Ballast Stresses on Hull

[katie 2]

In the process of completing a strip planked Freindship sloop (Pemequid 25'). Hull is glued and nailed red cedar (1"x1") with oak ribs placed approximately every 2' also nail fastened to the cedar. 2" floors are attached to each frame set and bolted to the yellow cedar keel and deadwood. Additional biaxial 4" glass tape has been placed on the interior planking in 6 different locations on each side, running from the keel to the sheer. The outside is of the hull is glassed and set in epoxy. The interior is sealed in epoxy. My question relates to the 2000lbs. of lead hanging on the keel....where do the stresses of this weight transfer up the hull? Where is the weight divided and which parts carry the which portion of the load? What roll does the glass sheating play and what about the nails in the planking?

[TR]

katie;

The load/stress of the ballast weight is transferred to the hull through the ballast bolts. Different things are done with the bolts in the hull, sometimes they have washers and nuts on top of the keel. This is okay if properly done. And sometimes they carry up through the floor timbers. This is better, but sometimes very difficult. It is the floor timbers which transfer the ballast load out into the hull planking. So if the ballast is only bolted to the keel, you must have substantial fastening between the keel and floors.

Hope that helps, Tad

[JimD]

Katie, here's a partial answer, concerning the nails, from Glen-Ls website:
To nail or not to nail.
With epoxy, nails are redundant. If you are building a boat with 3/4" thick or thicker planks, nails can be a great help in construction, allowing you to hold the planks in place without a lot of clamps... If using thickened epoxy, the purpose of the fastener is to hold the plank in place until glue sets. ..
As for the fg sheath, it adds a type of strength usually referred to as abrassion resistance, and also resistance to water absorption, but does not add structural strength. Haven't a clue as to what those strips of tape running vertically from backbone to sheer are required for if I understand and visualize this correctly.

And I'd feel a bit silly saying anything about the ballast except the obvious :D It hangs straight down due to the force of gravity and naturally would have a tendancy to break the boat in half. Its the designer's job to do the structural engineering to make sure that doesn't happen.

[Roger Cumming]

The strip planking creates a hull that is essentially a continuous shell. The ballast keel is attached to floors which are in turn connected to frames. The frames are attached to each plank of the hull - 1.e., each frame is nailed to the hull shell with nails spaced 1" on center. The force due to the weight of the ballast keel is transmitted to the hull shell via the floors and the frames. If the keel strikes a ledge an upward force will be exerted to the floors and in turn to the frames and in turn to the hull shell. These forces on the hull - downward due to the weight of the ballast keel, upward when the vessel grounds - are resisted by the hull and its bulkheads which act as stiffeners and help the hull keep its shape. If the stresses in the hull due to these forces exceed the hull's strength, the hull will rupture. Forces also can be applied to the hull shell directly if, for example, the vessel strikes a log or collides with another vessel or a dock. In this case the frames, floors and bulkheads serve to support the shell in resisting the force. Sometimes frames are cracked when a boat grounds. This is an indication that the stresses in the hull itself were not so great to rupture the hull, but were greater than could be resisted by the frames, so they cracked. When a vessel is properly designed, each of its components is proportioned according to the strength of its material and the magnitude of the expected forces. No component is larger (i.e., heavier) than it needs to be, and all are properly fastened together to transmit the expected loads. A boat designed for summer daysailing in good weather will be far different structurally from one which is to remain at sea even when the weather turns bad. This all sounds very simple and obvious. To do it, however - to design a strong vessel that is light and stiff enough to perform well while resisting its service loads (forces) - takes great skill.

[Aramas]

The construction method of your boat is a bit of a hybrid in that it combines traditional nailed strip planking with aspects of the stip plank composite method.

The edge nails in the strips and the glass contribute to transverse strength in much the same way as frames do, so you have that covered by the frames, the edge nails and the glass cloth.

