The Mysterious Neighbors — Frances K. Judd

"Stay away from The Witch . . . It's a jinx boat!"

Such a grim warning would have frightened a less courageous girl than Kay Tracey. But to this amateur detective the threat is only a challenge to make a trip on The Witch.
Kay and her friends, the Worth twins, peppy Betty and studious, quiet Wilma, board the trim little houseboat. Sailing close behind and always mooring near them, is the strange black Ajax, guarded day and night by a vicious watchdog. Two mysterious, unfriendly men are aboard, armed with guns and wary of any boat approaching them. At night, from the darkened Ajax, a flashlight winks a message in code and is answered by a beam from the shore. Can there be any connection between this sinister gang and the recent jewel robberies the police had been unable to solve? Kay, Wilma and Betty find that their holiday cruise has turned into a precarious chase. The trail leads the girls into danger on the Sharon River from which there seems to be no escape . . .