NAMBA October 2021 Web.pub

PROPWASH October 2021 Jet Boats (Continued from page 9) I was surprised at how well the boat ran. Maneuverability even without turn fins was impressive. I previously ran the boat with a surface prop and it was not competitive as a P limited mono. I decided the Calcraft was too big and decided to build an epoxy glass version of Štěpán’s sprint boat. I’m still in the pro- cess of finishing the boat, but you can see Štěpán’s videos on how the boat runs. Let’s look a little closer at water jet design. The idea is to use an axial flow pump to create thrust. The pressure needed lifts the water up from the boat’s bottom and forces it out a nozzle. The speed and weight of the water flowing out the nozzle creates thrust. This thrust is constant so the boat’s speed increases until its drag equals the thrust. There are a lot of details that matter, especially the inlet shape and boat bottom in front of the inlet. The standard water jet has one impeller. I’m not going to get into the internal design details. At the end I’ll print links to Štěpán’s videos and CG Trader pages for his testing. The important area for the installation is the bottom shape and intake alignment. The first picture shows the outline of the running water level projected on the bottom of a full-size sprint boat. 10 Hobby King water jet However, the sprint boats have found that a double impeller gives more reliable performance when there’s disturbed water at the inlet. Štěpán’s design uses two impellers. See his video on the subject for a comparison between one and two impellers. M30 water jet Running water line This picture shows the view of the inlet from the water flow direction. Both these pictures are of real boats and my model is a little simpler. Inlet