18x6 - 10 only means that you can use a 18x6 all the way to 18x10 prop... so like 18x6 or 18x8 or 18x9 or 18x10
In other words if you use a prop that's more pitch then "10"
Your probably over proped...
The 18 means 18 inches and the other number means the pitch of the prop ok.. L8ER