There are differences between radomes and open arrays:
1. Open arrays generally have tighter beam width for more precision.
2. Open is generally unsuitable for a sailboat due to things getting caught on the array like sails and halyards.
3. Open also takes more power and is larger by at least 1 foot than even the large (2ft.) radomes.
Get the 18" 2kw or 24" 4kw radome depending on your needs. Most folks are happy with 2kw units but you do get increased distance and precision with 4kw.
Full spec comparison here: http://www.koden-electronics.co.jp/P...ds50r63r-e.pdf