The prop you're describing is probably the 18" narrow blade prop. They also make a wide blade prop. I'm pretty sure the narrow blade was made for large scale air racing (T-6 style racing, I think!). The narrow blades allow for higher rpm's.
I currently use the 18-8 narrow blade on my IMAC plane. The pull is excellent. The only downside is the lack of any real downline braking. I've tried the wide blade as well, but it has quite a bit more mass in the blades and just didn't turn as well as I would have liked.
Just so you know, I currently use a Saito 180 with 30% high performance heli fuel---Excellent power! In my last plane, I used the same props with a Supertigre 2300 on 15% omega fuel---very good power as well!

Gary Hunt
Rantoul, Il.