The purpose in this exercise is to calculate the performance of the boat in
actual usage. After the boat is assembled, testing for efficiency is a good way to compare
new performance specifications and results against similar testing that may have to be
done as the boat and motor age. Marginal losses in performance are expected with age but
unless you know what the starting point was, you can't be sure where you might be in the
future. For the average production power boat, the manufacturer prints this type of
information, but rarely releases this data to the public. So it is a good idea to generate
this information yourself. Expending the effort to spec. out a family runabout isn't
usually worth as much as verifying the performance of a new High Performance rig.
As an aid to
more informed boating, we offer the following formulas. In many instances, you may wish to
convert your figures from statute miles to nautical miles or vice versa. A statute, or
land, mile is 5280 feet while a nautical mile is 6080 feet. (OK, rounded off a
bit, but easy to calculate) See formula #5.
1. The theoretical speed in MPH :
Shaft RPM x Propeller Pitch x 5 ÷ 5280
(6080, if using knots) = MPH (or knots)
2. Actual speed in MPH* :
3600 ÷ seconds per mile (stop watch time)
= MPH actual
3. Percentage of Efficiency :
Actual Speed ÷ Theoretical Speed = the
Percentage of Efficiency
4. Percentage of Slippage :
100% - the Percentage of Efficiency = Percentage
5. Conversion formulas between nautical and statute (land) miles:
a. MPH ÷ 1.152 = knots (nautical miles per hour)
b. Knots x 1.152 = statute miles per hour
1 knot = 1.152 miles
1 mile = .868 knots
*Actual speed must be determined on a measured mile.
6. To make a speed curve.
Measure speed on a measured mile at each RPM level in steps of 300 or 500 RPM.
Figure it out each time on the time-distance-speed formula. See Formula #2. Enter the
results on the following graph, and then plot the curve.