Trailer Boat Tail Aerodynamic and Collision Study

Aerodynamic Devices - Boat Tails

Technical Report CSTT-HVC-TR-169, December 2010
Prepared for:
Motor Vehicle Safety Directorate
Transport Canada
275 Slater St.
Ottawa, ON

Prepared by:
National Research Council Canada 
Centre for Surface
Transportation Technology

Jeff Patten, P. Eng.
Gordon Poole, P. Eng, PMP
William Mayda, P. Eng.
Alanna Wall, PhD

Executive Summary

Rear mounted trailer panels, also known as boat tails, have long been identified as devices having the potential to reduce aerodynamic drag, and hence fuel consumption. However, little work has been performed to determine how the installation of rear boat tails can affect a vehicle that strikes a boat-tail-equipped trailer from the rear.

The current Federal CMVSS 223 regulation stipulates the dimensional data that must be respected when devices are added to the end of new trailers which carry a rear impact guard (RIG). Specifically, any material added to the rear edge of the trailer that is less than 1900 mm (74.8 in) from the ground cannot extend more than 305 mm (12 inches) behind the plane of the rear impact guard. Once a trailer is in operation, operators must comply with the Provincial weights and dimension policies. At the time of publication, boat tail length is limited to 61 cm (2 ft) in all provinces, although some provinces do allow longer boat tails, with a permit.

Most commercially available boat tails in the US contravene the Canadian Motor Vehicle Safety Standard (CMVSS) 223 regulations with regard to the length of the boat tail behind the rear of the trailer and the height of the bottom edge of the boat tail above the ground. Therefore, Transport Canada wished to study the possible aerodynamic gains of boat tails and determine which types of vehicles, and what percentage of vehicles currently on Canadian roads, would strike a boat tail before striking the CMVSS 223 rear impact guard mounted to nearly all van semi-trailers. Additionally, it was of interest to determine if boat tails could accumulate snow, ice and debris and eject those products onto the road surface at a later time. The downstream visibility effects were also modelled.

The results were obtained via scale model wind tunnel testing and collision estimation using vehicular dimensional and census data. The visibility and flow modeling was performed using computational fluid dynamics (CFD).

The scale model experimental results indicate that the range of boat tails tested in this study (2 ft to 5 ft) can reduce tractor-trailer aerodynamic drag between 7.6% and 11.8% at 65 mph. This corresponds to an estimated reduction in fuel consumption between 4.7% and 7.3% or an estimated annual savings of between 2457 and 3797 litres ($2457 to $3797) for each tractor pulling a boat tail equipped van semi trailer. CO2 emissions would be reduced between 6707 kg and 10 366 kg per vehicle annually.

When considering the height of the boat tail, much of the aerodynamic drag reductions can be achieved by having at least some boat tail structure at the lower part of the trailer, even if it is significantly shorter than the higher section of the boat tail. A lack of structure at the bottom of the trailer significantly reduces the effectiveness of the boat tails. Boat tails are most effective if at least 75% of the rear edge is covered with a full length panel which corresponds to roughly 1800 mm from the ground on most van trailers.

Boat tail length does affect the potential for drag reduction, however, the overall design configuration of the boat tail plays a more significant role in determining the drag reduction potential. The most significant aerodynamic drag reduction occurs between zero and two feet. After two feet of boat tail length the savings continue to increase, but only incrementally.

Testing revealed that some two foot configurations provided more drag reduction than four foot panels of a different geometrical configuration.

Boat tail design, particularly the presence of a bottom panel, is more critical than the length of the side panels when considering aerodynamic gains for the same overall configuration. For example, a two foot design with a bottom panel provided a greater reduction in drag than a similar four foot design without a bottom panel in this study.

CMVSS 223 stipulates the regulations for boat tail panels applied to new trailers whereas the Provincial Vehicle Weights and Dimension policies apply to the application of aftermarket devices to existing trailers. It was therefore of interest to run the collision model with a boat tail that currently satisfies both the Federal and Provincial regulations. This provided a result of 0.5% of vehicles (passenger cars and light duty trucks) would strike the portion of the boat tail above 1900 mm, and these would all be windshield strikes. However, when boat tail material extends down the full height of the trailer to 110 cm above the ground, 44.7% of Canadian vehicles are at risk of impacting two foot boat tails before striking the rear impact guard of the trailer. It is important to note, however, that a significant portion (39.1%) of collisions would occur due to grille/hood strikes below 1900 mm. In comparison, if the commercially available four foot boat tails were fitted to van trailers, 33.6% of vehicles on Canadian roads would strike the boat tail before striking the rear impact guard, however the distinction between grille and windshield strikes cannot be defined with the data made available for this study.

In order to prevent at least 90% (an arbitrary value) of the vehicles on the roads from initial boat tail strikes (the sum of strike modes C2 and D), it is essential that square bottom 121 cm (4 ft) long boat tails be mounted higher than 1740 mm from the ground. Further, fewer than 10% of vehicles would collide (modes C1 + C2) with 90 cm (3 ft) boat tails, regardless of side panel height from the ground.

Many configurations that provide high drag reduction possibilities include boat tail material that is as low as 1100 mm from the ground which would cause a vehicle to strike the boat tail before, or instead of, the rear impact guard. However, for many configurations, this material is sufficiently short that the collisions are mostly grille/hood strikes.

While the bottom panel of boat tail configurations provide up to 20% of the overall aerodynamic benefit, their presence increases the risk of particulate accumulation (such as snow and ice) that could lead to dangerous shedding conditions.

All of the airflow analyses (experimental and simulation) presented in this work were based on uniform upstream conditions, that is, the effect of upstream wakes of moving vehicles and roadside objects was not taken into consideration. However, because a significant portion of commercial vehicular movement in Canada occurs within high traffic corridors where the flow field is highly distorted due to the presence of other vehicles on the road, it is recommended that further analysis of the effects of upstream wakes of moving vehicles be conducted to help quantify the real-world benefits of boat tails.

It is recommended that the effects of trailer rear out-swing and trailer conspicuity be further considered, although these effects were not included in this study.

The collision analysis presented in this report demonstrated that there would also be strikes between classes 4 through 8 trucks for nearly all boat tail designs considered in this study. Further, class 4, 5 and 6 straight trucks with cab over designs have the greatest potential for occupant injury if a horizontal boat tail structural member strikes and protrudes through the windshield. The data required to quantify the population of these strikes were not included in the data sources provided by Transport Canada, and it is therefore recommended that further study be undertaken to include these vehicle populations in the analysis. Based on the findings of this analysis, next steps could include the development of a windshield intrusion test protocol.

Transport Canada may also wish to refine the collision analysis tool to include estimates of hood height and slope (if the data can be obtained) and vehicle pitch-down due to braking, in addition to variable geometry boat tail panel designs (for example, side panels with cutouts).

  1. Notice: This document is disseminated under the sponsorship of Transport Canada, in the interest of information exchange. Transport Canada assumes no liability for its contents or use thereof. The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official policy of Transport Canada. Transport Canada does not endorse products or manufacturers. Trademarks or manufacturers’ names appear herein only if they are considered essential to the objective of this document. This report does not constitute a standard, specification or regulation.