they meant "navigable". They wanted to know why it is the latter. Does anyone know? Is there a reason?

Answer 1

I don't have an answer for why navigate yields navigable instead of navigatable; but in doing so, navigable is by no means a rule breaker among its closest peers. In fact, English has at least eight word pairs involving verbs with a -gate ending and adjectives with a -gable ending, as confirmed by The American Heritage Dictionary of the English Language, fifth edition (2010):

• investigate --> investigable

• irrigate --> irrigable

• litigate --> litigable

• mitigate --> mitigable

• navigate --> navigable

• obligate --> obligable

• propagate --> propagable

• segregate --> segregable

AHDEL doesn't list any similar -gable adjective form for a larger group of at least seventeen -gate verbs:

• abrogate

• aggregate

• arrogate

• castigate

• conjugate

• congregate

• corrugate

• delegate

• derogate

• elongate

• fumigate

• instigate

• interrogate

• relegate

• subjugate

• subrogate

• supererogate

But on the other hand, it doesn't list any adjectives of the form -gatable. For these reasons, it seems fair to say that, within the subset of -able adjectives associated with verbs ending in -gate, navigable follows the standard pattern—the same one that the other seven -able adjectives associated with -gate verbs follow.

Answer 2

Wiktionary defines navigatable as a misspelling of navigable made from the verb “navigate” (possibly nonstandard):

• 1992, Peter Sammartino and William Roberts, Sicily: An Informal History‎, page 45: Rivers were made navigatable, irrigation was improved, and the use of hydraulics was greatly increased.

• 1998, Flexible Query Answering Systems‎3, page 83: The second is that it has potentials for information exploration, because the displayed network is navigatable.

Probably because of the common usage of the suffix -able often applied to verbs in English:

common termination and word-forming element of English adjectives (typically based on verbs) and generally adding a notion of "capable of; allowed; worthy of; requiring; to be ______ed,"

Etymonline