You should be able to move your rowwing between boats easily. I have mine fastened in with "hand knobs" instead of traditional nuts, so no tools required to install or remove

>>>>>I'm having trouble finding an example of a dory with a drop in rowing unit...I enjoy rowing sliding seat and am ready to buy a unit but am a bit apprehensive considering the price.  The boat sails and rows perfectly as is, but speed and efficiency when rowing would be great.>>>>>

 Speaking as the Northeaster Dory's designer, I was startled at how well it does with a single sliding seat. It was fast, and I felt like it could keep going in huge waves with the sliding seat.  I didn't think such a large boat would feel fast with a single sliding seat, but it does.

(Tandem sliding seats work in that boat IF you don't install the sailing rig, otherwise, only one.  I've not rowed that permutation myself.)

>>>Does anyone (John) have some insight into how the drop in unit compliments the Northeaster Dory with a sailing rig?  Specifically, do the modifications inhibit movement or comfort when the unit is out of the boat (under sail)?>>>>>

Sailing versions of the Northeaster Dory accept a single sliding seat.  The seat has to move aft a few inches to accommodate the daggerboard trunk, which in turn requires special brackets which we'll provide for you.  

In theory you could still sail the boat with the slider in place, but in practice I think it'd be awkward and unfun.  It only takes a couple of seconds to dismount the rig from the sailing version, however.

>>>>>>>>My brother is building the tandem wherry, and he's got one unit already.  Could I move my dory unit between the two boats?  It would save a lot of money, no point in having 3 units between two people.>>>>>>>

One of your brother's sliding seat units will drop right onto the brackets in the Northeaster Dory.  So don't buy three.  

Northeaster Dory video, including sliding-seat action.

John,

thanks for the thorough reply...I will (some day soon) buy the sliding seat for the NE.D. and my brother can borrow it from time to time to tandem his tandem wherry...this is what I was hoping, thanks for the info.

I expect he'll be one of the first to build the tandem kit, as there was not yet a book when he ordered it...he's stitching it up today!  very exciting.