# How is the LWL achieved in design?

Discussion in 'Boat Design' started by Bluman, Mar 24, 2011.

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### BlumanNew Member

I am a long time sailor but new to the study of boat design. What part of the design process (which formulas or drawing rules) causes the LWL to be where it is. In other words, an asymmetrical shaped volume is going to float differently than a perfectly round shaped volume. How is it determined how that shape will sit in the water?

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### jehardimanSenior Member

The curve of displacement is used to determine the center of buoyancy. The weights are then developed to match the center of gravity to the center of buoyancy. If you have the weight and CoG but not the displacement and CoB (such as a flooding condition), then you develop the curve of displacement from the Bonjean curves.

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### Eric SponbergSenior Member

Back in the old days, before computers, naval architects established the length on the waterline very early on in the design process. Calculations for volume and center of buoyancy were done by hand using the Simpson's Rule calculation which typically relies upon 10, sometime 20, calculation stations, using 11 or 21 ordinates. Other numbers of stations are possible, but these were the typical spacings in boat and ship design. These calculations stations frequently became building stations such that frames were established at least at these volume calculation stations. Therefore, you needed a station spacing that was some easy multiple of the framing spacing. You would calculate the cross-sectional area of each station below the waterline, and then the Simpson's Rule procedure would integrate the areas over the length of the waterline to determine submerged volume and the center of that volume.

In the age of computers, we still need stations to determine cross-sectional areas at various stations, but we can cut these stations at arbitrary positions along the length. We are no longer married to a pre-set station spacing along the length. The computers do integration mostly by the Trapezoidal rule rather than the Simpson's Rule. The more stations you have, the more accurate the volume calculations. When developing the construction drawings, particularly for metal and wood-framed boats, then it is helpful to have a regular spacing for the frames, but these need not have any relation to the calculation stations.

This then means that the designer still has to have a pretty good idea of where he wants the waterline to be vertically so that he can control draft, displacement, and center of buoyancy when he shapes the hull, but he is no longer fixed to a pre-determined length. The length on the waterline becomes another variable that falls out of the design--it is what it is--rather than being a determinant going into the design.

That's progress, actually.

I hope that helps.

Eric

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### BlumanNew Member

Thanks to you both. The light has gone on for me, although now I've got to go back and start over on all the books I'm reading. I think I'm hooked.

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### BlumanNew Member

Eric, I just spent the morning going through your website. I have two things to say. First, I am an immediate fan of your G45 cat-ketch. Mind-blowing. Second, I am very humbled that you even took the time to reply to my post and for that I thank you.

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### jehardimanSenior Member

As Eric alluded to, compters handle most of the actual number crunching but you still need an organized approach to the overall design and drawing of the lines plan.

I start with a volume (displacement) to length curve (i.e. the curve of displacement) for the speed-length ratio I want/am given (these can be looked up in a good NA text). It is important to remember that this volume is the final displacement you are looking for, so make sure it includes all weights, variable loads, and outfitting. The curve of displacement gives the station areas and a midship section within the given beam and draft restrictions based upon standard types (cargo, combatant, speedboat, sail boat, etc.).

The LWL to speed places the midship location (again it can be looked up in a good text), then I put in a few waterlines, diagonals (and or bilge radii) and a profile to cover the "cargo" block buildup (ie saloon, galley, staterooms, container hold stacks, cargo holds, weapons & magazines, engine room and machinery spaces, etc.).

Then the cross-checking of stations/waterlines/diagonals to the displacement-length curve goes through a few cycles as I put in some control points (design waterline, deck edge, turn of the bilge, etc) for each station and begin my weights. Cycle and repeat until structure, decks, weights, powering, and prime mover are pretty settled. Then I finialize the shell and do a clean lines plan. After the lines plan is finished and all the limits (tonnage, length, beam, etc) are met, then I pull the offsets off.

Of course, a lot of that work today depends on how you are developing the drawing, by hand to be digitized or directly on the computer. Sometimes you need to work with (or around) any drafting program you are using. FWIW, I have not yet found a CAD program I like for developing a lines plan from scratch.

