I have a bias here cause I've owned a IP-26mkII and a IP-35.
First off comparing an IP-31 with a Tatran 37 is like comparing apples to green beans. Both are great sailboats but built to different sailing design spec's.
IP's are designed for room and cruising. Where as Tartan's are designed for club racing
Going to either will be a big step from your W W Potter 19.
Slowpoker did a great job laying out the differences in sailing characteristics.
You need to ask yourself a few questions.
What kind of sailing do you plan on doing? Crusing? Racing
Do you plan on overnighting on it or spending weeks on-board?
How many people do you plan on having on-board?
IP's are indeed roomy. My 26 had more room below than most Catalina
30's. But the price
she paid for this room was limited pointing ability. Plan on a 90 degree tack window with the IP's. Tartans have better than a 60 degree window.
My IP-35 has the room of most 40's, but she still doesn't point worth a damn.
IP's also have a shorter stick (mast). Meaning they are going to carry less canvas
. But this has a plus side. What canvas
they do carry is closer to the canter of effort. So they heel less and can carry the sails longer before reefing. To the best of my knowledge, neither of my IP's have heeled over 20 degrees. And I don't even think about reefing until the wind gets to the high teens, low 20's.
However, IP's do have some flaws.
Number one, and it's a biggie.... Chain Plates! The 31 will have the old chain plates and if not already done, with the age of her (1988), replacement costs need to be factored into the offer price. Replacement cost runs from $10k to $20K.
If you plan only on "protected water" sailing, their condition should not be too much of a concern. However, off shore is a whole nother mater.
In either case, a chain plate
failure can mean the loss of the rig.
Second issue are the tank's. IP used aluminum tanks
. Which are known to leak after a few decades. I believe the 31's tanks are easy to get to. But on IP's larger than the 31, tank replacement is a royal PITA!
Lastly, and not a flaw but a fact about any "full keel" boat, reverse maneuvering (backing) is a steep learning
If you have not already mastered using Prop Walk and Prop Thrust when backing (or any tight maneuvering for that matter), a full keel
boat will make you look like a drunken land lubber, when docking
Now for the good.
IP's are well built boats.
I know of no IPs that have or had a blister problem. IP did not use a "cored" deck
All factory attached deck hardware
have aluminum backing plates
Starting with the IP-31, all port lights are SS not plastic. Hatches are off the shelf units. Making repair
and replacement a snap.
Factory standing rigging is over sized.
With a full keel, all IP's have a shallower draft
than comparably sized boats. When grounded, they will only tip so much before resting on the hull
The "thru the knee's" steering
is a sore topic for some. But I like it because it is a very simple and robust "rack & pinon" steering
system. READ...No Cables
to maintain or adjust.
And did I mention interior
You won't be disappointed with an IP.
But heed the advise from A64Pilot, if you are looking for a day sailor, an IP will be overkill.