The glue between the strips keeps the water out smile.gif

Longitudinal strength is provided by the planking, deck, backbone and sheerclamp assemblies, and the outer glass skin would also contribute.

The whole thing is held together by the floors and sheer clamp.

Working out whether the sizing is adequate or not would be quite a job, and would have been done at the design stage. I could manage it in either conventional strip plank or strip plank composite, but I'd certainly struggle with the hybrid approach.

Should be a strong little sucker though smile.gif

[JimD]

The edge nails in the strips ...contribute to transverse strength in much the same way as frames I had always assumed that all those nails acted as little rebars throughout the planking adding to the monocoque nature of the hull, and was surprised that Glen-L says they are principally for ease of construction. I have plans for a Glen-L strip plank 22 footer that calls for nails every 2" aswell.

[pjwalsh]

I would agree with glenL on the nails primarily being for holding the planks in place until the glue sets - they provide little transverse reinforcement to the planking because they are placed close to the neutral axis and so add litle bending stiffness - in your construction scheme the frames are doing the work od transverse reinforcement for the planking. (edit: I am assuming the glass sheating is light fabric intended to resist abbrasion as opposed to heavy structural skins)

On the transfer of keel load ideally the keelbolts should go through substantial transverse floors that are attached to both the keelson stucture and the transverse framing system. It should be pretty well spelled out in your plans - if you are doing you own structural planning then I would look at the details used on similarly sized freindship sloops and use what has worked before. Your hybrid construction technique seems to be pretty much following conventional plank on frame approach and scantlings except for the edge gluing of the planking so I would think that the sizes and number of floors and keelbolts used in traditional construction would be appropriate.

[ 12-29-2003, 11:52 AM: Message edited by: pjwalsh ]

[John E Hardiman]

Originally posted by katie 2:
My question relates to the 2000lbs. of lead hanging on the keel....where do the stresses of this weight transfer up the hull? Where is the weight divided and which parts carry the which portion of the load? What roll does the glass sheating play and what about the nails in the planking?Quick primer on hull strength and the whys of framing.

A hull sees three distinct types of stress; I will discuss the 1st and the 3rd first because they are easiest to visualize.

Primary stress is caused by the hull acting as a rigid beam floating in water. If a wave crest was under the bow and stern and the waist in a trough, the vessel would want to bend. The strength of the skin (shell, deck and longitudinals) is sized to take this bending. But it will only work if the hull holds its form! Take a sheet of paper and cup it into a U shape: now bend the ends up. Notice how the center wants to spread out, the deck, beams, and thwarts prevent the hull from doing this. Now tape a piece of paper on to the center to hold the beam constant, then try bending it again. Notice now the hull flattens; this is what the frames do; they keep the hull from flattening.

Tertiary stresses are caused by the hydrostatic and hydrodynamic forces on the hull shell. For every foot of submersion, the pressure on the skin increases by 64 lbs per square foot. So at a draft of 3' the pressure is 192 lbs per sf. This is how the water floats the hull; these buoyancy forces exerted on the shell holds the vessel up. Additionally, as the hull moves through the water, and the waves past the hull; pressures are generated equal to 1/2 *rho* v^2; or ~2.9*K^2 where K is the speed in knots. So 2.9 lbs per sf at 1knt; 73 at 5knts; 290 at 10knts; 1160 lbs at 20knts. Due to the high speed of waves, “wave slap" is generally taken as 2000 lbs per sf and is added on to other speed related pressures. Frames and longitudinals are what transfer the shell hydro forces to the rest of the vessel. Think of the hull shell like a drum head; the frames and longitudinals are the rim and the shell the skin. The shell must be thick and strong enough so the pressure just doesn't tear out the shell, and the rim must be strong enough that it supports the shell. This is what the outer glass skin and the edge nailing is about; to support the skin loads and pass them to the frames/longitudinals.

