You need to check the serial number again. The Old Town canoe with the number 29039 is 16' long, built in 1913. Check both ends of the canoe -- the serial number should be stamped into the end of the stems at both ends of the canoe -- the stampngs are often poor, or covered by old varnish, paint and/or dirt. Posting pictures of the numbers here may help.
The OTCA model, like most Old Town models, was built in several lengths -- there certainly are 18' OTCAs.
For example (and I am just guessing about what the correct number of your canoe might be), The Old Town canoe with serial number 129039 is an 18 foot long, CS (common sense or standard) grade Otca model with open spruce gunwales, birch seats, decks and thwarts, and equipped with a keel. It was built between January and July 1940. The original exterior paint color was dark green. It was shipped to Ticonderoga, New York on may 26, 1941. A scan of this build record can be found by following the link behind the thumbnail image attached below.
This scan and several hundred thousand others were created with substantial grants from the Wooden Canoe Heritage Association (WCHA) and others. A description of the project to preserve these records is available at
http://www.wcha.org/ot_records/ if you want more details. I hope that you will join or renew your membership to the WCHA so that services like this can continue. See
http://www.wcha.org/about-the-wcha/ to learn more about the WCHA and
http://store.wcha.org/WCHA-New-Membership.html to join.
It is also possible that you could have another number or manufacturer if this description doesn't match your canoe. Feel free to reply here if you have any other questions.
Greg Nolan
President, WCHA