jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,753
Thinking about your exploring of speed, I read this quote/observation about sailing and sail boats.
Top Speed of the Flicka 20 (w/ Motor)
- Thread starter SailorElliot
- Start date
Speed through the water or speed over land? In Nautical miles/hour or Statute miles/hour? If you are measuring speed with a GPS you will need to make two measured runs in opposite courses and average the results to eliminate the effects of wind and currents. The formula for theorical hull speed is a very old formula designed for old wooden boats of the time. The factor of 1.34 is nothing more than a number plugged in to equate the formula results to the results they obtained by measuring with a knoted line in the water. That factor of 1.34 even though assigned it has within itself the data for a specific hull shape and displacement of a boat design of the era. At present hull shapes have changed in shape, materials and boat displacements and the formula is no longer as approximately accurate as it was when developed. I honestly doubt that your boat would have a theorical hull speed of 5.7 knots. I would venture to say a top theorical speed between 4.5 and 4.8 knots would be more reasonable. This would mean than when calculating time for a trip leg I would use a sustainable speed of 3.5 to 4 knots. I had a San Juan 21' at one time and even though it was a fairly fast boat in its class these are the speed numbers I got. One Knot is approximately 1.15 Miles per Hour so 4.5 knots would be 5.18 MPH. Statute Miles are used inland and Nautical miles at open sea. The mile markers will reflect Nautical or Statute depending on the waters you may find yourself. Speed is important as it has to do with range and time of travel and if running an engine it has to do with fuel consumption, refueling plans and estimated time of arrivals at destinations. Accurate speeds can be developed by tracking progress in a chart with hourly calculations. Sorry this probably does not answer your question of how fast your boat may go but hope it gives you an idea it was not a simple question.