What is the condition of the bottom of the boat? A clean bottom will allow the boat to move faster while a fouled bottom will add significant drag. Why 2,900 RPM, is that all you can get? If it is, two things, first check that the Tachometer is reporting the correct RPM and then check the size of the propeller. The maximum rated RPM for that engine is 3,800. A proper prop for that boat should allow the engine to reach close to Maximum rated RPM without exceeding it. If you do not know if the original prop has been replaced I would advice that you ascertain what prop do you actually have as far number of blades, size and pitch and then compare with other owners. There are also engine mechanical reasons why the rated RPM cannot be reached but that would entail engine diagnostics. Do not rely on the calculated speed formula as it is antiquated. the 1.34 factor was derived to adjust the Formula to the measured results of different boats. Since then the hull shape design and displacement of boats have changed significantly.
@sail sfbay has the right formula. Is that forumla perfect - NO, but it is pretty darn good for calm, flat water, the design of our typical boats on this forum, and the accuracy of the instruments we use to measure speed through the water,. A lot of factors can impact what you actually can achieve.