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Old 10-16-2011
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Hull Speed?

I am curious as to how accurate this is at determining your max sailing speed. My hull speed from the manufacturers states 6.5 knots...I can get to that pretty easily, I would have thought that it wouldn't be that easy to get to the max speed. This was actually close hauled into the wind...I know you can get way above that downwind and surfing the waves. This speed was from my GPS and that was averaged over 3 minutes so it wasn't just a spike at that 6.5 speed...it was an averaged speed. With some tweaks to sail trim and a very clean bottom I'd think I can get to 7.5. Where some manufacturers more conservative with rating the hull seed? Or is it maybe modern sails and rigging make it actually possible to sail above hull speed? Or am I thinking too much and just enjoy that I can get the boat moving like that!
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Old 10-16-2011
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Hull speed is a theorotical value. The constant varies from one boat to the other. As the stern f the boat gets wider the boat travels faster. With good sails and good wind, hull speed can easily be beaten.
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Old 10-16-2011
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Theoretical hull speed of a displacement hull= 1.34 X sqrt LWL


Other factors -

1) planing will take you above theoretical hull speed, usually downwind with a chute of some sort.

2) fouled bottom will slow you down.

You need a knotmeter, not a GPS when dealing with hull speed as it is through the water not over the ground. In addition you want a momentary speed not an average.
Faster and dhays like this.
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Old 10-16-2011
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As Jack said, the GPS is speed over ground (SOG) including effects of any current that might be there (aiding or hurting) A knotmeter is the better tool for 'boat speed', esp as a result of changes in sailtrim. Having both allows you to quantify the current and approx direction.

You'll get up to 'hull speed' sooner, typically, while beating because your apparent wind is the highest it can be for the wind speed... a clean boat will get there quite easily, as you've observed. Without a lot of breeze or swells truly exceeding hull speed is quite difficult.

Also some boats are said to 'extend' their waterline while sailing (as flat overhanging stern counters become submerged by the rising stern wave) therefore theoretically creating a longer LWL at that moment, perhaps 'skewing' the calculation.

Anyhow, I wouldn't get 'hung up' on the number.. find out what is comfortable for your boat (heel angle and apparent wind angle,)and use that number , whatever it is, as a target and steer for that when trying to optimize your VMG to windward. Polar diagrams may be available for your boat that would tell you what that optimum speed/angle would be. But even with that data you'd need to be certain your instruments are accurate.. not always easy to do.
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Hull speed as determined by the 1.34 x square rt of the LWL is not a hard number, just an approximation or rough guideline.

Below is a portion of an in interview with Gary Mull:



He was casually asked whether the maximum speed of his intriguing new boat design was 1.34 times the square root of the waterline length.
"I wish people would quit saying that," he retorted with intensity. "There's no such thing as a maximum speed under sail. There's a point at which the speed-versus-resistance curve begins to get very, very steep. At low speeds, a certain increase in horsepower gets you a fairly good increase in speed - but at high speeds, doubling the horsepower only gets you a very slight increase in speed. Usually somewhere around 1.34 times the square root of the waterline length - the sailing waterline, not the static waterline - that speed/ resistance curve starts to get very steep. But there's no absolute limit."

"But," he was asked, "doesn't the quarter wave start to build up higher than the cabintop?""No! That's not so!" he exclaimed. "I've never seen such a thing. That's all magazine talk. That's not naval architecture. I'm continually seeing this 'maximum speed under sail' or 'maximum speed-length ratio' or whatever-the-hell, and it's totally meaningless to naval architecture, as an absolute maximum. It does have meaning, because the speed-resistance curve does get very, very steep, as I say; but it seldom gets absolutely vertically asymptotic."
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Not sure if it has been mentioned, but "SOME" sailboat hulls do not follow the rule of the 1.34 time sqrt of WL. many of the newer planing hull designs are like power boats, you CAN go faster than the formula. On the otherhand, take a boat like the Pinta, Santa Maria, old ironsides etc, those will be lucky to hit the formula, and if they do go over, some designs DO sink into the water and eventually sink! I would SWAG that most boats built in the last 30 or so years, some as far back at 50-60 have the ability to go over the theoretical hull speed.

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Old 10-16-2011
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Quote:
Originally Posted by jackdale View Post
..

Other factors -

1) planing will take you above theoretical hull speed, usually downwind with a chute of some sort.

..
On a light cruising boat you don't need a spinnaker. Enough wind will do the job even without a spinnaker

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Old 10-16-2011
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Everybody knows, as well, that multihulls blow the hull speed numbers out of the water. And, naval architect Dave Gerr, director of the Westlawn school of naval architecture, says that relatively long skinny hulls are not bound by the theoretical hull speed number. OTOH, people who sail older designs like my Lyle Hess-designed Nor'Sea 27 do much better to expect no more than the theoretical hull speed and just be happy with it.
Jeff
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Everybody knows, as well, that multihulls blow the hull speed numbers out of the water.
Jeff
I believe the multi's plane. Theoretical hull speed applies only to displacement hulls.
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Quote:
Originally Posted by jackdale View Post
I believe the multi's plane. Theoretical hull speed applies only to displacement hulls.
I didnt know for sure if they planed, but I knew for sure they're fast.
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