Mystic — The four-and-a-half-year effort to restore the whaling ship Charles W. Morgan reached a milestone Friday afternoon as the last plank was wedged into the hull of the 172-year-old vessel.

More than 200 Mystic Seaport employees and spectators gathered in the museum shipyard to watch workers remove a 20-foot-long piece of yellow pine from a steam box that keeps the wood flexible. A forklift brought the plank up to scaffolding about 15 feet off the ground, where eight workers then carried the board to the last remaining opening and wedged and pounded it into place.

Finally, a shipyard worker drove a gold-plated spike into the plank as fellow workers shook hands and high-fived one another and the spectators below cheered.

Museum President Stephen White reminded the crowd that while the installation of the final plank was an important milestone, much work remains to be done, such as caulking and painting the ship before it's launched at a July 21 ceremony.

Work will then continue to prepare the world's last wooden whaling ship to sail to ports around New England late next spring.

"The Morgan is not done," White said Friday. "We have much yet to do to prepare her for her 38th voyage almost a year from today."

White said Friday was a proud day for the museum but an even prouder day for the shipyard workers.

"The only place this could happen is here at Mystic Seaport, thanks to all of them," he said as he called all the shipyard workers to come up and stand next to him during the ceremony.

Shipyard director Quentin Snediker said the crew members working on the ship all have skill and talent, but more importantly they have a reverence for the work that went into the ship in the past.

Highly skilled shipwrights and riggers have come from around the country to work on the $7 million restoration.

The launch, which will occur on the 172nd anniversary of the day the vessel was launched in New Bedford, Mass., will feature documentary filmmaker Ric Burns as the keynote speaker. After the launch, the Morgan will remain at the shipyard's lift dock, where shipyard workers will restore details of the ship and do all the rigging work. The ship is expected to remain open to visitors during the work.

In addition, modern elements will be added for the 2014 voyage, such as lifeboats, electronic navigation, a generator and a firefighting system. Much of that will be hidden from public view to preserve the ship's historic integrity and will be removed after the voyage.

Since the work began in November 2008, each piece of wood that has been removed and replaced has been meticulously documented with photographs, laser scans and X-rays. When the project is complete, the Morgan will still have between 15 and 18 percent of its original wood, including its keel.

The majority of the restoration has centered around the need to replace some of the deteriorated framing below the water line. The interior planking had to be removed to reach the framing, and some of it was replaced as well. Exterior planking was also replaced, with the last plank slated to be installed next month. The work is expected to preserve the 113-foot-long ship for the next 30 years.

Next spring the Morgan will first travel to New London, where it will spend three weeks preparing for the voyage and doing sea trials.

Escorted by a tugboat and the Roann, a fishing vessel restored at the Seaport, the Morgan will then sail to Newport, R.I., Vineyard Haven, Mass., New Bedford, Mass., the Cape Cod Canal, Provincetown, Mass., Stellwagen Bank in Massachusetts Bay, Boston, and back to New London before returning to the Seaport in September.

j.wojtas@theday.com