Notes on Skene's Elements of Yacht Design

[boradicus]

Hi all,

I am creating this page as an online note taking reference for myself, and hopefully it may be of some benefit to the rest of you. Comments are welcome from all.

I recently downloaded the free PDF book, Elements of Yacht Design, by Norman L. Skene, and provided by Cornell University's Library in downloadable PDF and ebook formats. I prefer the PDF format due to the fact that OCR technology still leaves much to be desired, and as a result, the PDF version is much clearer for me to read.

I am taking notes in order to better learn the principles of nautical design. Feel free to join in and add commentary as we go along.

Thanks!

 

[boradicus]

CHAPTER ONE:

- There are four general characteristics to yacht design:
* Seaworthiness
* Cabin Space
* Beauty
* Speed
At best, only two of these are usually successfully present in a single design.

- There are three kinds of resistance that a vessel encounters when passing through the water:
* Frictional
* Wave Making
* Eddy Making

Frictional resistance forms a large part of the resistance at low speeds and is decreased by reducing wetted surface area, and by having the immersed surface as smooth as possible.

Wave making resistance is the chief form of resistance at high speeds. The reduction of this form of resistance is accomplished by reducing displacement.

Eddy making resistance occurs at places where the streamlines terminate abruptly, such as at the stern, and the aft edges of the keel, rudder, etc. Fining the streamlining at these edges reduces the eddy making resistance of the corresponding surfaces.

For speed, if hull length is set, sail area may be increased; if sail area is limited, hull length may be increased.

 

[hoytedow]

Reading is fundamental.

Don't post the whole book.

 

[boradicus]

Chapter Two - Methods of Calculation

CHAPTER TWO:

-The majority of the calculations of the naval architect devolve to finding the area bounded by a curve and the center of mass relevant to that area.

-A simple method to find the area under a smooth curve is to use Simpson's method:

* ABCD is an rectangle bounded by a smooth curve from B to C.
* a is the line AB, e is the line CD, b c & d are lines in between and parallel to a & e, all with the uniform distance, s, in between.
* The area of ABDC = 1/3 s * (a + 4b + 2c + 4d + e)
* Simpson's Rule assumes the portion of of each segment of the curve to be an arc of a parabola tangent to the midpoint between the segments (e.g. a, b, etc).
* Simpson's Rule has a .2 percent margin of error in comparison to using calculus to determine the area.

It appears that the center of mass is found by taking the sum of the moments of each segment (a, b, etc) on either side of the central most segment, and then taking the difference between the sums for each side and dividing it by the area. The result is the distance to from the center most segment to the center of mass on the side with the greatest area, and then multiplying this by the distance, s, between each segment.

If someone could verify this I would be obliged! Thank you in advance!!!

- A fast way of finding the center of gravity of a trapezoid ABCD, would be to find the intersection of the midpoints of triangles ABD and ACD along lines parallel to AC and BD. I suppose this method could be used with less accuracy via the inscription of a trapezoid inside the of one half of a hull viewed cross-sectionally, perpendicular to its longitudinal axis.

- For determining approximate wetted surface area, Taylor's Mean Secant Method is purported to be the most accurate. Alternately, the Bilge Diagonal Method may be used foe "small" work.

I must say, I am a bit lost here, perhaps due to the lack of visual cues and familiarity of dealing with the topic matter. There must be a better way to approach finding the surface area of a hull using topology. Perhaps I need to get better educated .

-Moments of Inertia (both along the longitudinal axis and the transverse involve using "half-breadths" - and until I clear up that term, I will move ahead to other material).

-Figuring the Mass of the Hull is relatively simple, if not somewhat involved. Notably, he recommends weighing the vessel after construction to double check one's calculation of its mass.

 

[boradicus]

Post the Whole Book

Reading is fundamental.

Don't post the whole book.

Definitely not! This is more so I can determine a few key points I would need to remember as well as points I am going to have to revisit until I better understand !!!

 

[boradicus]

Chapter 3 - Displacement

CHAPTER 3:

- The displacement of a vessel is equivalent to the mass of the water displaced by the hull below the hull's waterline, and is commonly expressed in pounds.

- Displacement is determined by the lines of the yacht.

- There are approximately 35 cubic feet of salt water (and 36 of fresh water) in one ton.

- In one cubic foot of water, there are 64 pounds of salt water and 62.4 pounds of fresh water.

- The center of bouyance is the center of figure of the submerged vessel (I am not certain what he means by center of 'figure'). The force of gravity on the mass of the vessel and the center of bouyancy both act on this point, and are in the same vertical line or the vessel will alter its trim to bring them into line with each other.

- Design should entail making the sum of the mass of the various internal items of the yacht equal to the mass of its displacement at the waterline. I suppose this includes any passengers and luggage, equipment, and stored goods, as well as anchors, and other pieces and machinery of a fully functional vessel.

- The proper determination of the displacement of a vessel is the most important element of its design.

- Fineness of Design is determined by three coefficients:
* The Midsection Coefficient
* The Block Coefficient
* The Prismatic Coefficient (the most important one)

- Both the fore and after body should curve in the shape of a versed sine wave and a trochoid, respectively, longitudinally from the tip of the bow to the bilge, and similarly at the stern.

 

[boradicus]

Chapter 4 - The Lateral Plane

CHAPTER 4:

- The Lateral Plane is the vertical, longitudinal, underwater projection of the body of the vessel, the center of which is the point at which the lateral pressure of the water on a sailing boat, close hauled may be considered to be concentrated. It is considerably further forward at any given time of the center of figure of the lateral plane due to the fact that the vessel is moving forward through solid water. The wave that is consequently piled under the bow as a result of motion tends to increase in pressure in that region, and to decrease in pressure toward the stern.

