I made the beam narrower by multiplying the half bredth offsets by .825. That made it about 17% less but kept the shape true. To lengthen it I added 7.5% more to the form spacing deminsions it came out actually 21.5 feet in length. I would mention that making a Tandum 17 or 18 feet long would require a beam much wider than the original Sea Tour Double. If you shorten it 10% than add that 10% into the width. Shorter and wider is less performance however that may not be the issue. At least with a SOF you are not out much cost in your experimenting. Also By lashing the joins it is easy to take it back apart for corrections.

Good luck