The two boats are heading at each other head first. Both sensor and ir light source are on bow pointing forward.
We want to avoid head on collisions.
Do not need anything in stern.
Can’t use visible light. Headlights on boats are a no no as they blind other boats. Just red and green bow lights are permitted. Also visible lights could be detected on bridges street lights etc and would create false alarms.
The city skyline has plenty of lights
Ultrasound would not work well as the shells are extremely narrow and present very little profile to detect w US when moving head to head.