On a mooring at Shelter Island over Labor Day weekend this guy in fins and goggles swims over to us and asks for some help. He stated he has to ferry this guy's (stunning 70's, fresh Imron) Hinckley from there to Nantucket at first light and so he is removing some barnacles from the prop and checking out everything else under there. Came over to us since a tender dropped him off and now he can't climb back aboard (!!!). Long story short, of course I ferried him over in my dink and he went aboard. Next day I'm wondering how my prop looks, so I went for a dive. I had noted my speed had been off, but we had been towing a dink and had some bad waves and all I have is 10 HP for the 7800 lbs. Turns out I had 15-20 small (1/4" maybe) barnacles on that two bladed prop and clearing them did seem to make a clear difference. I would say 1/2-1 knot in fact. So for best late summer and fall sailing.....take a dive!