In order for any 2 lights to be distinguishable as separate lights they need to be of similar intensity and separated by a distance suitable to the needed visibility. In short, they need to be at least one yard (3 feet) apart for each mile of visibility required. 3' at 1 mile 6' at 2 miles 9' at 3 miles 12' at 4 miles 15' at 5 miles. Etc. The actual distance is more like about 40 inches, rather than 3 feet, but for sake of simplicity, feet will do. The other factor which affects visibility is height above ground level(water). Curvature of the earth and all that. 1' = 1.2 nautical miles 2' = 1.7 nm 3' = 2.0 nm 4' = 2.3 nm 5' = 2.6 nm 6' = 2.9 nm 8' = 3.3 nm 10' = 3.7 nm 12' = 4.1 nm 18' = 5.0 nm 26' = 6.0 nm 36' = 7.0 nm As you can see, the absolute distance your lights can be seen is derived from their height and that of the observer. The worst case from your standpoint is an observer whose eye is 1 foot above water. In practical terms your observer is 3 to 6 feet above water and your light is 3 to 6 feet above water or: 3 + 3 = 6 overall, which makes for 2.9 miles of visibility. This probably is the worst case for most small vessels in the 30-50 foot range. The best case is more like 12' + 12' or about 6 miles. For boats in the 50-70' range, the heights are about those of worst case for the next smaller group or about 6 miles and best case = 18'+36' or about 8.5 miles. Taking these numbers. 2.9 miles to 6 miles. 6 miles to 8.5 miles. For a sailboat with a 50' mast the visible distance is about 9 miles. Keep in mind that all your lights will not be at the same height. The masthead, normally would be the very highest. In order for the lights to visible separately vessels with lights expected to show: One mile, the separation is 3 feet, vertically or HORIZONTALLY from any other light. 2 miles = 6 feet 3 miles = 9 feet 5 miles = 15 feet. Notice that as a vessel gets smaller than 65' the physical space necessary to create this separation is also disappearing. In practical terms, lights must be at least 3 feet apart under any circumstances, in order to be seen Must be 6 feet apart to be seen separately at 2 miles. From this you can quickly determine the distance YOUR lights can be seen as separate, by simple inspection with a tape measure. Measure their height, from that you estimate the DISTANCE they can be seen, making a guess as to the height of the observer on another boat. Observers on larger vessels with their eyes at higher elevations will be able to see you further than smaller boats. ALL of this ignores the candle power or light output. Which must be sufficient to be visible in absolute terms. Figure 10 watts for 2 miles, white light or 1 mile for colored. Figure 25 watts for 3 miles, white light or 2 mile for colored. If you mount only the usual masthead, side and stern lights, you should have no major trouble finding space to locate them for separation and distance. If you add other lights for towing, not under command, etc. the problem becomes far trickier. Remember, vertical separation and HORIZONTAL. Capt. Mike Maurice Wilsonville, Near Portland Oregon