# Reverse Engineering (conversions And Modifications)

Discussion in 'Boat Design' started by viking north, Dec 25, 2010.

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### viking northVINLAND

Thanks to all from all for the support to keep this thread going. Thus far Mr.X has received enought info in that he can do a good job of taking his hull lines. Question, Once the lines are taken off his keelless and unknown waterline hull and presented to his architect/designer, is there enought info to determine the hulls prismatic coefficient number winthin reason to advise if the hull lends itself toward being a good all around sailer, a heavy cruiser or more in the motorsailer range.?

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### ScunthorpHull Tech

Viking if you know the volume of the vessel and the scantlings i.e. the thickness and material of the hull regardless of your intended ballast you should have a “draft” a water line. It follows that from there you should be able to find where the water line will go should you bolt on a couple of tons of lead engine and such and such. With basic stability calculations (similar triangles) you should be able to calculate for any add ons regardless of where they go. The existing hull is what you have it can’t be changed. By Prismatic Coefficient you mean (Cp) is the volume (V) divided by Lpp x Ax. The prismatic coefficient, Cp, describes the fineness of a hull and is the ratio between block and amidships coefficients. For a sailboat, this number will be as low as 0.48 for a boat designed for light winds, to 0.62 for a downwind sled. I don’t think my hull will be fine I would be interested to hear from the brainy bunch has on this one. (I know I know don’t do it) they told me that about ************ more lip stick for me pig please…

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### viking northVINLAND

Bertku, welcome to the thread, it's been a little hot at times but in the end i think it will be worth it. Given it a little time and i'm sure it'll get around to metacentre.Mr. X is working his way along one step at a time (smiley face)Hope his hull is suitable for something as we have to continue in the process. Geo.

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### ScunthorpHull Tech

Could I do better

Ok so here are a few some designed by professionals and I am guessing some not.
What is the pecking order
Naval architect, marine engineer, boat designer
who is king of the poop pile here anyway???

Do you know of an ugly boat?

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### viking northVINLAND

Scunthorp posting #62, informative, posting #64, ?????

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### BertKuSenior Member

Don't worry. I had my fair share with my thread "Did a dream got shattered" .

Anybody who knows : AROUND what axes does a boat heels. CB ? CG? crossing DWL with vertical 90 degree line?
Bert

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### YuriBJunior Member

Just one more conversion project,

Used common sence, no one naval architect was killed when giving directions.

For all the rest opinions, just pasting a quote:

One of the greatest sailing disasters in recent maritime history, the 1998 Sydney-Hobart Race, offered a number or lessons regarding the performance of sailboats and crews in heavy weather conditions. The 1998 Sydney to Hobart Race Review Committee report, summarized by Peter Bush, the committee chair, reported the following as one of the significant findings: "There is no evidence that any particular style or design of boat fared better or worse in the conditions. The age of yacht, age of design, construction method, construction material, high or low stability, heavy or light displacement, or rig type were not determining factors. Whether or not a yacht was hit by an extreme wave was a matter of chance."
(Ref: Rob Mundle in Fatal Storm, Publisher's Afterward p 249. International Marine/McGraw-Hill Camden, Maine.)

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### ScunthorpHull Tech

Exactly Yuri so what is the hierarchy NA, Engineer Boat designer? I would assert that it has been the trajectory of many self actualized amateurs which have fed the children of these noble men. “They” have gone without permission or degrees or understanding into the void as it is. Further, it is the “fear” the “ignorant wealthy” who watch from the shore competing only with the shallow attributes money can get them “bigger, faster, shiny” without really experiencing. I am not denouncing education, study, or fact. Not even wealth or power. I will however engage with arrogance, selfishness, and twits. I don’t know if I can contribute to the knowledge of those converting or re engineering or whatever we are trying to do here. I am however, unashamedly, willing to share my experiences of the process in figuring out this desire, this boat stuff. So “Let’s get her done”

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### BertKuSenior Member

Hi Scunthorp, What you wanted to say is now off your chest, can you now help me with a tiny bit of information.
I need that information to double check my righting arm calculations I have done on this "reverse whatever engineering" .

Around what axis heels a boat? CB, VCG or drawn waterline and vertical line ??? Various pictures in my books, shows different answers.
Bert

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It is not a fixed axis.

It is based upon the shape of the hull..you basically end up with a loci of the KMt at large angles.

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### ScunthorpHull Tech

Dude I am a mear mortal and there will be people (I hope) who can go into this as deap as you need. Try some of this and see if it makes sense.

