Why no modifications allowed?
Each Area Elimination is open to all single or multi-crew catamarans under 22 feet in length with a current US SAILING Portsmouth number. Boats with Provisional Portsmouth handicaps (e.g., those with brackets [ ] or parentheses ( ) around their rating), are ineligible to compete in Area Eliminations. Only boats in their original One-Design configuration are eligible. Modifications requiring Portsmouth “modification factors” are not allowed.
Statistically speaking, boats with provisional ratings (brackets and such) do not have enough data from which to establish a reliable figure. Therefore, the rating's accuracy is suspect. Modification factors are pretty much a 'rule of thumb' that tries to simplify the countless and complicated variations that we sailors can come up with. While the Portsmouth committee recognizes the need to establish some sort of figure for the modification factors and the rare boats, these numbers may not necessarily be as fair a representation of the capabilities. No handicap system will ensure all degrees of fairness but the Alter Cup attempts to make the qualifiers somewhat more level by not allowing boats that have unsupported ratings.
If you want to get the brackets off your boat rating - race more and make sure the race committee is submitting their results to the portsmouth committee!
To allow modifications (like the fat boy) and the others just makes a bad system worse!
There are some that believe that the eliminations should be sailed 1-design only. In principle I agree but, in reality it's just not feasible. There also those that believe exploiting a loophole is a test of sailing ablity (which is a load of crap). Not allowing mods eliminates that as an issue.
So the way I see it the "DP-N" with no mods is the line in the sand. Everything is a compromise.
It could be worse... we could be using PHRF!
Ding
Can't use D-PN either. Must use wind adjusted handicap ratings. The best of the Portsmouth numbers are those that are adjusted for wind. The only time you should use D-PN is in distance races or situations where the wind is expected to vary widely over the course of the race.
p.s. Trivia - What does the D in D-PN stand for?
That's a good point...I can only think of one regatta I attend (out of about 17 a year) that uses the Beaufort Ranges on the ratings. I don't know what the "D" stands for but I suspect it has something to do with this!
I'm going to guess the "D" stands for "designated," although some would probably say it stands for "damn." I think it should be a "B" standing for "basic" or "base." It's embarrassing that I don't know the answer to the question. And it is annoying that initials are used that people don't understand.
"D" does stand for designated. When we ran the Area D North eliminations last year, we looked into what we had adjusted at previous regatta's. We had people with Sq top sails that were'nt measured, taller mast, a highly modified Hobie 18 which also has a different rudder system, sq top main, and we think has been lengthened to 20'. We have boats with boomless sails that were modified from boats with booms(G-Cat 5.0). The list goes on and on, and this is just from the SEACATS fleet. Now all of these guys take a hit, but how much? Its very subjective, and you just dont want that in an Alter Cup qualifier, I agree that it should be sailed on a one design boat, but thats not going to happen around here, nor in many other places.
David Mosley
Well, that's silly. If this is the North American Portsmouth Yardstick, why wouldn't it be the NA-PN instead of relating to some sailing association in England? Actually, the "D" should stand for "Darline." 
I second that! D for Darlene!