WORD SPIRAL

Starting from 1, fill in the grid clockwise with four-letter words. The last letter of each word becomes the first letter of the next to reveal a type of warship. 1. Slightly open 2. Thoroughfare 3. Nobleman 4. Cheese 5. Major planet 6. Make dirty 7. Plant part 8. Not genuine 9. Otherwise 10. Egress 11. Aquatic bird 12. Undiluted 13. Conservative 14. Unit of length 15. Foolish 16. Small branch