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  #21  
Old 09-20-2012
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Re: Moment curves

Being as I have a bit of German in me......."I know NUUUTTTHHHEEENG!" said in my best Sgt Schultz voice with rolley eyes to boot!

My thought would have been the roll over point etc, but being as they are showing at degrees of heel......Could it be the imersed area of the hill thru the lines at the centerpoint?!?!? ie always one would have say 10,000 lbs of disp, the graphs show how that 10K is dispersed on ea side of the hull centerline.......my swag and thought.........OUCH........dang nabbit hartley, ye made me pull my brain electron by thinking........OUCH!......need to quite doing that!

Marty
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  #22  
Old 09-20-2012
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Re: Moment curves

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Originally Posted by Hartley18 View Post
About the curves: The engineer in me has always suspected that this had something to do with hull balance - perhaps drawn to show by the shaded areas that the finished product should steer straight at various angles of heel without any significant tendency to round up and show where those "forces" are placed along the hull... back in the days before autopilots when this was important. Again, that's just a WAG and I have no idea what the actual technical term for that is.

I'm sure the "c", "b" and "q" numbers must mean something... anyone seen Jeff lately?!?
That's the way it struck me - a bizarre mathematical construct to serve a somewhat similar purpose to the buttock lines on a hull drawing
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  #23  
Old 09-20-2012
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Re: Moment curves

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Originally Posted by SloopJonB View Post
That's the way it struck me - a bizarre mathematical construct to serve a somewhat similar purpose to the buttock lines on a hull drawing
If that's indeed what it is - how do you get that from the lines drawing? That's what I'd like to know...

I was hoping it might tell me something about stability, AVS, that sort of thing.. but maybe that comes from some other part of the design??
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Old 09-20-2012
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Re: Moment curves

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Originally Posted by Hartley18 View Post
If that's indeed what it is - how do you get that from the lines drawing? That's what I'd like to know...

I was hoping it might tell me something about stability, AVS, that sort of thing.. but maybe that comes from some other part of the design??
I was thinking about the use of the buttock lines to cross check for hull fairness and balance.
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  #25  
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Re: Moment curves

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Originally Posted by SloopJonB View Post
I was thinking about the use of the buttock lines to cross check for hull fairness and balance.
Maybe.. but the hull lines are shown quite clearly on other plans I have, so this is for something else again.

Given that this was produced by a skilled draftsman in the days before CAD, I can't believe that:
(a) they did it for no other purpose than to fatten the drawing package,
(b) they made them up.

...so the questions remain...
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Old 09-21-2012
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Re: Moment curves

I agree that there is no connection to the buttocks. This is an area distribution thing not a fairness thing. I think it was just another way, an unusual way, to try to get a feel for how the area curve changed as the boat heeled. But for me to really understand it I would have to sit down with the designer and see exactly how he calculated it. I would ask, "What does this tell you that a simple area curve wouldn't tell you at those angles of heel?"
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  #27  
Old 09-23-2012
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Re: Moment curves

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Originally Posted by bobperry View Post
I agree that there is no connection to the buttocks. This is an area distribution thing not a fairness thing. I think it was just another way, an unusual way, to try to get a feel for how the area curve changed as the boat heeled. But for me to really understand it I would have to sit down with the designer and see exactly how he calculated it. I would ask, "What does this tell you that a simple area curve wouldn't tell you at those angles of heel?"
Given that, unfortunately, he's been dead for over 40 years, that could be a mite difficult!...

To move this to the realms of the possible, Bob, what would a 'change in area curve as the boat heeled' tell you?
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Old 09-24-2012
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Re: Moment curves

Hart:
I have meeting this morning. I'll be back in about 3 hours. At that time I'll do my best to explain what I am talking about. You could prepare yourself for my explanation for finding the issue of GOB where I discuss how t read hull lines drawings. I don;t know the issue But you are going to need some understanding of basic hull form terms. Some of you are already familiar with this vocabulary. Meanwhile I'll continue to go over my response in my head so I can make it succinct and accurate.
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  #29  
Old 09-24-2012
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Re: Moment curves

