(ps. forgot to address your question on chafe with sentinal).
No, I have never seen any chafe associated to the sentinal. I guess it all depends on how/what is attaching it to the rode. I have see very fancy sentinal attaching schemes, with rollers and such (I think in westmarine catalog). But I just use a big brass "snap hook" which seems to be very smooth and the sentinal is never in one spot for very long. If there is any chafe that I see on the rode then its most likely from going through my windlass.