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?
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:
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.
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,"