Straight line through center at water line, overall inside toe rail bow to inside toe rail stern through center.boatingcruising:901643 said:How do you measure the waterline on a sailboat? I want to calculate the approximate best speed of my sailboat.
Do I measure the curve of the boat or just a straight line from the bow to the stern on the waterline?
Apples to oranges - hull speed is an approximation of how a displacement hull will move. In flat water it shouldn't be hard to reach (I can even get close in my boat, which has horrific hydrodynamics - motor in well below w/l, ballast bolted to side of keel...boatingcruising:901784 said:I get the feeling that reaching optimum hull speed is like getting the advertised gas millage in your car or obtaining the published speed of small aircraft at a set RPMs.
The GPS is giving your speed over ground. Hull speed is your theoretical maximum speed through the water. If we are doing 6.9 knots (hull speed for the Hunter 29) through the water, but the water itself if moving with say, 1 knot of current in the direction we are travelling, then the GPS would measure 7.9 knots of speed over ground. Current can help you go faster than hull speed and so can waves. As you slide down the face of the wave, your boat speed will increase. I once clocked 12 knots of SOG while surfing down 5-6' waves and my boat has a theoretical hull speed of 7.4 knots.So, if at 25' LWL, my hull speed calculates to 6.7 Kt. What is the best measure? I currently use a Garmin GPS to get my speed readings, but that is speed over land. How does that translate? I routinely exceed 6.7 with current and a decent wind, according to my Garmin.