Title pretty much says it. I am barely an armchair composer, but I would like to try my hand at some game music that requires some nautical theming. It seems to me that penny whistles and a celtic feel are close - but I would like to know what sort of keys and progressions I should be using to write original nautical music.
I've had a go writing some nautical-sounding music for games before. Andrew's answer about using Dorian is helpful - here's a few other tips I have.
For a piratey sound, accordions work well. They're characterised by a stride bass pattern (bass note on beats 1 and 3, chord on 2 and 4)
the folk-style melodies are heavily swung, lots of dotted notes.
Sticking to really simple folky chord sequences works well for a shanty sound. Try
- 4/4: | II - - - | I - - - | II - - - | II I II - | (like in "Drunken Sailor")
- or | I - - - | V - - - | I - - - | IV V I - | (for a happier tune)
For more of a Royal Navy feel: penny whistles, snare drums, straighter rhythms. "Sailor's Hornpipe" is worth a listen.
If you want to put drums in your pirate tune, something like a bodhran would probably sound most authentic.
The first thing that comes to mind is the mode or scale. A "generic" tune in those genres may often be in the Dorian mode (not the Greek version), either original or in transposition. Notably, the mode has the feel of a minor scale but without the leading tone (and with a raised sixth scale degree, when that note should arise). So, the typical dominant to tonic "circle of fifths" progression is a minor triad to minor triad, unlike in typical tonality. The raised sixth scale degree compared to natural minor can give the melody a fleeting Major feel when used.
The use of Dorian is by no means absolute of course, nor is it the sole generator of the style you seek. Meter and rhythmic patters depend on which style you are trying to emulate, for instance. Instrumentation also contributes to the character of the music, but it is certainly possible to have an entirely vocal composition in the styles you seek.
Many folk songs are simple and based on pentatonic scales. These songs historically were passed aurally (by listening and repeating), so they needed to be simple enough to be remembered.
The most basic pentatonic scale follows scale degrees 1, 2, 3, 5, 6 (and 1 and octave higher).
I believe 12/8 time shows up quite a bit in "piratey" music! I've seen this counted as both 12/8 and as 4/4 with triplets.