Just sunk 20k worth of merchant shipping with my S boat.
The last two ships, a large mechant worth 8000 and a medium merchant vessel worth about 5000 were both taken down with my deck gun.
The large merchant ended up doing 25% hull damage, and wounded some of my crew, but eventually went down. Got two medals and an extra two medals to disperse amongst the crew. I installed improved SD radar and finally got SJ radar.
Although the patrol lasted about 5 days longer due to not noticing my base had changed from Freemantle to the other naval base on the other side of Australia.
Edit: does sinking life rafts give you negative renown?