Deck beam Scarphs?

[NMBROOK]

I am posing this question for a second opinion.Whilst researching the deck construction for my Mordaunt 1681 build,I have come across conflicting material.Looking at the Lennox 'monograph',Richard Ensor shows deck beams with a single central scarph joint in some of the beams,however Peter Goodwin states that deck beam scarphing didn't start until well into the 18th century when large timbers became harder to source.Just thought I would 'throw this one out there' for other opinions.The largest span is around 8 metres.

 

Kind Regards

 

Nigel

[druxey]

I imagine at this time period 30' 0" lengths of timber were easily available, which is more than 8 meters. I suppose, depending on the geographic location of the shipyard, if only shorter lengths were available they would have scarphed them together. You are the master shipwright: you decide!

[NMBROOK]

That is a good point Druxey,both Lennox and Mordaunt were built at Deptford which was experiencing some timber supply issues at the time,this is covered in Richard Ensor's book.This could well explain his decision to include some randomly placed scarphs.I think if I follow suit and include some,but not on every beam

 

Kind Regards

 

Nigel

[wq3296]

Folks,

 

Nice post. From an engineering point of view, I can't see why builders of wooden ships would ever use timbers of that length for deck structure, even if available. No matter how you slice it and dice, intermediate support columns or bearing walls would absolutely be necessary under a very long deck beam because it would deflect a considerable amount just from its own weight. These are the same principals used in building design and construction today. See Longridge's book on the Victory - it looks like a forest below decks for all the columns in place. Accordingly, if columns are required anyway, why not use shorter deck beams designed for a specific acceptable deflection? Shorter beams would be easier to install and readily available. Further, except for the keel and other members that are fully supported, I expect that the scarcity of long timbers was not the reason why they weren't necessarily used for deck sub structure. The requirement for columns necessitated by the live and dead loads associated with sailing ships would over ride the need for super long deck timber, in my opinion. Note that ideally, the scarph joints would be horizontal type centered over column locations where you have minimum moment but maximum shear load, which is resisted by the reaction at the column.

 

I would suggest you use the scarphed joints and place them at regular intervals. I doubt they were placed randomly, particularly on a 17th century war ship. In some instances where long spans were necessary and columns were not workable, I expect deck beams would have been lapped side by side, with the length of lap made long enough to occur well past the point of maximum moment which would be center span. I am not sure how far structural design had advanced during the 17-18 centuries, but considering some of the cathedrals built during that time, I expect it was fairly advanced and would have extended to ship building.

 

Note that I am offering what I think are common sense solutions based on time tested engineering principals. Of course, there may be information available that says I'm all wet. However, if you think I'm all wet please provide hard evidence to support your position. Nice topic and thank you for bringing it up. Maybe we can all learn something.

 

wq3296

[druxey]

wq: Deck beam scarphs seem always to have been made in the vertical plane. Counterintuitive, I know! Certainly wide span beams had supporting pillars under them.

[wq3296]

Greetings Druxey,

 

Thank you for the response. We may be saying the same thing. To me, a horizontal scarph means that the beams are cut through horizontally so that the resulting combined beam has a top and a bottom piece. A vertical scarph results in a combined beam having a left side and right side. Is that how you see it, and did you mean mean to say vertical scarph in the same context as above?

 

wq3296  

[NMBROOK]

This is fun Wq 3296,when I say random,I mean occuring on random beams,not placed randomly on a beam.The scarph joints,whether single or double would be equal about the centreline.Coming from a structural engineering background,I hear what you are saying.In this instant the camber is radial and to allow water to drain off and does not drop out.I do know what you mean having built several bridges with Parabolic cambers that allow the structure to deflect to almost dead flat under load.The supporting structure of the deck,columns and hanging knees was designed to minimise any movement in the athwartships plane.I would say that cutting the scarphs horizontally would result in a weaker beam in the vertical plane,but that is just my opinion.If anyone fancies a go at finite element analysis then feel free   The main problem being with a tapered scarph is varying amounts of material either side of the beams neutral axis.

