Had no problem with Radio Shack cheapie aimed at the companionway.   Motion of the boat  does not come into play that way (long axis of the boat), only a body occupying the "stairwell" sets it off.  Russ S/V ASCENSION  Melbourne,FL   Be Holy or Die Trying"  St Stephen 33AD "Be joyful always; Pray constantly; Give thanks in all circumstances, for this is God’s will for you in Christ.....Test everything. Hold on to what is good. 1 Thessalonians 5:16 "Since you cannot do good to all, you are to pay special attention to those who, by the accidents of time, or place, or circumstances, are brought into closer connection with you." St Augustine