An RNLI Selsey spokesman said its all-weather lifeboat was requested by the UK Coastguard to assist a 'medical evacuation' from a 6,000 ton chemical tanker on the south east of Selsey at 6.40pm.
It added: "The Ellen Essberger had a crew member who required evacuation from the ship to a hospital. The ship had been in contact with a doctor via the UK Coastguard whoadvised evacuation by lifeboat as the patient had been suffering chest pains for several hours."
The spokesman said at 7.22pm two lifeboat crew members, one of which is a paramedic, were transferred to the ship to assess the casualty who was believed to be still having a heart attack.
The paramedic thought a transfer to the lifeboat by stretcher would make his condition worse so requested a helicopter lift, the spokesman said.
"The helicopter Rescue 175 arrived on scene at 8.17pm. The helicopter tried unsuccessfully for 20 mins to winch his winchman/paramedic down before at 8.42pm this was completed using a hi line transfer," it added.
The RNLI Selsey spokesman said the helicopter returned at 9.50pm but the paramedic instructed the pilot that the casualty was now in a critical state so would not be suitable for airlifting off.
"Both paramedics suspected the man had suffered a neurological event," it added.
"The helicopter returned to base leaving his winchman aboard the ship."
The spokesman said a decision was made for the ship to head for the pilot station at the entrance to the Solent where a pilot was taken aboard the ship.
"The ship then proceeded into the Solent to a position south of Portsmouth Harbour where the ship came to a stop.
"The lifeboat came alongside the Ellen Essberger and at 11.50pm took off the helicopter winchman, two lifeboat crew members and the casualty in a stretcher using the ships companion ladder.
"A course was set for the Camber docks in Portsmouth Harbour where an ambulance was waiting with local CG officers the casualty was handed over to the ambulance crew at 12.15am Sunday."