 

Kind Regards

 

Nigel

[trippwj]

I seem to recollect that the keel was assembled with vertical scarphs - that is, the two keel pieces had a left and a right piece vice one atop the other - in order to maximize the strength to avoid the weakness inherent in the other style.  it may have been in Sutherland (1711) or Rees that I saw that tidbit.  Will need to do some digging.  In the case of the deck beams, where particularly for a man or war they carried significant weight, it would make sense to scarph the same manner.

[NMBROOK]

Totally in agreement Wayne,it is interesting though the French chose to do their scarphs differently,however they did employ far more sophisticated technologies to ensure lower hull stiffness.Gaeton's Fleuron is an ideal showcase for this.

 

Kind Regards

 

Nigel

[druxey]

wq: I did mean left and right, as viewed from above!

[trippwj]

Recognizing that these are from a much later era than the original query, here are a couple of snippets from Fincham (1825) - An Introductory Outline of the Practice of Ship-building available via Google Books in PDF (I have not been able to locate an edition with the plates as yet but continue to seek same)

 

Concerning the Keel (Fincham, 1825 – page 9)

 

4. As the keel cannot be obtained in one piece, as to length, several pieces are bolted together lengthways, by what-is called a side or vertical coak scarph; the scarphs being in length about three times the depth of the keel. The coaks are for the support of the bolts, especially to resist the strain when the butts of the scarphs are being caulked; they are one-half the length of the scarph, and their breadth one-third its depth.

.

5. The scarphs are bolted with from six to eight bolts; eight, from frigates upwards, and six to smaller vessels: half of the bolts are driven from each lip side, with a ring upon the head, and clenched upon a ring on the opposite side.

 .

6. The French and most other nations have flat or horizontal scarphs; but as these scarphs tend to weaken the keel, in the direction in which it is most subject to strain, more than the side scarphs, the English mode is preferable; for the keel bends vertically, which brings a tension on the upper or lower fibres, according as hogging or sagging takes place, which fibres are cut off, in a greater number in these scarphs, to let in the lips; and when sagging takes place there is a tendency to open the joint at the lower lip; this opening will cause the scarphs to leak, except a  stopwater be placed at the intersection of the joint of the scarph with the outer edge of the garboard seam, or by increasing the length of the scarphs.

 

Concerning Deck beams (Fincham, 1825 page 70) –

 

169. The beams are distinguished into single pieces, two ( b ), three ( c ), and sometimes four piece beams (f and g): the length of the beams and the timbers that can be provided to make them will determine the number of pieces they are to be composed of, which should always be as few as possible; for the quantity of timber required to make them will be increased with the number of pieces, because the number of scarphs is increased.

 

170. When a beam is made or composed of more than one piece, the pieces are united together with vertical scarphs. If in two pieces ( b ), the scarph is 1/3, if in three pieces ( c )  1/4, and when in four pieces (f and g) 1/5 the length of the beam.

 

171. The scarphs are distinguished into right and left hand scarphs, and are named by the hand that is on the side of the angle, or the side from which the wood to form the scarph is taken off; when at the side end, the face is towards the scarph and looking upon the upper surface; they are bo1ted with from seven to nine bolts; so as to make their distances apart from 16 to 18 inches, placed alternately. about 2 1/2 to 3 1/2 inches from the upper  and lower part of the beams. An equal number of them is driven from each lip side and clenched upon the opposite; in addition to these bolts, one nail is driven into each lip on the opposite edge to the nearest bolt, and one bolt is frequently driven up and down in each lip to prevent its splitting.

Edited May 6, 2014 by trippwj

[wq3296]

Greetings Nigel, Et Al,

 

Good discussion all around. My thoughts:

 

Not disputing the information provided because it is reasonable. However, see latest issue of Ships in Scale for the article on L'Hermione, Photo 9, page 17. Photo shows new keel on its side, ready for placement. Note the horizontal scarph. Many other good photos in this article showing her in frame. She's French built.

