I agree that scanmar was being cautious in advising against using the vane under power. I have had the engine running with the vane in the water and it was not harmed. But, the norsea only had a 13hp engine so the force of the prop wash is relatively small. Since they spec the vane for boats up to around 40 feet I suspect they are more concerned about the prop wash from bigger boats. The forces on the vane when sailing fast are not small, especially if the boat is not perfectly balanced.
One of the things I like about the capehorn is that it can be installed off center. There is a boat like my new one here in the bay with on installed to port and, in photos, it does not look like the vane even sticks out behind the boat at all. Its a canoe stern as opposed to a flat transom boat.
With the vane installed off cener the prop wash problem is probably reduced. But, the capehorn literature does not say you need to worry about an off center installation if you want to use the vane to steer under power.
Another factor that may allow the capehorn to deal with propwash is that the vane is not short and wide like the monitor. Its long and thin. Much of the vane would normally be out of reach of the prop wash and the part of the vane in the prop wash would not have that large of a surface area to absorb energy from the prop wash.
I have not heard anyone here in the bay say anything but good things about their monitor vanes. The one exception is a member of our yacht club who did a circumnavigation in a custom designed cold molded 50 footer. He tried a monitor vane and it could not function in the wake of his big spade rudder. He said he would have had to install two of them (one port and one starboard) to be able to use a windvane on his boat. He really wanted it to work. An autopilot to steer a boat that big can really suck some amps. It was not really a monitor problem, any vane would have had the same problem.
One of the reasons I did not pursue the issue with the monitor on the norsea was that the small autopilot that I would have used on the vane did a perfectly fine job of steering the boat under power when I hooked it to the tiller (even when I mounted it much farther forward than the manufacturer specified.) Under power the tiller pilot did not need the range of motion that it needed to steer under sail. On the aft cabin norsea it would have been a pain to be constantly climbing back to work with the tiller pilot mounted on the vane. I did consider using a flexible engine control cable to transmit the tiller pilot motion back to the vane, but always decided to go sailing instead of screwing around with it.