Nanda - you Really Really don't want to solo an 18 footer or even 17 footer tandem - maybe if you are just going out to a farm pond, but otherwise not - way too much surface area for the wind to blow you around - it also takes more horsepower to get the bigger boats up to cruising speed

the Wenona Prism and Wilderness are both solo canoes - can't be used for tandem for any practical purposes

I've once used a solo boat for tandem on a local lake - you have to sit on the floor and it is harder to paddle that way - also decreases stability