(Editor's note: This story has been edited to correct an error.)

Groton — A preliminary estimate released by the town this week shows the Groton Town police boat sustained more than $18,000 worth of damage in an Aug. 31 accident on the Thames River.

The lower unit of one of the 27-foot boat's two 300-horsepower outboard motors was torn off when the boat struck rocks outside the channel north of the Mohegan Pequot Bridge in the Montville area, according to an incident report compiled by Capt. Steven Sinagra, head of the department's marine patrol unit.

The report was made available Thursday.

The accident occurred shortly before noon while being operated by Sgt. Kelly Crandall, who was training under the direction of certified boat operator Sgt. Douglas Hoffman, according to the report.

After leaving the no-wake zone near the bridge and heading north, Crandall had accelerated to get the boat to plane and throttled back . Both officers then spotted what appeared to be a log in the water, the report states. Crandall turned west to avoid the object and struck a submerged dike at the edge of the channel.

Hoffman said the boat was traveling at 23 knots at the time of the accident. Sinagra said he spoke with a Norwich police officer who was also operating a boat in the area at the time and confirmed the rocks were not visible prior to low tide.

Groton Town Manager Mark Oefinger said the initial damage estimate is from Spicer's Marina. The town's insurance adjuster is preparing its own estimate.

Oefinger said he has included the report of the accident for Town Council members in advance of any possible discussion at next week's meeting.

The boat was purchased in 2012 with a portion of a $433,000 federal port security grant.