its difficult on some boats unless you are running a speed app, to know your exact speed, on Water rail there is a line on the rev counter marked 5mph, now depending on tide conditions that can be anything from 4 mph to 6 mph (or more if heading down south through great yarmouth, I was doing 6.3 mph on little more than tickover last week.)
If you ever hire a Marthams boat, you dont even get the help of a rev counter.
but I have noticed on water rail, that there is no appreciable wash until you are doing 0ver 2400 rpm, ie 6mph through the water, and then maybe 4" of wash at the bank. at 1600rpm (supposedly around 4 mph) the wash is barely noticable, when I was going through horning i was making rpm for 4mph, and travelling (by gps) at 4.6 going with the tide. its really tricky because water rail has a harmonic frequency between 1200rpm and 1600 rpm that means the whole boat shudders and things come crashing off the worktops, so its back off until the shudder starts then nudge forward until it stops, and thats as slow as you can go without dropping back to tickover.
Strangely most of the Martham boats I have used have a similar issue at low revs just above tickover, the engines seem not to have a happy spot wher ethey can do 4 mph, its 2.8 mph or 4.2 mph, with not an option between thats not going to shake the boat apart.