How hot do you want the heat for grilling jumbo prawns on the barbeque (barbie)? Do you cover the barbeque with the jumbo prawns on it? When are the grilling jumbo prawns cooked properly?
For grilling modest sized jumbo prawns on the barbeque, the best thing is to grill on a medium to medium high heat if you are using gas (a medium distance if you are grilling the prawns with a charcoal barbeque). You want them to sear on the outside.
Jumbo Prawns on the Skewer - un-marinated and raw
Since you want this grill searing to take place, you also want to avoid covering the prawn skewers as they grill. A few minutes each side when they are skewered to lie flat on the grill should be ample. Only for very very large jumbo prawns will you want to cover them as they are cooking - in case they need extra cooking time and will get too dried out on grill side and not cooked enough on the inside.
When grilling jumbo prawns with the shells on, you can use a higher heat which will make the shells crispier and easier to eat and add more texture. The intact shells will keep the grilling jumbo prawns from drying out excessively while they are grilling on the barbeque.