This is also sadly brute-forcish:

Code:
#!/usr/bin/ruby

def find_num(x, try, max)
    # Output intermediary steps if you want
    # puts "#{x},#{try},#{max}"
    return false if x % 5 != 1
    return true if try == max
    return x if find_num( (x-1) * 4/5, try+1, max)
end

1.upto(99999999999) do |x|
    if(ans = find_num(x, 1, 5))
        puts "Answer: #{ans}"
        break
    end
end
Assuming this program works (and I have no idea if it does) you can figure out an arbitrary number of pirates greater than 1. The answer to 8 pirates is (SPOILER)390621. The answer to 9 pirates is (SPOILER)1953121. The answer to 10 pirates is (SPOILER)9765621. This program appears to take exponentially more time as you increase the number of pirates linearly, which means it's a sucky algorithm. 11 pirates would take many minutes to solve.