"When a ship is heeled, the center of buoyancy of the ship moves laterally. The point at which a vertical line through the heeled center of buoyancy crosses the line through the original, vertical center of buoyancy is the metacenter. The metacentre remains directly above the centre of buoyancy regardless of the tilt of a floating body, such as a ship. In the diagram the two Bs show the centers of buoyancy of a ship in the upright and heeled condition and M is the metacenter. The metacenter is considered to be fixed for small angles of heel; however, at larger angles of heel the metacenter can no longer be considered fixed and other means must be found to calculate the ship's stability.
The metacenter can be calculated using the formulae:

KM = KB + BM

Where B is the center of buoyancy, I is the Second moment of area of the waterplane in meters4 and V is the volume of displacement in meters3. KM is the distance from the keel (bottom middle section of the ship) to the metacentre.

Initially the second moment of area increases as the surface area increases, increasing BM, so Mφ moves to the opposite side, thus increasing the stability arm. When the deck is flooded, the stability arm rapidly decreases.The center of buoyancy, is the center of the volume of water which the hull displaces. This point is referred to as B in naval architecture. The center of gravity of the ship itself is known as G in naval architecture. When a ship is stable, the center of buoyancy is vertically in-line with the center of gravity of the ship.

The metacenter is the point where the lines intersect (at angle φ) of the upward force of buoyancy of φ ± dφ. When the ship is vertical it lies above the center of gravity and so moves in the opposite direction of heel as the ship rolls. The metacenter is known as M
The distance between the center of gravity and the metacenter is called the metacentric height, and is usually between one and two meters. This distance is also abbreviated as GM. As the ship heels over, the center of gravity generally remains fixed with respect to the ship because it just depends upon position of the ship's weight and cargo, but the surface area increases, increasing BMφ. The metacenter, Mφ, moves up and sideways in the opposite direction in which the ship has rolled and is no longer directly over the center of gravity.

The righting force on the ship is then caused by gravity pulling down on the hull, effectively acting on its center of gravity, and the buoyancy pushing the hull upwards; effectively acting along the vertical line passing through the center of buoyancy and the metacenter above it. This creates a torque which rotates the hull upright again and is proportional to the horizontal distance between the center of gravity and the metacenter. The metacentric height is important because the righting force is proportional to the metacentric height times the sine of the angle of heel.

When setting a common reference for the centers, the molded (within the plate or planking) line of the keel (K) is generally chosen; thus, the reference heights are:

KB - Center of Buoyancy
KG - Center of Gravity
KM - Metacenter

Righting arm
Distance GZ is the righting arm: a notional lever through which the force of buoyancy acts.Sailing vessels are designed to operate with a higher degree of heel than motorized vessels and the righting torque at extreme angles is of high importance. This is expressed as the righting arm (known also as GZ — see diagram): the horizontal distance between the center of buoyancy and the center of gravity.

GZ = GM sin φ

Monohulled sailing vessels are designed to have a positive righting arm (the limit of positive stability) at anything up to 120º of heel, although as little as 90º (masts flat to the surface) is acceptable. As the displacement of the hull at any particular degree of list is not proportional, calculations can be difficult and the concept was not introduced formally into naval architecture until about 1970"

I hope this is helpfull if not then perhaps you can re state your question or what you are trying to achive or perhaps our NA's can jump in cheers.

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### viking northVINLAND

Ok while the boys are off into higher forms of math. lets us back yard types keep an eye on what Mr.X's next move is. We got only one answer to the last question in that is yes the designer can get the prismatic coe. but to do so it will cost Mr.X time and money as the hulls weight(displacement) a factor of waterline which in turn as you have read in Skenes is a factor of determining prismatic coe. will have to be calculated thru the detail process of determining the hulls weight from the materials it's built from. Which now brings us to the purpose of the question in the first place which is to drive home how to save money by supplying as much info to the designer as possible. As mentioned in several past posts WEIGH the hull and here is where that info when supplied with your hull lines to the designer will make finding the waterline easier and thru calculation one of the magic numbers(prismatic coe.) you need less costly. By the way this is just a working or draft waterline but as you will see plays an important role in the continuation of the build. Later today lets pose another question, this one a little more challenging in that it's resultant magic number plays a couple of roles in Mr. X's build. Geo.

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### BertKuSenior Member

Yes,ineed it is not fixed. I figured that out in the meantime. Therefore were all the pictures differently in the books and with Wikipidea. Thanks goodness, my calculations were thus at the end correct.
No fun to fiddle with pins and wire to get volume.
Thanks
Bert

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### BertKuSenior Member

Sorry Viking my apology , will not interfere anymore
Bert

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### BertKuSenior Member

Viking, I am thinking aloud now. This is what I would have done.
1) make some very good quality photo's of your hull , sideway and top etc.
2) borrow a material thickness tool, and measure the thickness of the hull wall.
3) make photo copies of the photo's on see-through paper
4) build something as close as one can with a program like Delfship etc.
5) place the see-through paper over what you have created with the software program on the screen and adjust untill you are happy.
5) let Deltship decide for you what kind of "lucky dip"you have bought, where the CB, CVG and Cp is.
But I have never played with those boat design software, thus I don't know how easy it is.
Bert

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