Boat-tail design

[walke2jd]

I have a 2002 Saturn SL that I have been aeromodding. I'd like to add a boat tail and had an aero question concerning its design.

Would a boat-tail that only tapered vertically and not horizontally still add a large aero improvement? In case that wasnt clear, I would like it to taper from the roof and underbody, but not get skinnier between the tail-lights. Also, the magic number for slope is 10 degrees correct?

Thanks,

Jason

[SVOboy]

Yep, still a benefit, but less since you're decreasing the overall wake less, right?

Also, you can increase the 10 degree more if you have attached flow, so it doesn't need to be a constant 10, but it should begin somewhere around there.

good luck

[basjoos]

A boattail tapered only horizontally OR vertically would greatly reduce the drag, but has an increased wetted area compared with a horizontally AND vertically tapered boattail, so it would have slightly higher drag. It is obviously a successful solution, witness the multitude of horizontal boattails (rays, flatfish) and vertical boattails (sunfish, angelfish) in nature. The Aptera has a combination of horizontal and vertical tapers. The Avion has a only a vertical taper to its boattail.

There is an advantage to having some horizontal taper to the boattail. Since it sticks out some distance behind the rear axle and moves in an opposite direction compared with the front of the vehicle, having some horizontal taper would make it less likely for the tail to clip nearby objects when making a sharp turn.

[skyl4rk]

Tapering both horizontally and vertically will add some strength to the structure when the parts are fastened together. Putting a curve in flat panels and then attaching them makes them stiff.

[MetroMPG]

Another consideration:

Tapering in one "plane" only will mean you've got very sharp transitions from top to side of the boat tail. If the angle isn't perfect, you'll end up forming vortices at that junction which will increase drag (or negate some portion of the effect of the taper).

Ideally, you want generously rounded "shoulders" so pressure recovery is gradual.

[Frank Lee]

But most of all, it would look better.

[Bicycle Bob]

Depending on many factors, I'd aim for about a 15 deg slope on top and about 4 or the bottom. Leaving the sides full-width lets you use a smidgen more taper top and bottom, and the extra surface area was quite popular with the designer of the B.A.T. series, which began the fashion for tailfins.

[evolutionmovement]

Sorry to be the design Nazi, but the tail fins began before the BAT cars when Cadillac designers incorporated them into the 1948 models, echoing the tail fins of the Lockheed P-38 Lightning, though Bertone took it to another level, for sure.

[Bicycle Bob]

Ah, yes. Cadillac used them for styling, and then Bertone tried a practical application which didn't pay off, but boosted the fashion functions.