WASHINGTON (AP) _ The spinning propellers of power boat motors cause thousands of deaths and injuries that could be prevented with the addition of protective devices, a consumer group said Thursday.

The Institute for Injury Reduction called for ''propguard'' devices to be installed. A Coast Guard analysis in 1979 showed as many as 3,400 injuries are caused a year by the propellers.

It said the leading makers are Mercury Marine of Waukegan, Ill., and Mercury Marine, based in Skokie, Ill.

No one from either company could be reached for comment when calls were made late in the day.