As a trailer travels, it leaves a wide low-pressure zone (wake region) behind it; the vehicle is effectively pulled backwards by this vacuum. Boat tail panels added to the rear edges and a diffuser under the chassis gradually recover the flow and reduce this loss.
How Does a Boat Tail Work?
Three or four slightly inward-angled panels placed around the rear doors allow the flow leaving the body to converge without separating. European regulations have paved the way for this technology by permitting folding aerodynamic devices that exceed the vehicle's maximum length under certain conditions; the panels fold at the loading dock and deploy on the road.
The Diffuser's Contribution
The diffuser surface, angled upwards behind the rear axle group under the chassis, releases the underbody flow into the wake region in a controlled manner. Although its contribution on its own is limited, when used together with side skirts and a boat tail it increases the total gain significantly; the combination effect has been validated by real-world road measurements.
- Best result: side skirt + boat tail + diffuser combination
- Panel angles should be optimised in the 10-15 degree band
- Folding mechanisms must not obstruct dock operations
- The gain is directly proportional to time spent at cruising speed
References
- Van Raemdonck, G. M. R. & Urila, I. (2019). A Study in Options to Improve Aerodynamic Profile of Heavy-Duty Vehicles in Europe. Sustainability, 11(19), 5519. DOI: 10.3390/su11195519
- Madhusudhanan, A. K., Na, X., Boies, A. & Cebon, D. (2021). Effects of semi-trailer modifications on HGV fuel consumption. Transportation Research Part D: Transport and Environment, 92, 102717. DOI: 10.1016/j.trd.2021.102717