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post #11 of 13 Old 07-27-2012
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Re: How do you measure the waterline

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Originally Posted by rbrasi View Post
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.
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.

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Annapolis, MD

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post #12 of 13 Old 07-27-2012
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Re: How do you measure the waterline

Rbrasi, I see that you are from Henderson and I assume you sail in Lake Mead. Is there much river current in the lake? What is your drift rate on a calm day – you could use that as a constant to adjust from speed over ground to boat speed. Don’t get too hung up over theoretical boat speed (THS). After all, it is theoretical and was developed in the age of square riggers which we all can agree have an entirely different hull form. The basic principal is displacement boats have to push water out of the way to move, and after a certain point, they can’t push the water away any faster. As you approach THS, a standing wave will develop at the bow and the stern. In effect leaving your boat in a “hole” between the two. At that point, it requires an enormous amount of energy to pull your boat out of that “hole”. This is when power boats step up onto a plane. About the only way you can do this in a sailboat is to surf a wave or in your case, a wake. Unfortunately you need something along the line of 10-15 kts of boat speed to sustain a surf so for guys like us, the thrill is fleeting. Use your THS speed as a bench mark, kind of like breaking the sound barrier and don’t stress if you aren’t matching it all the time.
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post #13 of 13 Old 07-27-2012
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Re: How do you measure the waterline

I live in Henderson, but have never sailed anywhere but the Pacific Ocean. My boat resides in Marina Del Rey, Ca. I kind of answered my own question there, didn't I?

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