Re: water surging
Not being tacky but if you didn't have that gauge you wouldn't be worrying about it. Just like idiot lights vs gauges. If you didn't have an oil pressure gauge you wouldn't be worrying about it dropping at idle during hot weather driving.
Most engines of today's design have scoops on the lower unit that use ram water in addition to impeller pressure to produce engine cooling. You can see the indentations on the lower unit preceding the holes....just like air induction on airplanes. The ram pressure helps the water flow when the impeller becomes a centrifugal pump at higher rpms rather than a displacement pump when at the lower speeds.
So since you have 2 forces working here and over the operational range and conditions of a lower unit there will be a lot of different forces at work. Surely you will have variances.
