New hull speed equation

Discussion in 'Sailboats' started by JakeWood, Aug 7, 2023.

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JakeWoodNew Member

I asked ChatgGPT to put together a formula for true hull speed taking the most important factors into account. I am wondering what you guys think about it. The equation might be complete garbage but that is why I am posting it here - for some of you guys that know naval architecture way better than I do (I don't really know much at all). Here’s how the conversation went:

Me: Write an equation for true boat speed for a sailboat, using the following variables: length of water line, half angle of entrance, sail area to displacement ratio, and stern slope angle

ChatGPT:
The equation below provides an approximation that incorporates the mentioned factors:

Vmax = 1.34 * √(LWL * SDR * cos(SA) * cos(SSA))

where: Vmax is the true maximum hull speed in knots, LWL is the length of the waterline in feet, SDR is the sail area to displacement ratio (a dimensionless factor), SA is the half angle of entrance in radians (measured in degrees and converted to radians), SSA is the stern slope angle in radians (measured in degrees and converted to radians), cos(x) is the cosine function.

To use this formula, you'll need to ensure that all angle values (half angle of entrance and stern slope angle) are converted from degrees to radians before plugging them into the equation.

Last edited: Aug 7, 2023
2. Joined: Jan 2006
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That equation looks impressive but don't bet your bankroll on it. There are far too many other variables that will effect the max velocity.

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JakeWoodNew Member

Not betting anything on it! It would be interesting to run it against polar curves from testing to see if it worked. But, like I said, this is way out of my league. AI was the source. May be in the wheelhouse of some people around here though.

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AlikSenior Member

Phase of the Moon should be also included in formula... together with those angles

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jehardimanSenior Member

LoL, it is a mixture of all the button sorting formulas. Not here, not there.

Edit to add: Is JakeWood a real user, or is it AI?...really, only 2 post and both in this thread?...Occam's Razor...

Last edited: Aug 8, 2023
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JakeWoodNew Member

Ha. Real user, but that is a great theory. I like the one about the moon phases too. I am new to the forum and I found it after reading Busting the hull speed myth - Yachting Monthly https://www.yachtingmonthly.com/sailing-skills/busting-the-hull-speed-myth-82832 and trying to see if anyone had taken this further. I couldn't find any, so I gave the AI a try. The article got me thinking that the equation should be doable - not to a level of perfection but at least closer than the old 1.34 equation. Anyway, in full disclosure, I'm a physician and a fairly novice sailor, but I'm always curious about these types of things. I'll take it as a compliment that you thought I was AI

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jehardimanSenior Member

Being recognized as something that has less than normal human intelligence is not a compliment....If you are real, I would not recommend using AI for anything, as AI was almost universally applied to hydrodynamics (as far back as the 1970's) and failed miserably.

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JakeWoodNew Member

I was just being politely funny about being accused of not being real. BTW, I don't think AI in the 1970's would be equivalent to AI 50 years later. Either way, I wasn't really looking for a discussion over whether or not I'm real or AI is a good source for anything. I was just looking for opinions as to whether that was a better or even OK formula for getting closer to the right number. If you have thoughts on that, I'd be happy to hear them, but let's go ahead and move on from the discussion of my realness.

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jehardimanSenior Member

You are an AI, as you have no idea about what you are speaking. Those who lived through that era would not be so flippant. Go mastrabuate somewhere else.

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wet feetSenior Member

Our new poster may or may not be a collection of electronic components.The formula presented might have a few hurdles to overcome to explain this:

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alan craigSenior Member

How sad that on a forum like this, with plenty of NA onboard, all we get is silly responses.

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CarlosK2Senior Member

I think it is very useful to distinguish between

(A) "Hull Speed in the real world" which is the speed that many sailboats reach upwind = 0.35 Froude

(B) the so-called "Hull Speed" which is the speed reached upwind by dinghies in the 70s of the last century and the old America's Cup Yachts: 0.40 Froude

and

(C) "Carene Speed" which is the speed that a sailboat reaches downwind if it hoists enough sails and without taking into account small waves, "Surf-riding", or big waves, "Surf-gliding":

SLR = 8.26 / (D/L)^0.311

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CarlosK2Senior Member

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CarlosK2Senior Member

Beowulf

Hull Speed: 10 Knots
Carene Speed: 20 Knots

Surf-riding: 22-24-26 Knots
Surf-gliding: improbable because may be need a 30 meter high Wave:

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CarlosK2Senior Member

Beowulf

33 Tons
80 LWL

60 D/L

SLR = 8.26 / 60^0.311 = 2.3 SLR (Speed Length Ratio)

2.3 x Squared Root 80 = 20 Knots

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Gerr Marine Naval Architects and Yacht Designers https://gerrmarine.com/

the Author of this magnificent formula I think I remember is Dave Gerr

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I am quoting the numbers for Steve Dashew's Beowulf Monster from memory and they are surely not exactly correct; but I think the idea is understood

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