Posts: 32935
Location: Rhinelander, Wisconsin | -------------------------------
One reason that boat porpoises at that speed is weight distribution and the lift at that rpm to the pad. The boat is trying to lift to the pad, but can't quite get it done and literally falls off again, causing the boat to 'bounce'. At about 3800 to 4000, the engine needs to be trimmed down. One way to fix the issue is a TH Marine 5 degree motor wedge, giving you 5 degrees more negative tuck. Another is to try a Rev 4 prop which may give you enough lift to power through the porpoise trimmed down, and no 'bounce' will be noticed.
The vented prop allows for gasses to escape quickly giving you a better hole shot, and with engine height correct should be sealed up by water flow.
http://www.mercurymarine.com/propellers/advantage/technologies/pvs/
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Take a look at that link. According to Merc, the time the vents come into play is powering up by allowing exhaust gasses to flow over the outside of the prop increasing RPMs and shortening hole shot; once you are underway, the vents are sealed by head pressure with water flowing across the hub.
Three blades are way faster than 4, so a 3 blade prop will offer a better top end and slower hole shot. Advanced designs like the Rev 4 offer more 'lift' that allows the hull to have less wetted surface and therefore better top end despite the fact you are spinning four blades. With that also comes better ability to keep the nose of the boat down while the boat is coming onto the pad, which is in that 4000 rpm range. The rev 4 is an ideal wheel for big walleye boats like the 690, 620, and 621.
64 is danged fast for that boat running a stock 175 and standard load.
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