Portsmouth-based Able Seaman Daniel Goffey had been following the sailor up the ladder when he grabbed her behind - then 'sniggered' as she slapped his hands away.
The 25-year-old pervert sexually assaulted the crew member after drinking alcohol at a navy barbecue onboard £3.2bn aircraft carrier HMS Prince of Wales.
His victim, who can't be named for legal reasons, had been trying to get rid of 'intoxicated' AB Goffey and was left feeling 'very uncomfortable'.
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Now, after denying sexual assault, AB Goffey has been convicted following a three day trial at Bulford Military Court, Wiltshire.
The engineering technician admitted he could be 'very confident' after a few drinks but insisted he didn't grope the sailor.
Bulford Military Court heard a number of mess parties broke out across the ship after the organised deck barbecue finished.
AB Goffey arrived at his victim's mess because he had 'run out of booze' and wanted to persuade her and others to go to one of the other parties, the court was told.
The female sailor and others tried to draw AB Goffey away by going to another area of the ship, which meant going up a ladder.
The court heard that as the group went up the ladder one-by-one, AB Goffey sexually assaulted the sailor.
Giving evidence, she said: ‘We all went up single-file.
‘I felt two hands on either side of my butt-cheeks for about two seconds.
‘I turned around to push him away and he just sniggered.
‘I slapped his hands away.’
AB Goffey denied touching the woman's behind while he was following her up a ladder because he would not have been able to balance himself.
He said: ‘You have to grab the handrails to go up those ladders.
‘Everyone was drunk so you would definitely have needed to grab the handles.’
AB Goffey, now understood to be based at HMS Nelson, in Portsmouth, will be sentenced for sexual assault at a later date.
AB Goffey was cleared of two other counts of sexual assault. He had been accused of touching another female sailor's bottom and kissing her.