Kurt -
I concur with your conclusion regarding the type IXD, but I think the 35% to 40% increase in dimensions between the type VIIC and the GATO class might have made a difference. I suppose it would depend on which of the following factors was crucial to the aircrew noticing the sub (differentiating the sub from background) -- motion, contrasting color or brightness, or angular dimension of the sub from the observation point. My experience as a lookout indicated the relative significance of those factors was in the order listed, and with size influencing only the last, your overall assessment is probably correct.

As an aside, I conducted water clarity studies professionally to determine how transparency influences the heat balance (loss or retention of solar energy) and productivity (depth at which light is adequate to support photosynthesis) of various bodies of water. We used a standard diameter disk about the size of a large plate or small platter. The disk is divided into quadrants with alternate quadrants colored black and white. The disk is supported by a measured cord attached to its center and slowly lowered into the water until it just disappears from view at the surface. That depth (measured on the cord) is recorded, and the disk is then lowered another meter or two. The disk is then slowly raised until it can first be visually detected from the surface. That depth is also measured on the cord and recorded. The latter depth is invariably less than the former; and the two are averaged for the standardized measurement of clarity. Watching people use the disk, one can note the use of movement to verify their observations.

Applying this to the question at hand, I suspect the real trick is causing the lookout to notice whatever visual cues are available. Perhaps the lookout first noticed a sheen of leaking fuel, an unusual surface turbulence, or unusual behavior of marine life before he actually saw the boat. Lookout training tends to be neglected when other sensors provide most of the contact information. Historical anecdotes describe some dramatic differences in the abilities of the sailors of different nations to visually detect opposing forces under similar circumstances. Japanese airmen may have been better trained in nautical observation techniques than their allied counterparts, or it may simply be easier to stay attentive under more pleasant flying conditions. =AL=