Here's a link to one groups view of boats and orcas

http://www.sanjuanislander.com/groups/whales/orca-relief.shtml

and a link to the only study I can find on the impact of boat engine noise

http://www.orcarelief.org/docs/erbe_boatnoise.pdf

Looks like most of the data concerns large engines 150 hp or higher. One has to wonder how trolling motors or large motors idling at trolling speed fit into the equation.

I'm also reminded about the video that is on youtube showing the Orca coming up alongside a sport fisher and taking half the salmon that was hooked. Doesn't look to me like the orca has much concern over someone moving at trolling speed. There are a lot of other sport fishers visible in the video, too.