Not unusual to lose signal above 50 mph as the hulls tend to create an air pocket, if you check underneath with a straight edge you will see that the very bottom of the hull tends to be a bit concave in the last bit around the sump which aids in handling I believe. My 91 P2 had that, and my 01 P3 has it too. I do not worry about the depth readings when going that fast anyway, if I want to check something out I slow down and survey the area. I know some other boat brands will read above 50 but not many, I have been in about 15 different brands over the years with partners, and have only seen a couple that would read consistently above 50-55 mph.
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