Lifeguards made a series of rescues this week at Lucy Vincent Beach in Chilmark, where rip currents have created unsafe swimming conditions in spots, Chilmark beach superintendent Martina Mastromonaco said yesterday. On Monday, two women were rescued by lifeguards after currents pulled them out to sea.

Mrs. Mastromonaco said there was a rescue on Tuesday and an additional series of rescues on Wednesday. She said the trouble appears to be happening at a specific time in the tidal cycle. On Monday a rescue took place shortly after 10:30 a.m.; there was a rescue on Tuesday at around 11:30 a.m. and one on Wednesday at noon. Each of those times was about two and a half hours after low tide. “It is predictable at this point,” Mrs. Mastromonaco said. The tide is about an hour later every day.

Two women, who are sisters and summer visitors, were rescued by two lifeguards who are brothers on Monday morning. The lifeguards are Bennet Schwab, 17, and Evan, 20, both from Chilmark. Bennet is a senior at the Martha’s Vineyard Regional High School; Evan is in his third year of college.

Mrs. Mastromonaco said the women were swept out about 50 feet from the shore by the time they were rescued. The brothers used torpedos, floating buoy-like devices. An unknown experienced swimmer also tried to help.

On Tuesday, six people were rescued from the water in a scene of minor confusion. “It was difficult trying to figure out who needed help,” she said.

And again on Wednesday, “We had three people [rescued],” she said. She said Lynn Murphy, 32, of Chilmark and a seasoned head lifeguard, participated in the rescue along with Darren Stobie, 20, of Vineyard Haven.

“A 12-year-old got into trouble. His grandfather had gone into the water to help the child and he too needed to be rescued,” Mrs. Mastromonaco said. Juliet Burkett, 20, a third lifeguard from Chilmark, provided backup.

After the incident, Mrs. Mastromonaco said there was a touching scene. “The little boy hugged Mr. Stobie and said: ‘You saved my life,’” she said.

Lifeguards have a flag system for marking troubled waters: green is okay, yellow is caution and red is danger.

Rip currents and riptides are different and occur at unpredictable times and locations. “Environmental factors can help you predict as the tide changes,” Mrs. Mastromonaco said.

She said the ocean can sometimes be deceptively calm, and swimmers often look for places where waves are low and the water looks smooth.

There was one particular spot on Wednesday that fit the description. “But that is the place where the rip current is working, because it is pulling the water out,” Mrs. Mastromonaco said. “Yesterday we had no waves, but as soon as you got in, you got pulled out to the right,” she said.

This is Mrs. Mastromonaco’s 16th summer on the beach. She said a number of people have told her this summer that the currents are different. She thinks it may be due to the movement of sand and beach, following heavy erosion this year.

“Currents are so strong here. There is a tidal pool, which is created by a big sand bar,” Mrs. Mastromonaco said. At low tide the tidal pool is easy to watch and enjoy. But as the tide rises, the pool might not be so friendly, she said. She believes an offshore sand bar is contributing to the treacherous currents.

Swimmers caught in a rip current are advised to swim parallel to the beach until they find a spot to swim ashore. Do not fight the current by swimming against it.

Mrs. Mastromonaco had high praise for the lifeguards at Lucy Vincent this week. And she offered one more piece of advice for swimmers:

“Learn how to signal for help. Waving your hands is a call for help,” she said. And to keep things clear, she urged swimmers not to wave to their friends from the water. “Someone may interpret what you are doing as a call for help, when you are really saying hi,” she said.