The question is what do area curves tell us about a boat's performance?
First off I had better define "area curves". Boats are laid out with a series of tranverse stations, i.e. station 0 at the cutwater and station 10 at the intersection of the counter or stern profile with the DWL. I call this the "buttwater" because I think it sounds funny and is kind of accutrate. Armed with a ten station breakdown and some calculus the designer can determine the volume and distribution of the volume of a new design. Today the computer does it all. Years ago we did it all by hand with a slide rule or a calculator. An area curve shows the distribution of volue in a hull by plotting the area of each station along a horizontal line, usually the plan view centerline of the lines drawing. The curve is only a graphic dislay of what the numbers tell you. You do not need the curve, but it is fun to look at and can help especially when you overlay them for similar length designs. You can heel the boat over, maintaining the LCB/VCG alignment and also look at what happens to the area curve. Now you have a "heeled area curve".

Take a tube. Cut it in half lengthwise and put identical points on each end. Call it, for argments sake, a long, very skinny boat hull. With points on both ends it's a "ouble ender". As you heel this hull over it will rotate around the center of the arc with the centerline of the half tube staying parallel to the DWL. But boat hulls are not tubes cut in half. Most boat hulls are complex, asymetrical fore and aft shapes, i.er. the bow is pointy the sten is wide.


The closest thing we have in a real boat to our tube is a double ender ,like the Westsail 32. It's fatter with a far more complex sectional shape but fore and aft it is almost symetrical, i.e. it's hard to tell the bow from the stern once you ignore the curved stem and the straight stern post. A curve of areas for this hull will almost be symetrical around station 5. Or, damn close to it.

You see the same thing in long, skinny boats like 12 meters or 6 meters. Fore and aft they are almost close to symetrical. So, as they heel over they alomost rotate around a line running parallel to the DWL. Keep in mind the LCB, Longitudinal Cneter of Bouynacy will always stay right above the LCG, Longitudinal Center of Gravity. Always. If it does not, then the hull will automatically alter the fore and aft trim so the LCB is right over the LCG.

This is where it gets tricky.

If you take a modern beamy boat with a big fat fanny and a really pointy bow then you have the antithesis of our "tube boat". The fore and aft distribution of volume is going to be scewed well aft with the LCB being around, say 54.5% of the LWL aft of Sta 0. As this very asymetrical beamy boat heels over the volume immersed forward will not be enough to keep the boat rotating parallel to the DWL. The bow will need to immerse more than the stern to pick up new volume and when the bow goes down the stern comes up. Always. I call his "rolling down". The result is the waterlines that stayed almost symetrical fore and aft in the Westsail now become very asymetrical and distorted. Our once almost syemtrical "heeled waterline" now has a big bulge to leeward and is almost straight on the weather side. We are shoving a very strange, asymetrical shape through the water.

Plotting heeled curves of areas can help the designer predict just how much the boat is going to roll down when heeled. If you had a boat with a really full bow and a picnhed stern the exact oppropite would happen. The bow would pitch up as the boat heeled and the stern would roll down. But today we don;t see boats like that.

If you were doing a gran prix race boat you may want to do a study on heeled conditions to determine changes in Cp, LCB movement and wetted surface. Distorted heeled waterlines can give a boat wild changes in personality as it heels. You've heard people say, "You have to sail it flat." The boat with distorted heeled waterlines is not the same shaped boat at 25 degrees of heel as it is a 5 degree of heel. While with a boat like the old Valiant 40 the boat loved to be driven hard and tipped on it's ear. The balanced hull shape of the V 40 means that the hull shape does not change much as the boat heels through 30 degrees. The bow does not roll down and the stern does not pitch up. This is why for a well behaved boat I like to start the design proces by determining just how skinny I can make the boat. Skinny boats have better manners, generally speaking.

I've probably over simplified this. And, no doubt diagrams and illustrations would be a huge help. Check out Steve Killing's book YACHT DESIGN EXPLAINED. It's the best.

Before you come back and attack me think twice. Do some extra research and if you still think I'm wrong bring it on. But I warn you, I didn't invent these relationships. I have only observed them. Hope this has helped a bit.

Time to walk the dogs.
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  #30  
Old 09-24-2012
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Re: Moment curves

Thanks, Bob! I think I understand that.

One question: Do you still think these "moment curves" might be another way of saying the same thing as "area curves" do?? (Sorry, I've seen plenty of lines plans and even understand them bit, but I don't think I've ever seen an area curve..)
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