Section line questions

[Don Case]

On my drawings of the Discovery the section lines are labeled, starting at the maximum beam being 0 and going toward the bow they are (B), (D), B, D, F etc.  Why are the center few bracketed? Also the stern most one is A.P. and the one at the stem is F.P. What do those designations mean?

[Roger Pellett]

AP and FP stand for after perpendicular and forward perpendicular.  They define the dimension, length between perpendiculars or LBP.  In simple terms LBP is the “length that floats the boat,” although there are hull forms where this is not true.  In making a lines drawing, the naval architect first draws a baseline, then the FP and AP.  The space between is then divided into stations.  These stations do not necessarily match frame locations as frame shapes were developed from the full sized lines layed out on the mold loft floor.

 

Roger

[Don Case]

Thanks Roger. Yes, I knew that the stations didn't match the frame locations but I do have to keep reminding myself. Do you know why some station designations are in brackets? It's almost like they were an afterthought.

[allanyed]

From David Steel's The Shipwrights Vade Mecum 

 

DEAD-FLAT.

A name given to that timber or frame which has the greatest breadth and capacity in the ship, and which is generally called the midship bend. In those ships where there are several frames or timbers of equal breadth or capacity, that which is in the middle should be always considered as dead-flat, and distinguished as such by the character ['+' surrounded by a circle]. The timbers before dead-flat are marked A, B, C, &c. in order; and those abaft dead-flat by the figures 1, 2, 3, &c. The timbers adjacent to dead-flat, and which have no rising, are distinguished by the characters (A) (B) &c. and (1) (2) &c.

 

Allan

[Don Case]

Well thank you Allan. Now that I've learned something my day hasn't been a total waste. 😃 But, doesn't that mean that if all of them aren't in brackets then there must a bit of bend in the keel?

[allanyed]

Don,

Not sure what you mean.  The station line numbers along the keel are  a reference to their location along the keel, they have nothing to do with the keel itself.  The body plan gives the shape of the hull at each of these stations.  You can study some drawings that show both body plan and profile and you will see the station line numbers on each view.     There are plenty to choose from on the Wikimedia site and the RMG Collections site from which to choose.   https://commons.wikimedia.org/wiki/Category:Ship_plans_of_the_Royal_Museums_Greenwich  

If you are still unsure, please post a sketch or drawing of why you think  station lines would have any connection with the shape of the keel.

Allan

Edited January 24, 2021 by allanyed

[Don Case]

Because you said that the terms in brackets referred to a section of the ship that was dead flat. I just assumed that if that section was dead flat, the rest must be slightly curved. I'm a little confused now 

[Roger Pellett]

OK! Naval Architecture 101

 

The hull’s shape Is defined by a specialized drawing called a “lines drawing.”  If you’re familiar with mechanical drawings, the lines drawing is an orthographic projection showing the hull in three views; the top view is called the “half breadth,” the side view the “sheer,” and the end view the “body plan.”  To save drafting effort since the hull is symmetrical only one side of the hull is shown on the half breadth drawing.  The body plan appears to be odd to some as one side shows the view looking forward and the other looking aft.

 

The hull’s shape is further defined by contour lines that appear as curves in one view and straight lines in the other two.  The half breadth shows a series of curves known as WATERLINES running fore and aft.  The body plan shows the SECTIONS as curves, and the sheer shows a series of curves known as BUTTOCKS.  Buttocks often do not show on very old drawings so they need not concern us.

 

For hull classic Eighteenth hull forms, the waterlines are (mostly) convex arcs that come to a point at the bow and stern.  It therefore stands to reason that there is a point where the waterlines are at their widest point.  The section at this point is called the “deadflat.”  

 

It is also possible that the hull shape will feature an area where the body plan sections are identical.  This would naturally happen at them widest area of the hull. Modern day Naval Architects call this a parallel midbody.  This would mean that there is more than one deadflat section.

 

Roger

[Jaager]

4 hours ago, Roger Pellett said:

These stations do not necessarily match frame locations as frame shapes were developed from the full sized lines layed out on the mold loft floor.

Roger,

I have a definite bias about this, but in all of my observations, I find that the stations more than match frame locations. They define them.

In every instance that I have encountered, a station line is the midline of a bend (paired frames). 

For POF framing of a model, where the lofting methods in current general use is employed, the stations are all but useless. This is certainly the case if a bend is glued up before a pattern is mounted then final shaping is done.  This certainly an incentive to discount the importance of stations

 

I see that there was a major change in framing and lofting after about 1860.  Most of the books that we reference about lofting and framing were written after this change occurred. Most were published near or after 1900.   Iron and steel require a different level of precision than does wood.  After 1860 iron and steel quickly began to rule.  The old shipwrights who knew the old methods were no more.  The methods required to shape and iron or steel frame had "infected" those who built using wood.  At least those in the major industrial yards that influenced the book authors. 

 

My proposed explanation for how it was most often done, before 1860::

It is my thesis that lofting was more of a guideline.  On the loft floor, what was enlarged to full size and used to produce the frame patterns was only the stations.

For the midship bend, the pattern could be used to define the complete bend since both frames had no bevel.  At the other stations only the down bevel frame was cut and erected.  The actual bevel was cut on the ways using battens resting on each station as a guide.  Once that was satisfactory, the up bevel frames were shaped using the same patterns and the eye of the experienced shipwrights. The intervening bends and (pretty much only in England) the single filling frames were preliminarily (rough) shaped on the ground and then finished on the ways using the battens.

