Miscellaneous Musings from Don MacPherson, Technical Director
Ocean Tech Expo is a small conference and expo for the submersible community. It was held last week in Newport, RI.
Most of the major players in ROV and AUV vehicles were exhibiting, and I was very pleased to see many of our HydroComp thruster design clients exhibiting vehicles. I had the opportunity to speak to the conference attendees, as part of a SNAME session, with a presentation entitled “Contemporary Thruster Design for Submersibles”.
But enough about that… One thing nagged me the whole drive back to New Hampshire.
As we sit in the designer’s chair, we miss having to deal with the messy practical side of thruster operation – and yet, the practical application of thruster operation should affect design decisions. Let me share some a few things that I saw regarding the “care and feeding of your small thruster”.
First, the picture below is a unit we designed for a vehicle operator (that was eventually passed back up the chain for adoption by the manufacturer). It has proven to be a very nice performer for the vehicle.
I want to illustrate a few small deficiencies that, while certainly not of any major concern, might help to make the thruster an even better performer.
Nozzle-propeller symmetry
In a ducted thruster, tip clearance affects efficiency, as does keeping the propeller fully symmetric with the nozzle. Small tip clearances are better, but this requires tighter tolerances and more dimensional stability.
The image here confirms that the nozzle is not axi-symmetric with the propeller. There is greater clearance to the top blade than to the bottom.
Is this critical? Probably. Remember that for an increase in tip clearance of 1% of diameter, you will see some 3% reduction in efficiency. Combine that with the imbalance in developed thrust, and the practical implications can be meaningful.
How can you avoid nozzle-propeller asymmetry? Make sure that the nozzle attachments (i.e., struts) have sufficient strength and stiffness. As you can see from the first picture, the attachments are made from a relatively flexible plastic material. They should be changed to a stiffer material.
Viscous drag and roughness
At such a small size, roughness can be huge. The propeller is made from a metal that has had a chance to corrode a bit. Keep your propellers and nozzles clean and smooth. Having said that, be careful that any polishing or buffing of the propeller does not alter the shape of the propeller, particularly on the nose.
Nozzle edges
A part of the design of the nozzle is a trailing edge shape that promotes separation per an intentional design objective. So, the shape of the nozzle should also be clean and smooth, and the trailing edge should also be sharp.
This picture is a detail of the trailing edge. You can see how it has been roughened from impact and handling. Take a minute with some fine grit sandpaper to return the trailing edge to a sharp corner.
Flow discontinuities
Take a look again at the top picture. You can see cutouts and holes in the nozzle that allow for different fittings. When these fittings are not required,however, it would be helpful to fill these to improve water flow. Any discontinuity is just costing you thrust or battery life.
Summary
These small considerations may not “make or break” a design, but they are relatively simple things that can keep your thruster healthy and well fed.
Filed under: Engineering, Events, Hydrodynamics
