Notes on Skene's Elements of Yacht Design

Smart.

No, it is determined by the mass of the vessel and by the water density. The lines are irrelevant.

That's an old trochoid-wave theory which was used in the past for giving shape to sailboats and some powerboats. Not in use anymore, as it has been replaced by hydrodynamic knowledge coming from research on various parent hull forms. But it is ok to learn the trochoid-theory as well, both for personal culture and because it will help you explain the hull shapes of many classic yachts.

Didn't get this one. Could be true for some particular hull shapes, but not as a general rule.

Cheers

 

Hoyt, what a great idea! Okay, I can do this! Let me recapitulate some of my questions and quandaries below:

1. In Chapter 2 - what does he mean by a half breadth, and how does this translate into an equation for solving the moments of inertia? I take it that a "station" is a vertical line through the hull at which a given moment of inertia is calculated - is that correct?

2. Also, in Chapter 2, any help with either the Bilge Method or the Mean Secant Method for determination of the wetted area would be appreciated. Maybe just a better explanation or a 3D perspective drawing would help to clear up where I am getting fuzzy on things. I can sort of visualize what he is talking about but I believe that an equation with a clear explanation of the variables would untangle things a little for me.

3. From Chapter 3, I believe that I have finally figured out how he is determining the Center of Bouyancy, in that it is the center on all axes between the waterline and the draught of the vessel - is that more or less correct?

4. Also from Chapter 3, he goes to great length to explain why the bow and stern ought to be longitudinally shaped as a "versed sine wave," and a trochoid, respectively, yet his example of his 30' cruising yacht does not seem to make much use of this design consideration. Is the wave theory that he is talking about still valid in today's thinking? Also, I was not very clear about the formation of the trochoid. Although lengths of the wave formed by the trochoid seem to be a function of the changing "chord" (that he calls the diameter) of the half circle. Again, a formula with variables would help clear this up considerably!

5. In Chapter 4, there is no drawing of the lateral plane, and so while I am able to sort of visualize it, I am not sure what I should visualize it in relation to. A drawing of the lateral plane and how it relates to other relevant forces might be helpful to me here. I suspect that some of these things might eventually become more clear the further I read in the text, but this was a bit fuzzy. For one, in my thinking, a plane is not usually something that is "shaped." The other thing about the determination of the lateral plane would be what he means by the percentages he uses. Is he talking about the forward bound of the lateral plane being set at a certain percentage aft of the further most point of the bow at the waterline - or what is he talking about? This older original edition may not be as clear as some of the later edited editions. Again, a drawing would be quite helpful!

6. I am going to have to table questions for Chapter Five until I actually sit down and try to work through an example to see if I have understood well enough what he is saying there.

7. In chapter Six I have two questions. The first is what does he mean by "winging out?" I am making the intuitive assumption from the sound of the term that he means that one should evenly distribute mass along the curve from the bilge of the hull to the waterline so that at any given angle of heel (up to where stability is neutral???) has the same vertical mass. Is my understanding of this close? I am assuming what he is saying is that the hull should be able to heel easily within a certain predetermined arc. But he is not being clear to me where the safe bounds of that arc lie. I would think there ought to be some margin for safety in not bringing the hull to heel too closely to the point at which its stability is neutral, and therefore easily compromised with respect to its righting arm. The other question I have is what he means by "h" being "the vertical distance between centers of effort and centers of resistance." Again, a picture, or at least a clearer description would help immensely!


Okay, I believe that's it for now! Thanks for all helps and aids well in advance!!!

 

Those are great questions, Boradicus. I don't have any great answers.

 

Oh - with respect to "winging out," maybe I should have said that I think he means that at any given heel that the hull has an equivalent "righting arm" - not mass. Is that right?

 

Thanks for the idea!!!

 

daiquiri - thanks for the response! That really helped clear up a few questions - especially about the current use of what he calls "wave theory."

What is the real method for determining Center of Bouyancy?

Thanks!!!

 

Here is a definition of half breadth: http://dictionary.reference.com/browse/half-breadth plan

"half-breadth plan [haf-bredth, -bretth, -breth, hahf-] Show IPA
noun Naval Architecture .
a diagrammatic plan of one half of the hull of a vessel divided lengthwise amidships, showing water lines, stations, diagonals, and bow and buttock lines."

 

Boradicus, may I suggest you to start with this book instead: Principles of Yacht Design - by Larsson, Eliasson: http://www.amazon.com/gp/offer-listing/0071487697/ref=dp_olp_used?ie=UTF8&condition=used
It is all explained so well in there, with a lots of nice pictures and drawings.

Cheers

 

Thanks! What is the bilge diagonal between stations (Ch. 2)?

 

Thanks! I have Marchaj's Sailing Theory And Practice on order - do you think the basics are covered equally well in that text? Thanks in advance.

 

Don't know - don't have that one.

 

Daiquiri - thanks again !

What I understand both the Bilge Diagonal Method and the Taylor Secant Method to do is to approximate the mean through a curve in the form of a straight line. Would it be correct to say that the convergence of the series of these mean lines would then approximate the length of the curve? Then, having determined within reason the length of the curve (from bilge to gunnel, ostensibly) one could then multiply this by the LOA to get the surface area of the half-breadth of a vessel?

But what about the curve along the LOA?

I suppose, as well, that the surface area of the stern (perpendicular to the longitudinal surface area) is of little value for the determination of wetted surface area for understanding resistance to forward motion.

If these assumptions are in essence correct, then what would remain for me to understand is how the convergence of the series works for building up the length of the vertical curve of a half-breadth of the hull.

Thank you in advance, once again!

 

here.Ok, I got some clarity on the Lateral Plane

 

I found a link, here, that explains fineness in terms that make better sense to me.

 

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