Gavin Atkin has produced an interesting design for a rowing skiff called Julie, flat bottomed, which at nearly 16ft long is about as long as you can get out of two sheets of 8ft by 4ft ply.
She should be a reasonably fast and stable river boat, nice for fishing or just mooching o'er the stream.
One thing I disagree with Gav over, however. He says a boat that long is not car-toppable. Phooey! I used to pop my 19ft Otter skiff on the roof rack all the time until we got a new car and my dear lady wife started worrying about scratching the paintwork with the outriggers.
With Gav's Julie, removing the rowlocks removes all risk of tramlines on the roof of the car, and a line from the bow to the front bumper removes all risk of the boat coming adrift, however long. The picture shows Gav's model placed on a glasses case that is unnervingly similar in size and shape to my estate car, which shows it will be perfectly car toppable. All you have to do is put it the other way up and strap it down securely.
So build her light, I say, and you will be able to row faster as well!