Designing a low horsepower, low speed jet from scratch.

Primary Goal

• Use 10 HP gas or electric motor
• Maximum thrust @ about 8 MPH
• Large passages through jet to minimize clogging
• Relative high RPM to enhance weed chopping

We selected 2400 RPM and a 5 ¾” diameter impeller as a starting point. We calculated the entrance and exit angles of the impeller and stator blades. (Stator in the blade assembly aft of the impeller that catches the water as it comes off the impeller blades and takes the twist out so the water exits the nozzle straight.) These shapes were laid out in AutoCAD and checked for fit and form.  We then made a solid model drawing of each piece. These drawings were changed to STL files and sent to our 3-D printer

A full scale ABS plastic impeller & stator were made (more on our 3D printer late in this article). I then took the plastic impeller to a stainless steel loss wax foundry and dipped the ABS in ceramic, melted out the ABS and poured a stainless steel impeller.

Stator being made on 3D printer

One of the NAMJ test boats is an aluminum 14’ John Boat. The middle of the transom was cut out and we welded in a 6” diameter elbow shape to form the jet drive intake.  A thrust bearing was fabricated on the front of the intake to house the impeller shaft. After some modifications to the flooring we were able to build an engine bed to house a 23 HP Honda V2 air cooled engine. A trip to a local Lawn and Garden grave yard yielded a 24” splined double U-joint drive shaft. The jet was mounted so low in the hull that the crank shaft to impeller shaft offset was 3 ½”.  Looks like the U-joints will handle that angle OK.

The plan is to run the plastic stator in the first test at maybe up to 15HP to check performance. By measuring thrust at different RPM’s we can tell how the jet is performing. Also the stator is fitted with a ¼” pipe just forward of the stator vanes. I will install our triangle pitot tube in there. The tube has 3 very small openings toward the bottom end; one on the leading edge of the triangle and one flush on each adjacent side. These holes are connected to small tubes that run up through the triangle shaped Pitot tube and out through the ¼” pipe. By attaching vacuum/pressure gauges to the tubes and turning the Pitot tube you can measure the angle of water flow into the stator.  This will tell me if my stator blade front edge is correct.

We can ballast the boat so the jet stream is above the water. This will enable us to check for rotation of water exiting the nozzle, which will tell us about the aft edge of the stator vane.

Also, we will make different size nozzles to ensure we have the correct size for the thrust and speed we are looking attain.

Plans are to run this in the next day or so when we get a break in the weather.

Will let you know how things progress.

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