Unfortunately it is the nature of the beast that jet drives aren't as good as props in the rough, that is what you get for having no drive parts below the planing surfaces, but on the plus side the draft of water needed to run is much reduced and it is much harder to break an impeller than a prop.
Jet drives are high volume low pressure pumps, and are nor self priming, when the intake comes out of the water it can't pull enough vacuum to pull water up the intake, you have to wait for the boat to come off the plane and settle down in the water enough that the boat's displacement fills the intake with water and the impeller now in water can pull more water up the tube.
There are a few things you can easily do to improve things. Firstly you can round over the front edge of the inlet lip so that at speed the boundry layer of water that sticks to the bottom of the hull can smoothly run up into the tube. Conversely sharpening the rear edge of the inlet lip will make more of the water that hits the lip go up the tube instead of back along the bottom of the hull. The biggest improvement would com if you built a scoop at the back of the lip to force water up the tube as soon as it touches back down on the water, but it is something sticking down to get broken if you run white water with it, and it will add some drag at high speed. Finally you could also remove the grill which would ease the flow of water up the tube, but obviously at the cost of an increased chance of sacking foreign objects up the tube and breaking the impeller.
Paul Upton-Taylor, Greased Weasel Racing.
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