How Do Over-the-Road Miles Equate to Engine Hours on a Boat?
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Statute Miles Versus Nautical Miles
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While there is no direct correlation between hours of engine time on a boat and statute miles driven on a highway, there is a relationship between statute miles and nautical miles. A statute mile is 5280 feet in length. A nautical mile, the distance of 1 second of longitude when measured at the equator equals 6076 feet. Dividing the nautical mile distance by the statute mile distance (6076 ÷ 5280) yields an answer of 1.15. Therefore, a nautical mile = 1.15 road miles. If your boat is equipped with a LORAN or GPS unit and you enter the coordinates of your location when departing, checking the unit in exactly one hour will indicate how many nautical miles you have traveled. Multiplying this number by 1.15 will yield the equivalent number of "road miles."
The National Institute of Standards and Technology (NIST) is the governmental organization that determines the U.S. definition of a statute mile and a nautical mile. The organization was formerly know as the National Bureau of Standards.
Keep in mind, however, that calculating road miles directly from running time on the engine is not possible.
Running the Engine Does Not Equal Going Someplace
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Marine engines, particularly marine diesels, run smoother if they're allowed to warm up before getting underway. As an example, a 22-horsepower Yanmar diesel requires a warming period of 10 to 15 minutes. The hours meter on the engine panel will record the operating time, but the boat has not traveled anywhere. Additionally, a boat may not be running at top speed, so after an hour of running the motor, and including the warm-up period, it may have traveled less distance than its top speed would allow it to. The sad reality is that unlike an automobile, a boat lacks an odometer.
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Horespower Does Not Equate to Distance Traveled
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Unlike traveling on a highway, which is basically flat, traveling in a boat means dealing with waves and swells. Attempting to measure distance traveled based upon the power output of the engine is impossible. In dead calm, perfectly flat conditions, a boat traveling at top speed will cover the maximum possible distance it's capable of per hour. However, if that same boat is motoring into heavy seas, the distance traveled will be reduced, even if the engine is still running at its maximum. So while the vessel's running time and power output are identical, the actual distance traveled will vary significantly. This is the other reason engine hours can never be accurately equated to road miles. There are simply too many variables to factor in.
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