Originally Posted by Bayfly
They are designed to hold water around the prop when it's elevated, thus it will support your effort to maintain water pressure, while letting you run more shallow. If there is no water, then there cannot be water pressure!
I use a ShallowBlaster on a 200 HPDI mounted on a tunnel hull and it preforms, as intended.
Your water pressure does not come from the prop. Water comes from the intake grates on the sides of the cases or in some instances, holes in the nose of the case. These devices WILL NOT help you keep water pressure. Once you are on plane, they do nothing.
I also disagree that they are designed to "hold water around the prop" when its elevated. Once you are up on plane and the foil/plate/etc. is out of the water, its just along for the ride. They do "hold" water around the prop on initial holeshot and, act as a large trim tab to keep the bow down for better holeshot.
I run a full tunnel boat and have my engine mounted higher than anyone here. I have tried it all!