1.1. To spell a short vowel sound, only one letter is needed:

at

red

it

hot

up

1.2.

To spell a long sound you must add a second vowel. The second may be next to the first, in the VVC pattern (boat, maid, cue, etc.) or it may be separated from the first one by a consonant in the VCV pattern (made, ride, tide, etc.). If the second vowel is separated from the first by two spaces, it does not affect the first one. This is the VCCV pattern in which the first vowel remains short. Thus, doubling a consonant can be called "protecting" a short vowel because it prevents an incoming vowel from getting close enough to the first one to change its sound from short to long: 

maid,  made,  but  madder;

dine,  diner,  but dinner.