Strategies for trolling speeds
As posted prior, the average is 7-8 knots. As posted by Mako Matt, it also depends on how each lure works in the existing sea conditions...You need them to work right. Big Marlin lures need faster speeds, while swimming split bill ballyhoos need much slower speeds.
But here is a rule of thumb that a professional taught me a while ago. Most tuna actually like it slow. Even a green machine on a bird presents it'self nicely at 5 knots in most sea conditions. Giants love swimming baits at 3.0 - 4.0 knots, and they can swim much faster than their smaller cousins.
I beleive you'll hook more fish trolling a slower speeds BUT, when you need to cover a lot of water to find the fish, fast speeds will do best at that job...Once you are convinced you're in an area where there is fish, I suggest you slow down to 5 - 7 knots. Not only will it keep you in the area for a longer time, but as I said, I think the fish like it better...That's provided that your lures are working well...Green machines, small clones, ballyhoos ( either skirted, naked, or split bill ), hard body deep diving swimming plugs and most spreader bars are all good at slower speeds.
I actually troll at 8-9 knots, depending upon the water conditions, until I find them. Then I slow down to 5 - 6 knots when I'm convinced I'm in the glory hole.................Howie