It could also be that the bottom is of the type that does not generate a strong response?
I mean probably the returning signal is a proxy for
radio hardness of the bottom. Say rock can be read at 100 m but ooze at only 80?
Our in-hull mounted (but thru
hull type) Ray reads to about the same depth as your
sensor but it did read to about 120 meters back in
Sweden.
So, my guess is age/condition of the
sensor may count or else there is a big variation in how strong an echo you get depending on what the bottom is made out of.
b.