The problem with hashing is that you do not just want a number, but a number that is between 0 and the size of your table and that, over a reasonable distributi

The problem with hashing is that you do not just want a number, but a number that is between 0 and the size of your table and that, over a reasonable distribution of objects, gives numbers that are uniformely distributed in that interval. You also want to avoid some obvious collisions (say, if you just add the letters of a word then all anagrams will hash into the same place, which is bad). Converting a word into base 26 will give you a huge number. Over six or seven characters and your hash does not fit into a 32-bit integer, and over 13 characters it will not fit into a 64-bit word. So he chooses to multiply by number that is prime in relation to the table size because that would definitely avoid cycles (say, if you've got a table of size 16 and double the number each time there are many positions into which words with that prefix will never hash, which can be a very bad idea).