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Discussion Starter #1
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?
 

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Unless this is a custom one-off build you should be able to find the LWL easily online. What kind of boat specifically?
 

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Go to sailboatdata.com.
Theres a Hunter 290 listed with a 26.92 waterline.
One foot one way or the other is maybe one tenth of a knot so unless you are into serious racing, it really doesn't matter much.
 

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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?
Straight line through center at water line, overall inside toe rail bow to inside toe rail stern through center.
 

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Discussion Starter #6
Tks everyone
My goal is to simply have some idea of what the optimum performance of the sailboat would be so I can have a goal to shoot for. I am not into racing, at least not yet!

so if the waterline is 26.92 than the sq root is about 5.18 ( i used the calculator) so the optimum speed should be about 6.95 say 7kn. I hope I can learn to get at least 5.5 - 6 out of the boat.
 

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If you ever get to hull speed , please tell us the conditions
 

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I am new to sailing but based on conversations with other sailors 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.

FORMUL: Optimum Hull Speed = Incredible Accomplishment
 

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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.
 

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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.
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...

What builders/designers advertise is not hull speed, but polars - which ought to be based on their real world measurements (tho designers may post calculated polars on a design before it's built)

Ability to maintain hull soeed in varying conditions and over a sustained period of time is much trickier than hitting it for a few minutes 135 off the wind on a nice day :)
 

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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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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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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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