The highest capacity bow shackles will be made from galvanized alloy steel rather than galvanized steel. The combination of alloys is what increases the strength of the D shackle. Once you have the perfect product for you, you can expect to use it for winching applications and even light tow strap use. Once you install them, you will be able to keep them in place permanently and use them when the situation calls for it. The ability to not have to worry about storage is a great benefit of D shackles. To ensure the pin does not unscrew over time, it is wise to use an unobtrusive wire tie through the hole and pin to keep it securely in place.
Keep in mind that some bow shackles look more like a horseshoe shape with a pin connecting the two ends. This shape difference does not intrinsically effective load capacity—it is simply a different design. If you want the highest capacity D shackles on the rear of your vehicle, you should explore all shapes, but look for galvanized alloy steel.