 

There was a note on one of HIC plans for a class of USN warships where the sisters were being built at different east coast yards. The frames patterns were to be made by the loft team at the lead yard and shipped to the other yards along with the plans.   Patterns for 12-20 stations is much more practical to ship than 120-200 patterns representing both faces of every frame.

 

With my first POF hull (Kate Cory - Hahn style), I came to really dislike point plot lofting.  I found that getting a curve to connect the plotted points was error prone and inconsistent  from frame to frame.  For me, the Hahn method  (forming a glued up frame horse shoe of wide timber planks and fixing a pattern)  involves an unacceptable amount of waste of expensive wood.  After a long search and several false trails, I developed a new way.  The timbers are shaped as individuals, but include the bevels on both faces.  There is also some extra needed for alignment of the stack of frames between each pair of stations.  The spaces have temporary wood filling them.  The space wood is cheap Pine and it is held by a bond that is easily released.  The Old Boys would have used low cost and rectangular chocks to fill part of the spaces.   My method of frame assembly that only needs the already existing stations to define my frame shapes.  The logic of this process as probably being similar to actual practice became clear to me.   I thought/think that those Old Boys, for whom time and effort was vital,  would not miss the reduced time and expense that this way of doing it represents.  I have an additional advantage over the batten guide method.  Because the pattern is already on each timber,  I can position a pattern with both stations on it in precise alignment on both faces of the stack of frames.  The bevel for both faces is precisely in place.  I can shape the stack off the hull as a unit.   This is much easier and less error prone in lofting than the ca 1900 published methods. 

[Don Case]

2 hours ago, Roger Pellett said:

OK! Naval Architecture 101

 

The hull’s shape Is defined by a specialized drawing called a “lines drawing.”  If you’re familiar with mechanical drawings, the lines drawing is an orthographic projection showing the hull in three views; the top view is called the “half breadth,” the side view the “sheer,” and the end view the “body plan.”  To save drafting effort since the hull is symmetrical only one side of the hull is shown on the half breadth drawing.  The body plan appears to be odd to some as one side shows the view looking forward and the other looking aft.

 

The hull’s shape is further defined by contour lines that appear as curves in one view and straight lines in the other two.  The half breadth shows a series of curves known as WATERLINES running fore and aft.  The body plan shows the SECTIONS as curves, and the sheer shows a series of curves known as BUTTOCKS.  Buttocks often do not show on very old drawings so they need not concern us.

 

For hull classic Eighteenth hull forms, the waterlines are (mostly) convex arcs that come to a point at the bow and stern.  It therefore stands to reason that there is a point where the waterlines are at their widest point.  The section at this point is called the “deadflat.”  

 

It is also possible that the hull shape will feature an area where the body plan sections are identical.  This would naturally happen at them widest area of the hull. Modern day Naval Architects call this a parallel midbody.  This would mean that there is more than one deadflat section.

 

Roger

I guess I was taking the language too literal. Who would have thought that deadflat didn't mean flat. Kinda like floor and ceiling I guess. I'll pick it up again. I used to have a passing knowledge of this stuff. I can forget a lot in 20 years. There's been a few time since I signed up here where I've said to myself, "You knew that." I hope I don't get to be a pain in the butt, I tend to ask a lot of questions.

Thanks for all the answers I've gotten.

[Jaager]

2 hours ago, Don Case said:

Who would have thought that deadflat didn't mean flat. Kinda like floor and ceiling

From your comments, it appears that you have a misconception about what it is that his being described.

Deadflat does mean flat.  It is about understanding what it is that is flat. 

At the midship - generally this is the frames about 40% of the distance from the FP.  Almost never is the midship half the distance along the length.

If a plank is placed against the outer face the frames here, it is parallel with the keel.  There is no bevel, so it is flat.  Beveling is tricky and bothersome to do, so it gained an accolade "deadflat: instead of just "flat".

 

In old shell first hulls and early frame first hulls, the first inside reinforcement  timber across the keel defined the inside bottom. If they were close enough together to walk on or had boards placed across them, they were the floor.

 

If the inside of the frame timbers are covered with planking, this serves to seal the inside of the hull.  It is a sealing.  No Webster's or Oxford and imprecise spelling  becomes ceiling.  If you think on it, the layer of material fixed to the underside of the beams for the floor above a room does sort of seal it also.

Edited January 25, 2021 by Jaager

[Roger Pellett]

Jaager,

 

Thanks for your clarification.  My post was intended to introduce Neophytes to the mysteries of ship drafting and was based general principals.  My Naval Architecture education occurred in the early ‘60s, prior to the dawn of CADD.  We were still required to manually make a lines drawing from a table of offsets with the finished drawing India ink on vellum.  What a mess!  The ship in question had a steel hull and if lines drawing sections were a multiple of frame spacing we had no way of knowing.

 

I certainly agree that there were many framing conventions in the wooden ship era most of which were certainly connected to construction techniques, a subject usually ignored.  I agree that many ships were built by lofting a minimum number of frames and then adding filler frames between, all dubbed to produce a fair hull.

 

This may account for differences in performance of sister ships built to the same lines.