 

Now that I've had a chance to think about it, vertical scarphs make perfect sense, particularly for scarphs that are unsupported by a column or other structure. A vertical scarph seems as though it would be better at resisting deflection. The location of the neutral axis would not change because the two full depth beams are apt to act together. With a horizontal scarph, each beam member would have its own neutral axis and may act independently, depending on how they were fastened. For supported scarph joints, it probably doesn't matter because the support would take the load.

 

In Longridge's book, he has a section on scarphs and reports that, on British ships, the keels had vertical scarphs but keelsons had horizontal scarph. Our discussion here is about deck beams, but I guess the type of scarph used by different builders for various applications was subjective.

 

Thank you all for a good discussion from which I benefited. What's next?

 

wq3296

[NMBROOK]

Hi Wq

I too find the whole structural design aspect of shipbuilding fascinating.My preferred period of the late 17th Century being a 'grey area' for reference material,I think necessitates some understanding of how things 'worked' in order to make a 'feasible' reproduction in the absence of full detail drawings.I think the keelson if different in that it acts as a clamp to hold the base of the frames (riders) to the keel.On thing I cannot get my head around is what determined where columns were placed.I know that they weren't present under every beam,but what rule governed which?Looking through my material last night revealed no answer.Aligning the columns to act with the knees would make sense,but if this is true,none of the author's of my books have stipulated that is the case.

 

Kind Regards

 

Nigel

[wq3296]

Greetings Nigel,

 

I guess it would depend on how far structural analysis as a practical design tool had progressed during that time period. My sense is that it hadn't progressed enough to where the designers/builders were calculating the placement of columns, knees, etc. This leaves us with "rule of thumb" design. I have read, in one of Chappelle's books I think, that major aspects of ship design was based on proportion. Even spar diameter was related to length of the spar, which was related to overall mast height, which was related to ship LOA, and so on. So, if you knew the LOA of a particular class of ship, you were off to the races and could pretty much design the major dimensions for a new ship. Plus, let's not forget good old design by previous experience - also known as trial and error. A good example of this gone wrong is the Vasa sinking, which I am sure you know about.

 

As with building design, girders span between columns, and beams span between girders. I guess the ship equivalent would scantlings, pillars, and carlins. You could use a modern timber design manual to arrive at reasonable spans for a given timber cross section based on the wood you are using from the kit. This should range you in close enough for what you want to do. Grog time.

 

wq3296

[trippwj]

Thought I would throw in an illustration from Rees' Cyclopedia (1820) of the deck beam scarphing on a 74 gun ship.

 

[jud]

Deck beams do more than keeping the deck up, posts can do that, When a ships bow hits the next swell with the bow canted down from the swell she is riding, the upper ends of the ribs, because the planking is in compression, will want to move out because of those forces. When she crests a swell, both ends are not supported as well as amidships so the planking goes into tension and tries to push the ribs together. Without those deck beams the ship would tear its self apart. All of the structural members of a floating ship are constantly changing from tension to compression and all beams have dual rolls, one to keep the ship together and placed so the crew can live aboard. Because of the constant changing of the forces working on a ship, joints are well thought out.

jud

[wq3296]

Greetings Jud,

 

It is understood that deck beams do more than hold up the decks. In fact, they function as a diaphragm to brace the perimeter of a ship: much the same function as the floor structure in a building.

 

wq3296 

[popeye2sea]

Would not the position of posts be determined somewhat by the placement of various furniture and gear (i.e. pumps, capstans, etc.) about the decks.

[NMBROOK]

Hi Henry

Yes I agree the positions of the columns are affected by the deck furniture,however I am sure it can't be that simple?Or can it

 

Kind Regards

 

Nigel

[Maury S]

You guys are such an incredible source of info.!!!

Maury