on dimensions

[Snoo973]

ok now i know someone there is going to get annoyed, but i cant help asking (thats how u learn things right?). I know the "you cant scale a boat up and down as u like" story, i just wonder: in the past builders didnt really have blueprints or anything and they started from an half hull model, then this boat could have been either 20' or 40' on depending on what was in the builder mind. The thing worked just because of the know-how of the builder and a lot of guessing then?
Ive read a few times on the net about pilot cutters built up half size the original and they kept speed and stability, its a quality of their kind of design or just a case?

[WayGray]

I am no world expert, but have designed and built five boats (#6 will be built this summer). For guidance I always look at existing designs for comparison. I have found that you can scale designs up or down by 10-15% without too many problems. The easiest way to scale is to simply space the frames closer or further apart; that doesn't alter the design too much.

I would really question whether a design could be scaled by as much as 50% and not require some fairly radical (but perhaps hidden) adjustments to keep curvatures and displacement in balance.

[A. Mason]

I'll take a stab at answering your question.

In past generations, boat builders began their apprenticeships while still children and basically grew up immersed in the theory and methodology of boat building.

My grandfather apprenticed when he was around 10 years old with his uncle. They learned by observing their elders first, then doing for themselves. In those days, builders usually stuck with a particular type of design that functioned well in their local area.

Many of the earliest Monterey purse seiners were built by immigrant Italian boatbuilders who used neither half-models nor blueprints. Basically they mimiced the construction methods used for generations in their native area of Italy. They adopted the materials readily available in California but used the principles they had been taught as children.

You might call it guesswork though it was really a lot more than that. Some of it was a bit of experimentation based on a general set of principles that had worked for a long time. I've no doubt they had a failure or two along the way. I believe the key was thorough familiarity with a specific type of design.

Think of a person who can walk into a kitchen, grab a bunch of ingredients, and whip together a delicious meal without the aid of a recipe - same principle.

Anita

[Roger LaPrelle]

Something to keep in mind is that simply changing the spacing between frames is not scaling, it is stretching. When you stretch, proportions change and it is automatically a new design. When you scale, you stretch in all three dimensions to the same scale. A 10% increase in length represents a 33% increase in scale (1.1 x 1.1 x 1.1 = 1.33, approximately). However, scantlings have to increase as well, and , not being a Naval Architect, I don't know how to calculate this increase. There are many similar designs of varying lengths available, especially if you are looking for a Gaff Cutter, for example. Let the designers assume the headaches of the larger (or smaller) version of the design.

Oops, soapbox syndrome again...

Roger

[Dave Fleming]

Agree with Anita, the emperical knowledge that shipwrights gained in the past was a big factor in the sucess of a new or altered design. It does no good 'just' to be a good mechanic. What was necessary was getting out on deep blue and seeing first hand how things work or don't work.

If you do the search thing, you will come across the thread in this particular Forum on how Capt'n Nat designed a new vessel.
IE: Hand Carved Model, custom take off device, offsets recorded in small notebook, passed on to head loftsman, lofted full size, gone over bye the Capt'n and Loftsman, corrections made and noted and then and only then was a vessel built.

There are rules to be followed in the scaling up or down of a known design and I would suggest you do a bit of research on this aspect of design/alteration before beginning work.
We are fortunate to have several Designers/NA's that read and post to the Forums. I would encourage you to seek them out with an:
Attn: Designers/NA's
as title of a thread and then include as much in the way of particulars regarding the design and your intended modifications and see what they have to say.

[Bruce Taylor]

There's a lot of truth in Anita's remarks, I think.

I spent a couple of weeks down in Carriacou, Grenada, recently, where boats have long been built from half-hull models carved by guys who don't necessarily know how to calculate a righting moment or a curve of areas. The lines of the largest hulls are strikingly similar, at a glance, to the lines of the smallest ones. However, you may be sure that they are not simply scaled up and down in a mechanical way. The people who build these things grew up within the tradition, and have a deep, largely intuitive understanding of their vessels. They have rules of thumb ("length should be four times beam", for example), but they rely largely on what they've gathered from growing up around boats, watching grandpa build boats, and generally soaking up the tradition.

Here's something I was reading the other day. It's from Douglas Whynott's book about Joel White, _A Unit of Time, A Unit of Water_:

"Warming my hands over the stove, I tell Junior about Nite Bird and ask if it's true that his grandfather designed the catboat. He says that he did...'I have the model here,' Junior says...

He takes down a wooden carving, a half-model, and there's Nite Bird, though 12 inches long. A chid's toy to some eyes, it's crescent-shaped and beamy, and has a smooth, hand-worn surface. Several lines are scribed into it. Like Nite Bird, this shape, this sculpture, has a dreamy feel to it. I ask what tools they used to make them.

