You have a thru-hub exhaust if there are exhaust holes to the engine gearcase across the prop shaft. Non thru-hub is reliable across the prop hub. Non Thru-hub Exhaust Used by DIYers and experts, Yamaha outboard https://henricotr161005.boyblogguide.com/29190445/the-single-best-strategy-to-use-for-small-yamaha-outboard-motors