A pair of J shaped flails with a centre "fan" finger is the best combination for grass, rushes, and occasional dense stuff.
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I found travel speed was more than double that of the whale tail flail I had been using. The swinging J blades chop the stuff up better, and it composts down faster too.
In grass whale tails won't leave the slight corrugations that the J type does.That's same machine as we have, I often wonder if whale type flails would leave a better finish?
Cheers. Will check them out.either way, flails direct will probably be the best and cheapest place to get them
Ive got 2 sets for our trimmer now, Y flails for summertime ditch mowing and a pair of sharpenable "twist" flails for winter hedge work