- The contour of the lateral plane of the boat when it is heeled is different from when it is erect. The center of bouyancy also has an effect upon the position of the lateral plane.

- Although difficult to pinpoint, its location can be defined as being between certain limits for the purpose of placing a corresponding plan of sail. This distance can be approximated as being 56 to 57 percent of the forward waterline (does this mean that percentage of the LWL toward the stern from the point of the forward water line???).

- A centerboard boat requires less lateral plane than does a keel boat.

- Rudders can be functionally ignored for the purpose of figuring lateral plane because they have little bearing (no pun intended).

- Modern designs have a lateral plane with a shape concentrated both fore and aft, which contributes to more efficiency, less wetted surface area, and quicker turning potential. Carried to too high a degree this will contributes to a lack of seaworthiness, and for this reason should be avoided on cruising vessels. In order for a boat to lie hove to well the lateral plane must be well spread out in a fore and aft direction.

- The forward portion of the lateral plane is the most effective for lateral resistance; therefore the leading edge should be made as long as possible.

 

[PAR]

Boradicus, many of us are well versed with Skene's classic text. Also, since it's debut over a century ago and it's most recent revision being some 70 years back, much of the information is at the very least, crowded by modern innovation and technological improvements. Some consider the text nearly valueless, though I agree with Bray, who wrote the introduction to the latest reprint in 2001 and think there's plenty of useful information, contained within, I'm not sure how your regurgitation, of what you perceive are the chapter highlights, can best served in this format. Particularly in light of the obvious prejudices (by Skene) shown in some of it and the huge crevasse that isn't covered, in actual modern craft, which are unlike the "modern craft" he speaks of (CCA racers of the 1940's era).

 

[boradicus]

PAR, thank you for your response. I suppose, in the same way that others have pages devoted to boat projects, this a "learning" project page for myself, and I hope for other new comers to the field, which granted, there probably are few comparatively! While I could remove the page, being a new comer myself, I was hopefully not too selfishly hoping for insight from others as I try my hand at learning a subject I have very little knowledge about... My hopes are that by taking some notes about things that make sense and other things that I find to be more obscure, that along the way somehow I will be better able to identify why some parts of Skene are outdated, and why some of it is still quite relevant. But I suppose at the heart of it all, I am really just quite eager to begin learning! I have ordered Marchaj's SAILING THEORY AND PRACTICE and I am just immersing myself in whatever I can get my hands on in the meanwhile!!

 

[boradicus]

I should probably invite you to clarify what you mean by Skene's prejudices, as it would highlight better for me a sort of road map in terms of better defined land marks to look for while learning design. Thanks again!

 

[boradicus]

Chapter 5 - Design

CHAPTER 5:

- First, fix all principle dimensions (LOA, LWL, BOA, BWL, Draught, Lateral Plane Area, and other proportions)

-There are three principle yacht classes:
* Keel boats with outside ballasts
* Centerboard boats with inside ballasts
* Semi-keel boats with centerboards and outside ballasts

- Using the LWL and the vessel classification, one can then look up in Skene's tables the corresponding beam at the waterline, figure the draught, freeboard, and displacement, and then proceed to determine the proper area at the midsection for this displacement. Skene gives a suitable prismatic coefficient for a cruiser as being .50 to .52. Then the lateral plane is calculated.

- A drawing is then made, inclusive of the sail plan, and deck plan. Skene notes that it is generally good to have the widest point on deck abaft. Making a model will help to clarify points of difficulty in the design process.

 

[boradicus]

Chapter 6 - Stability

Chapter 6:

- The Center of Bouyancy appears to be the midpoint between the beam intersected by the midpoint between the waterline and the draught.

- The angle of heel is the angle that exists between the water's surface and the normal waterline of the vessel (when not heeled), elevated above the water's surface. As long as G is below M (see the diagram) the stability is positive (the vessel will return to the erect position).

- The statical stability of the yacht is equal to "a," the righting arm, multiplied by the hull displacement. This can be figured for various angles of heel. More stable vessels have a shorter distance (height) between G and M.

-This theoretical measure of stability changes dramatically when a yacht is at sea and supported by the motion of waves, and must be considered to be comparative rather than quantitative.

- Values for wind pressure may be calculated for each moment of heeling: Displacement * righting-arm = sail-area * h * pressure * cos(heeling angle), where h is the vertical distance between centers of effort and centers of resistance (what does this mean?). :?:

- While the above are all measures of static stability, dynamical stability is the integral of the curve of statical stability, and is the amount of force needed to "bring" the yacht to heel at a given angle.

- The period of oscillation is of importance with relation to speed; which is the reason for "winging out" the ballast.

- It is also important to know the longitudinal values relating to stability as well.

 

[groper]

Bora, download a free program called "freeship". Learn to use that and you will gain a whole lot of information in a short time. It will automate all of teh hydrostatic calculations for you, generate a lines plan, give you your CoG and many other hydrostatic values such as the stability numbers etc. You can assign thickness and density to all your surfaces and this will give you weights and areas etc. It will also develop your panels (lay them out flat) so you can cut them from your chosen material etc.

Not saying it does everything for you, but leaning to use it will speed up your progress and get you familiar with some of the basic design concepts and you can see how changing the shape of things effects hydrostatics etc...

 

[boradicus]

Thanks, Groper. I will definitely look into it. I have delftship already, and am attempting to learn how to use it. I am kind of like an old crab, going sideways back and forth, slowly crawling forward !

 

[hoytedow]

Perhaps the forum will be better served if you were to post specific questions about the PDF rather than utilizing the laborious practice of spoon-feeding it page by page. Just sayin'.