Avoid This Grilling Mistake That Leads To Dried-Out Potatoes
If you want to bake potatoes on the grill, it may take an hour or more until they're soft inside. With such a long cooking time, there's always a risk that they might dry out to some extent. Celebrity chef Robbie Shoults, who owns Marshall, Texas' Bear Creek Smokehouse, Marshall Mercantile, and High Horse 1898, says that you can keep this from happening if you don't expose your spuds to direct heat.
As Shoults explains to Mashed, "Placing the potatoes directly over the heat source on a grill can dry them out." If you have a grill equipped with shelves, he recommends that you cook the food on the top shelf. Furthermore, he suggests turning off the burners on one side of the grill and then placing the potatoes in this cool zone.
Shoults also reminds us to coat the potatoes with butter or oil before cooking, because this barrier can help lock in moisture. Before you oil your potatoes (or grill grates), though, consider using mayonnaise instead. Not only is this condiment more flavorful, but it will also stick to your potato instead of dripping off and potentially causing a flare-up as oil might.
Here's what to do if you have a charcoal grill
Robbie Shoults' suggestions about using the top shelf or turning off some burners are excellent if you're using a propane grill. Even if you have a basic charcoal grill, though, you can (and should) still cook your potatoes over indirect heat.
The difference between direct and indirect grilling is that, in the first case, you place whatever you're grilling right over the open flame. While this is great for getting a quick sear on a grilled flank steak, it won't work for potatoes, since they may char on the outside while remaining raw inside. With indirect heat, you're essentially using your grill like an oven.
The way to set up a charcoal grill for indirect heat cooking is to pile the coals on one side and leave the other side empty. If you have a large grill, you can even pile them to each side, leaving a coal-free strip running down the middle. Light the coals, then put the potatoes on the grates over the empty spot. If you have a two-zone setup with coals covering half of the grill, you'll need to rotate the potatoes halfway through the cooking time so that both sides of the potato have equal heat exposure. The benefits of a three-zone setup mean that the potatoes will get indirect heat from the coals on both sides, so there's no need to turn them.