Hey Trevor,

I took a quick look at the NE Dory and with the mast so far forward if you tried to implement a jib boom, your "J" measurement would be way too small for a jib any larger than a handkerchief.  The main photo for the NED on this site shows John sailing with a jib.  Also, a jib might get fouled on the sprit boom/yard when tacking depending on the gooseneck/snotter arrangement.  This may be why they specifically go with a sloop rig.  There are other factors too like combined center of effort, etc. and how high it is compared to the righting moment.

Skully presented some good reasons one does not see jibs with spirit rigs, but you should look back through the forum - there are several discussions why a jib is not good on an unstayed rig. Without stays it is very difficult to control the luff tension on a jib. If the jib luff sags too much, all the jib is doing is pulling the boat sideways, adding nothing to forward momentum and everything to sideways slide and heel.

That being said, Reuel Parker, in his "Sharpie Book" on page 114 describes a Maryland crabbing skiff with a spirit main and a balanced jib. I will leave it to Reuel to explain it. I also notice the mast is very raked - which might help with luff tension, although the balanced boom on the jib would help alot with that.

Any which way, working to weather with a jib on an unstayed rig would be a very trying exercise.

Ahhh... Excellent point.  I hadn't even thought about the stayed vs. unstayed thing.  Sloop rigs usually require some sort of tension aft to counteract the tension of the jib luff/forestay.  That can be aft swept shrouds (ala B&R rig) which are quite a bit easier to fit in vs. a full blow backstay which is not practical on a small boat.