is there a formula that anyone knows of that you can use to work out the speed of the boat?

if you mean a formula based on hull shape and wet surface area, hull wieght, prop size, angle and depth, cell count, cell capacity, specific gravity of water, size of the chop, gearing, motor size and RPM, ambient temperature and barometric pressure, and other widely varying and sometimes inconsistently measured variables, then the answer is NO.

simply beg, borrow, or steal a GPS to measure speed on any given day, or measure the elapsed time taken between two points (of known distance apart) on the water.

this is the formula used to calculate velocity:

v = d/t

where v is velocity measured in metres per second (standard international unit), d is distance (between those 2 points) meaured in metres, and t is time measured (with a stopwatch) in seconds.

find a unit converter using google to turn meters per second into MPH or KMPH