exactly
you have a boat that has took a lot to get it as perfect as possible..if someone went a bought a stock boat bolted on the same exact engine at same rpm's and just jumped in the
boat and ran it, it probably be 10 maybe 15mph slower.

the variables are what make it slower, not faster, you might get a day when the weather is very hot and humid and the boat is a touch slower, or waves on the river slowing it down etc.

but none of these things will make your boat faster, not a chance, its only by making the percentages lower than speed is gained with the same rpm gears and prop.
if someone come along and says he has same engine at same revs with same size prop and is getting 80mph plus, you know its not possible with the given figures.


by using a set of figures to start with and charting exactly what you do, this helps to know what props for certain conditions as well to race with.
you might have a prop that's 4mph slower on a hot smooth day,but its the fastest when in rough water etc, like tyres on a race car..