'Oh, a knife, a spokeshave or a plane, and some sandpaper," Junior says.

...I say, 'It seems that someone grows up around the water, and sees boats from the time he was a kid, and then someday he creates a boat in his mind and then carves it out.'

'That's about it,' Junior replies."

[ 02-09-2003, 09:16 PM: Message edited by: Bruce Taylor ]

[pjwalsh]

Any design is in large part due to the past experience of the builder/designer - varying the dimensions of any design by a factor of two will result in major changes in behavior.

There are basic scaling relationships that can be used as guides when altering dimensions:

stability or righting moment is a result of the volume of water displaced (L^3)and the distance between the centers (L) of the displaced volume and the center of mass of the vessel. In general it varies as L^4.

The heeling moment from wind pressure on the sails is the result of sail area (L^2)and the distance between the center of pressure and the center of mass of the vessel (L). It varies as L^3

Sail area varies as L^2

Resistance varies as L^3

So, making a reduction of 50% in all dimensions will result in a vessel with
1/2 the length
1/2 the beam
1/8 the volume
1/4 the sail area
1/8 the heeling moment
1/16 of the righting moment
1/8 of the resistance

as compared to the original.

this is an idealized comparison with limitations, but it illustrates the basic idea that stability decreases with size much more quickly than the heeling moment of the sails. clearly a change in size of this magnitude will require changes in the relationships between sail area, length, draft, and beam - not to mention the sizing of structural members.

while you may get by with 10-15% magnitude changes with little detriment to performance and safety the result would be better if the altered design were considered in its own right.

Boats built in the past by rule of thumb were generally succesful because these relationships had been worked out for specific hull and rig types within specific size ranges over long periods of time.

[Snoo973]

thanks all, but dont misunderstand me guys, i didnt mean to throw down their skills in creating boats, i envy them for knowing what they knew and for their instinct in making it a real thing, its honestly fascinating to me, when i said guess i didnt mean throwing a coin in the hair, i was just referring to their ability to distinguish whats good and what wasnt, not on the base of some software calculations (and said that i dont mean to offend any designer thats reading either)

[Bruce Taylor]

It's quite wonderful that successful boats can be built -- have been built for centuries -- without the mathematical and engineering know-how that an N.A. brings to the job. I suppose I'd want to stress, though, that these boats don't defy the principles on which modern boat design is based. They conform to those principles, for the most part, but arrive at them by a different process. You can't fool Mother Nature...as pjwalsh says, halving the beam and length of a boat will reduce volume to 1/8th. That's going to be true no matter who is designing the boat, or what tools the designer is using.

[WayGray]

Sorry, I didn't mean to offend anyone by including changing the distance between frames as a means of "scaling" its size. I should have referred to it merely as "alteration". But it can get you that longer canoe to fit more gear or that shorter boat that will fit in your existing garage if the change isn't too much.

[whb]

Hey guys don't worry so much about people here being touchy. There are qualified NA's and designers here that cringe sometimes but most of them are pretty good about explaining things to us laypeople. The type of questioning that we do is what makes us potential builders, renovators, explorers etc. If we weren't that type we would simply let someone else build us a f&*^&%^%$^ boat and be done with it. As Anita says there were mistakes upon the way but they were learned from. smile.gif

Howard

[A. Mason]

The all time classic example of taking alterations to a sound design beyond its limits was the 16th[?] century Swedish warship "Vasa." The designer and builders created a sound design but the king decided he knew better and demanded all sorts of alterations. In that time and place, you didn't argue with the king. The designer committed suicide and the "Vasa" sank next to the dock in Stockholm harbor. It is a fascinating true event documenting when sound principles are tinkered with way too much. [Sorry, I don't have any website links handy.]

Anita

[Snoo973]

yup i knew the Vasa story. Here theres a link:
http://www.vasamuseet.se/indexeng.html

[John E Hardiman]

It is important to notice what has been said:


Originally posted by pjwalsh:

Sail area varies as L^2

Resistance varies as L^3

So, making a reduction of 50% in all dimensions will result in a vessel with
1/2 the length
1/2 the beam
1/8 the volume
1/4 the sail area
1/8 the heeling moment
1/16 of the righting moment
1/8 of the resistance

as compared to the original.

The converse is also true is scaling up, but to the inverse proportions. Anyway...

Notice the ratio of heeling moment to righting moment. As you scale down, the beam must become wider to hold the same sailarea:displacement ratio. Conversely, as you scale up the ability to carry sail increases faster so you can begin to add more sail or make the ship narrower to increase sailarea:displacement ratio. This is important in the development of ships and the reason behind the choice of iron as a building material and fiberglass as a boatbuilding material. You can just make a more powerful (i.e. sailarea:displacement) hull with those materials because you can build a longer narrower hull for the same weight and sailarea.