Last edited: Mar 25, 2011
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### Eric SponbergSenior Member

You are welcome. My pleasure.

Eric

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### BlumanNew Member

Thanks Jehardiman. You have mentioned many terms that I have come across but still lack a full understanding of, so I am using the replies on this thread as sort of a study guide. Can any of you NA's recommend a NA primer that I could start with for study? I've been reading books on boat design, but often they don't go into the theories that deeply that you senior members a mentioning. On the other hand, I'm strictly an amateur and don't think I can dive into a full blown engineer's treatise and get much out of it.

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### rxcompositeSenior Member

I hope Eric asnwers this.

I am a boat designer and have 3 Naval Architects colleagues. The Master degreed NA says The LWL is the most important and should be determined first and should be divided evenly for simplicity of simpsons rule adding that any section (or half section) can be added later if it is above waterline. That I should start 0 at the bow. That I could divide uneven only when strength calculation fails.

The 2nd senior Naval architect however says I should start 0 at the rudder post and progress towards the bow. When I showed him the book drawing, he argued that those are stations and he was referring to the numbering of frames. Further that stations are used for calculating volumes and LWL is determined from the rudder post to the bow intersection at WL.

The 3rd NA however started his 0 number at midship and assigned + numbers towards the bow and negative numbers towards the stern. His explanation, the midship is the most important section as this is where the midship section drawing is taken from.

Which one is the correct and accepted way in the field of Naval Engineering or more specifically what is the International Standard?

Was the 2nd NA correct when he said the frame numbers and station numbers are different?

I have doubts about the 3rd NA, but he is also a licensed practitioner so no arguement.

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### Eric SponbergSenior Member

rxcomposite,

I don't think there is an international standard on how to number stations, or in which direction. It is like drawing a boat a ship, do you draw it bow pointing to the right (preferred in America) or pointing to the left (preferred in Europe and in Oz-NZ)? It does not really matter as long as you are consistent throughout your hydrostatic and stability calculations and your construction drawings.

Frames are typically numbered different than station numbers. Stations are for calculating hydrostatics and stability, and frames are for construction. They can coincide, or not, depending on the whims of the designer. Again, be consistent and make sure all things are marked properly on the drawings.

Here is an example of how my drawings have evolved over my career. When I was in college and early in my career, I used 10 stations (11 ordinates) with station 0 at the forward end of the waterline. Hydrostatics and stability calculations were done by hand and based on those stations. Now, with the computer, I put station 0 at the forward peak of the stem--the furthest forward extremity of the hull, and the stations are arbitrarily spaced. When there is a longitudinal discontinuity, lke at the forward and after ends of a keel, you need more stations closely spaced. Where there is little change in shape along the hull, the stations can be fewer and more widely spaced. They are numbered consecutively, but have meaning only for the hydrostatic and stability calculations in the computer file. They have no meaning for the boat's construction. If I or the client want a lines plan, I typically show the lines at the frame numbers if a metal boat, or at arbitrary stations if a composite or wood-epoxy boat. On metal boats, the frames as lines stations are helpful because I need the frame shape for the construction drawings. Typically, every frame gets drawn, and notes are added for important construction notes and for all the welding specifications. The frame numbers, if a metal boat, will also start at frame 0 to correspond to station 0 at the top of the stem, but then they follow a regular spacing that is easy to build. If it is a composite or wood-epoxy boat, there may be few to no frames, and so frame numbering becomes immaterial.

Do what seems to be the common practice where you work. Stay consistent so that the drawings are easily understood.

I hope that helps.

Eric

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### Tim BSenior Member

I recently did a project where I was told that a certain sensor was located between two particular frames... that's great, but where is it? X,Y,Z co-ordinates?

Where you put the origin is entirely up to you. I tend to use midships, centreline, DWL with X forward, Z up, Y to port. However, for newly drawn hulls I don't worry about whether the origin is exactly in the middle of the extents, as it doesn't actually matter. As long as you don't move it, and measure everything relative to it, you are fine.

