So what is the difference between ‘ship’ and ‘sheep’?
1. It is not about length.
Many students think that the difference between ‘ship’ and ‘sheep’ is the length – ‘sheep’ is longer because it has two vowels in the spelling. This is not, in fact, completely true and can lead to students making words with /i:/ unnaturally long. Vowel sounds change in length depending on the level of stress placed on them and the following sounds. If any of /p, t, k, s, z, ʃ, tʃ/ follow the vowel, it will become shorter. Since ‘sheep’ ends in a /p/ sound, the length of the vowel is actually reduced; there is not very much difference between the length of ‘ship’ and ‘sheep’ for this reason.
Other pairs of /ɪ/ /i:/ words clearly show a difference in length, for example ‘lid’ and ‘lead’ as the long vowel is not reduced due to the following /d/ sound.
Consider the following: if we say the word ‘ship’ and use a very long /ɪ/ sound, does it change into an animal? Likewise when we pronounce ‘sheep’ very quickly with a shortened /i:/ does it change into a boat? The answer is no in both cases. A vowel is determined by position, not length.
2. It is about position.
When you pronounce ‘sheep’, your jaw should be nearly closed, your tongue very far forwards in the mouth and your lips spread. To pronounce ‘ship’, your tongue should be further back in the mouth, your jaw should be more open (close-mid) and your lips should be more relaxed. On a vowel grid, the two sounds are shown below:
3. Mother tongue interference can cause the problem.
Many languages (including all Slavic & Latin languages) only have one vowel sound in this area of the mouth. If your language has only one, it is quite likely that you will use that vowel for both /ɪ/ and /i:/ in English, causing a pronunciation error. You can overcome this by learning the English mouth positions and mastering short, long and reduced vowel lengths.