As Anita and others pointed out; in the old apprentices/watermen learned the "proper" ratio of length:beam:depth for the type wooden craft they were building and these were passed down with subtle variation from generation to generation.

As the science of Naval Architecture developed from the 17th century on, a better feel for what could be accomplished began to grow. The USS CONSTITUTION was of extreme length:beam:depth for her age and had novel diagnonal beams in her hull to support the great length. Later clippers and wooden racing boats were diagonally strapped with iron to support the great length of hull and overhangs. Modern materials; steel, aluminum, and composite; are just the present development in the continous push to develop speed on the water. Nothing new....

As a final note, we should add the Mary Rose and Great Eastern and the circa 1800 Royal Navy sloops and Baltimore Clippers to the list of ships who were pushed beyond their material design limits.

[pippo]

Larsson and Eljiarson (spelling?), in "Principles of Yacht Design", say that such a transformation is not linear. If you guys are interested I might post the relevant transformation factors here.

[Snoo973]

Pippo, se puoi sarebbe tanto di guadagnato, grazie. (uhhmmm translation: Pippo if u can, do it, thanks)

[pjwalsh]

pippo,

actually the scaling factors I cited are not linear L^2 is quadratic, L^3 cubic etc...

that is kinda the reason that stability increases faster than heeling moment from the sails with an across the board linear increase in all the dimensions (yep, inverse is also true). My example is a sort of an artificial construct because you could change only beam, or length, or draft or sail area or some combination of them - at this point the specific shape of the hull and sail area and everything else is a factor - I am talking very rough approximations here... this is not sophisticated computer based engineering, I was actually thinking in terms of doubling all the dimensions of a wood shed resulting in 8 times more wood capacity etc.. just a pencil for computing gear.

On the ability of the builders of the past to create succesful designs without modern design tools - I would say their techniques were not much less sophisticated than those used today - just slower to implement.
I used a crude dimensional analysis to come up with some idea of the effects of changing dimensions - traditional builders using models and other less mathematical design methods might not see these relationships in terms of the numbers but you cann be sure that they appreciate the effects of changes in scale and the implications of selectively adjusting beam, draft, length or sail area - boatbuilders are often very sophisticated in their understanding of the dynamics of their vessels. Even seemingly crude building traditions like the guys on the beach in Carricou. Really, I see the development of traditinal forms as very refined and creative engineering. In an environment mostly driven by the needs of the commercial application of the vessel there is little tolerance of negative features. If it doesn't work, breaks, is unsafe, or is unprofitable the feature will surely be discarded.

[John E Hardiman]

Pippo is correct on the subtleties though. the are not exact.

Righting moment is a good example. Righting moment is a function of KG, KB, and BM. Now KG and KB vary directly with the length. BM on the other hand is a function of the waterplane inertia and the volume and does vary to L^3. Now as BM>>(KB-KG), it is close enough to L^3 not to matter for large vessels, but is of prime importance when trying to scale up a small boat where the crew is a large portion of the weight of the boat.

[pippo]

Of course they're not. What I tried to say though is that also one-dimensional measurements (beam, lwl, etc.) do not scale up linearly in a balanced, correct "inflating" process. In other words, some deformation must occur...
I'll try to post the transformations as soon as I can.


Originally posted by pjwalsh:
actually the scaling factors I cited are not linear L^2 is quadratic, L^3 cubic etc...

[JimD]

Originally posted by pippo:
Of course they're not. What I tried to say though is that also one-dimensional measurements (beam, lwl, etc.) do not scale up linearly in a balanced, correct "inflating" process. In other words, some deformation must occur...
I'll try to post the transformations as soon as I can.

</font><blockquote>quote:</font><hr />Originally posted by pjwalsh:
actually the scaling factors I cited are not linear L^2 is quadratic, L^3 cubic etc...
</font>[/QUOTE]And this is the reason a slightly larger hull can carry significantly more sail, in general. So if you try to significantly scale down a big boat using simple math the smaller version will be way over canvassed and probably too narrow at the beam.

[pippo]

Here I am as promised. I will quote "Principles of Yacht Design" by Larsson and Eliasson, International Marine, Camden, Maine.
Chapter 2, pages 11-12:
"...scaling up a boat linearly does not produce a design compatible with good performance and stability".
Assuming the lenght L, the scaling factors are as follows:

Sail area: L^1.85
Beam, depth, freeboard: L^0.70
Keel & rudder span, chord, thickness: L^0.70
Volumes: hull L^2.4, keel L^2.1

There's much more in the table (derived factors, secondary relations) but this should be a good start. Those transformations define a proportionally equivalent series of yachts in terms of performance and stability.