Incidentally, openDynamics upcoming hydrostatics engine uses the integral over a triangle mesh. Therefore, instead of using tens of sections, I use a few thousand triangles, and let the computer handle the hard work (including using a dynamics method in 3DOF to solve for heave/pitch/roll).

This probably doesn't clarify anything,

Tim B.

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### jehardimanSenior Member

Let me say that I believe that anyone with a lick of common sense and a fair hand with tools should be able to build a boat. Boatbuilding is the second oldest profession (I place it before priest and after strumpet which I think is a good fit; right between "I'm in it for the money" and "you gotta have faith") so if you really want to stay an amateur you are in good company. However, if you want to dance on the edge you have to know where the edge is, and why it is there.

FWIW, I think good boatbuilding, and naval architecture in particular, is one of the last "guild" systems. You have to serve an apprenticeship to really grasp all the factors that make up a good boat. This is not to say that anyone can't build a good boat, but that it is all too easy to build a poor one. A then again, it used to be pretty hard to make a deadly boat...easy to make a poor one, but hard to make a deadly one. Boatbuilding used to be a slow evolutionary process where new designs followed closely the 'good' old designs. However, with today’s light weight and fabrication critical materials, high power engines, and access to "internet how-tos" it is quite possible to build a boat radically departed from tried designs and appropriate dimensional relationships.

Following basic books like "The Nature of Boats" (Gerr), "Skene's Elements of Yacht Design" (Skene and Kinney), or any of the other "look-up chart" books will keep someone who doesn't want to get in too deep towards the middle as they distill previous design trends. Getting a little closer to the edge are books that lay out the basic theory behind naval architecture and structures like 'Introduction to Naval Architecture" (Tupper), "Basic Ship Design" (Rawson & Tupper), "Principles of Naval Architecture" (SNAME), and "Principles of Yacht Design" (Larsson & Eliasson). For the real hard core ship and boat design there really is no substitute for engineering classes. Bleeding edge design is a combination of theory, lab, and practical experience closely coupled with material and fabrication development. And it is very easy to dance off the cliff and possibly kill someone with an "unknown unknown" when pushing design that far. The fortunes and lives of men reside on the naval architects’ decisions and you should understand when you cross the line and why.

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### rxcompositeSenior Member

Thanks Eric. That means all 3 NA's are correct and it is a matter of preference.

I did not work with all of them at the same time so the conflict of practice arose. I guess if the three of them would work together, there would be endless discussion very early on during the lines plan drawing.

The bow direction was also a good point of knowledge as some NA would present bow pointing to the right and some would point to the left. I see this in much of the drawings presented to us.

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This is a very interesting read. Mainly from the point of view that clearly each of us work in different fields of naval architecture, and hence arrive at different ways of “doing our job”.

In my field, commercial high-speed vessels, this is the first parameter we select, the waterline length, as this provides the Fn. We need to know the Fn so we can ascertain what the main performance drivers which leads to the selection of the hull form based upon the L/D ratio.

I agree. However the closest I have found is Paramarine, with their XT Curves, this is amazing, very easy to draw a simple or complex LP. I tend to hate the parent child tree of “attributes”...they are all written by computer programmers, not naval architects. Thus the nomenclature is often so abstract it is meaningless. We use very clear and precise words/terms in naval architecture, I have yet to come across a program that uses them!

For me, …see above.

The caveat being, I think this relates to which field of naval architecture you are in and hence which are the most important parameters that drive the design. Hence as Eric says, there is no standard, one does the most appropriate in the filed/discipline you’re in.

In our field though, once we have the hull with the Lwl that satisfies the SOR, the midship location is important. The main reason is that this determines the location of the LCB. Some hulls we use are best suited with an LCB of 8% aft..some approximately 0%...so for us, the location of midhsips is important.

However, I do agree that…

So long as the location of the datum does not move…everything is consistent. Until you try and compare the performance with another hull that is correct of course, gets a bit messy correcting datums!

Whole heartily agree.

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### rxcompositeSenior Member

I always thought prostitutution is the first or oldest profession.

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