Bowie knives are essentially built for adventure. Adventure, of course, oftentimes means inconvenience. This sometimes means having a dull knife and no bench stone on which to restore its edge. You can still sharpen your knife, but it will take some technique and the right tools. You have several options for portable sharpeners.
The easiest way to provide yourself with a field sharpener suitable for Bowie knives is to buy a cheap whetstone. These come in very small sizes. Oil is usually used in sharpening, but this is not necessary. It will take longer to sharpen on a stone without oil, however, and you’ll have to wash away the metal flakes from the stone using something else. Any nearby water will do fine for keeping the stone clean of debris.
Consider the problem with using a standard bench stone. You have to hold the blade at the same angle throughout the grinding process. However, with a rod system, all you have to do is move the stone over the blade of your knife to refresh that edge. However, for some systems, larger blades, such as a Bowie knife, will require that you reposition your rod periodically to get the entire blade.
There are also stones made out of ceramics and other materials which make find sharpeners for a Bowie knife. Oftentimes, these stones come with their own instructions and are more abrasive than a traditional whetstone. These stones can be used most anywhere, as they’re generally very small. Some knives come with such stones contained in an extra pouch sewn to the sheath. Be sure to use oil, if the stone calls for it, though many of these stones do not require as much.
The problem here is that you can scratch your blade. However, other problems exist here, as well. For instance, you can easily grind away part of the guide on the stone, and you will also find the process a bit cumbersome. Therefore, rod systems are likely your best bet. Other choices do exist, but they can actually cause more damage to your Bowie knife than what you will do with a guide or even with a regular whetstone.