Given the doubts generated by the formation of the plural of words ending in -y (convoy, jersey, gibberish), the Royal Spanish Academy explains the three cases that can occur.
Source: Fundéu RAE
1. Law-laws
Spanish nouns and adjectives or those fully adapted to Spanish that end in -y preceded by a vowel form the plural by adding the syllable -es, which, as Grammar indicates, implies that y acquires consonant value. In this way, the plural laws, oxen, viceroys and convoys have been consolidated in the language from law, ox, viceroy and convoy, voices that have followed the phonetic laws of Spanish in their evolution.
So in ‘Several machinery convoys were intercepted and attacked’ it would have been appropriate to write ‘Several machinery convoys were intercepted and attacked’.
Also in our RAE section: Window cleaner, in a word, with s and without quotes
2. Sweater-sweaters
As an exception to the above rule, some nouns and adjectives ending in - and preceded by a vowel, normally of foreign origin and later incorporated into Spanish, maintain the diphthong and form the plural by converting the y to i and adding an s, without altering by This is the sound of the word: the y of the singular and the i of the plural are pronounced the same (gay-gais or jersey-jerséis).
In this case, remember that the -y in word-final position is not considered a vowel for the purposes of graphic accentuation. Thus, jersey does not have an accent because it is acute and ends in a consonant other than -no -s, but jerséis does because it is acute and ends in -s. On the other hand, the flat variant yérsey is written with a tilde because it does not end in -n, -s or a vowel, and its plural is yerseis, without a tilde because it is flat ending in -s.
Therefore, what is appropriate is “He wanted to innovate with knitted jumpers that are even more modern than they seem” and not “He wanted to innovate with knitted jumpers that are even more modern than they seem.”
It may interest you: “Solo” and “este”, do they have accents or not in any case?
3. Gibberish-gibberishes/guirigáis
Finally, there is a reduced group of nouns ending in -y preceded by a vowel that admit the two previous plurals, with preference for the addition of the ese; This is the case, for example, of guirigay or stay, whose plurals are guirigáis/guirigayes and estáis/stayes, respectively.
Thus, examples such as “He liked nothing more than to follow with the camera the gibberish that his characters rode” or “A stays schooner is a sailing ship with two or more masts” are appropriate.