1. The Tesla Valvular Conduite, uses the coanda effect to restrict the flow of liquid down a conduit in one direction. This is perfect but too complex to manufacture.
2. The Thrust Augmenter. Technology usually found in Pulse Jets, this can also increase the thrust of the engine.
Step 1: How a Thrust Augmentor Works
A Thrust augmentor is placed a short distance after the exhaust directly inline with the opening. It has a converging intake followed by a diverging section.
As the fluid is forced out of the exhaust it enters the opening to the thrust augmentor draging in surrounding fluid from the local environment as it does so. This increased mass of fluid then accelerates out as the augmentor diverges.
At this point the pulse jet sucks in and this creates toroidal vortices in the augmentors diverging section which expand to the walls of the augmentor and travel along as fluid pistons, pushing fresh fluid infront and pulling in fresh fluid behind. Such a ‘piston’ is a more effective than a simple flat pressure front because it stays together over a much longer distance before degrading. In addition, its swirl also appears to act as a rotary pump, drawing even more fluid with it. All this air adds to the reaction mass and greatly increases thrust.
This means the fluid traveling down the augmentor has significant inertia capable of resisting the sucking action of the engine thus the intake fluid must come from else where. The shape and possition of the thrust augmentor mean that the intake comes from the front of the boat and in effect, sucks it forwards.
This means that all of the work done by the engine should be used to move the boat forwards ultimately increasing overall thrust and hence the boats velocity.