Now the hard one, Secondary stresses. Secondary stresses are how the vessels loads (propulsion, keels, rig, cargo & passengers, tanks, dynamic pitch and roll, etc) are passed to the hull so that they are supported by the tertiary hydro loads, but also accommodate the +/- primary bending stresses. This is normally accomplished by dividing the vessel up into structural "compartments" or "blocks"; each block containing the necessary structure to keep the block rigid under maximum load. A traditional mid-sized sailboat is normally divided into three such blocks. The bow, forward of the mast supporting bulkhead/partners; the cabin, aft of the partners and forward of the cabin end/cockpit bulkhead; and the cockpit/lazarette. Bulkheads are good points to break up stresses because you can analyze the block with only shear forces at the bulkheads. Normally, forces such as rig, keel, and propulsion loads are lead to a hard point designed to take the load, this hard point normally has additional “stringers” (fore and aft) and “deep frames” (athawartships) let into the structure to transfer the load throughout the structural block. For a keel, the floors are the deep frames, and the structural keel (as opposed to the deadwood or fin keel) with or without a keelson functions as a stringer. In lightly skinned vessels (such as steel or ‘glass), the longitudinals play a significant part in distributing the load; in heavily skinned vessels (most wooden vessels) the shell is thickened enough to not require longitudinals.

Once all three stresses are determined, the combination of all load cases is analyzed and the maximum stresses are determined in each member. Each member is then evaluated to determine if it is strong enough within the limits provided by its criticality (like a safety factor of 1.2 for shell plating above the waterline or 5 for rigging foundations). Now in the days before massive computing capability, this was done by following “good design practice” from existing successful vessels or “the rules” that others laid down with only a few calculations for the critical pieces. Even then, because of the poor ability to fasten things together, structural failures were common. These were either accepted (look at how many of the old racing yachts were totally worn out in 2-3 years, stripped of their hardware and keels, and burned for the fastenings) or you massively overbuilt (see any old wooden ship).

Now to answer your question;

When upright, the stress due to the weight of the keel comes up through the keel bolts and attempts to pull the floors down; the floors are attached to the planking and the frames (also attached to the planking) which resists this load due to the pressure differential between the inside and the outside of the planking. The ‘glass tapes may aid in this load; providing the load path from the upper planking to the keel/floors (think of two equal lengths of rubber band and string knotted together at each end, as you pull on them the rubber/wood stretches but the string/glass doesn’t, eventually the string/glass carries most of the load). The load each bolt carries (for both normal and impact loads) and therefore each floor is determined by the bolting pattern (if you need to ask how to do that calculation, e-mail me and I’ll try to point you to an example). Care must be taken to ensure that the pattern doesn’t overload a few bolts with many bolts “sleeping”.

The more important question is what happens in a knockdown. Now in addition to a down force, there is a moment. The keel will attempt not to pull the floors down, but to rotate them. This is how floors and frames get broken. Depending on the keel width, attachment, and bolt pattern, several things may happen. If everything is designed properly nothing will happen, and the load will have been picked up and transferred to the planking via the frames. Otherwise, the keel bolts may stretch and/or break off the keel. The floors may crush. The hull may be stove in as the keel point loads the outside of the root.

How do you prevent such things?….Good design! Wide floors, a good keel bolt pattern not all on centerline, structural keel root fillets, etc

[George Roberts]

katie 2 ---

I don't know how much detail you wanted anyone to go into ...

If you bed the lead keel to the hull with an adhesive, you will get a different load distribution than if you just use bolts.

If you tighten the keel bolt nuts to different torques, you will get a different load distribution.

In any case, if you had asked this question prior to building, you could have built with the load paths in mind.

[Aramas]

Regarding the nails being to close to the centre to do the duty of frames, while that may be obvious from an engineers point of view, it doesn't explain how so many near frameless nailed strip boats have done good service for decades. I seem to recall Chappelle and Bolger took a less theoretical view.

Try it with a sample.