GRP/Fibreglass. A good material for lightweight offshore boats and, provided any knocks and scratches are touched up on a regular basis, will give many years of service. We do not recommend the use on large inland waterways boats though - not really tough enough for "Contact Boating".
Funny, but six years later
Google gives this link first of all - in answer to g.r.p. hull.
Thank you tdw, you wombat - you're a cult favorite when grumpy, I guess!
it is the first link because none of the other articles use the period punctuation in the abbreviation ... it's all in the Keywords! Something to consider if you are building a web site and want your web page to show up first in a google search ;-)