Yep, @GPSguru beat me to it... you're both right. Below taken from the Orkney site in respect of a Longliner, which superceded the Strikeliner I believe.
Bottom line though is that whilst 22 knots may technically be achievable, the factory will have recorded this with the lightest crew they could find, and with no gear on board, with a following wind and with the tide. I'd argue these probably aren't real world conditions.
Regardless of this, your revs are still low. As @Maverick has suggested, check your throttle opens fully at the engine, and if it does, then check your fuel lines and filter are allowing full flow of fuel. When you get to 4300 rpm, does the engine struggle to get there, or does it zip round but not go any higher?