That's a can of worms...
When I bought my D in '88 I had a SR Mariner knot meter installed. I was amazed at how fast the boat was...so amazed I didn't believe the knot meter. So, I ran a measured mile...the numbers agreed to within a fraction of a knot...still didn't believe it, so I bought a knot stick...numbers still agreed. Finally, when I got my GPS I was convinced.It is quite easy to achieve 6.5 to 7 knots if the boat is tuned the way the manual describes.However, using that tuning, the C has excess weather helm, the mast falls off to leeward in all but the lightest conditions and it is very hard to make it go fast without stalling the stock rudder and rounding up.I have since tightened the shrouds, eliminated the mast rake, tuned in a little pre-bend and replaced the rudder. Since then, 7.5 to 8 is possible on a close to broad reach in ideal conditions.My knot meter goes to 10 kts and when surfing we can keep it pegged...I have seen GPS readings of 12 while surfing.Balance, tuning, fore and aft trim, sail trim and keeping the boat no more than 15 degrees over seem to be the key to getting max. performance from the boat.Dave Gerr (one of the most widely respected naval architects practicing today), puts the maximum speed of the 26C under sail (non-surfing) at 9.5 knots. I have never achieved that...but it is something to aspire to.The D usually PHRFs around 216 to 219 and the S usually PHRFs around 219 to 222, suggesting that the D is potentially marginally faster.BTW, I estimate my sailing weight to be approximately 3500 lbs.