An Old Worn Out Boat, Prime Candidate for Cold Molding?

@WhiteRabbet,

I have worked out in my mind an engineering basis for cold-molding over carvel.

We need to us use some imagination here!

Lets look at a typical plank on frame image (note this is not the only structural arrangement):



Notes:

• The planks are fastened to the frames and the frames are supported by the longitudinal members (Keel/Bilge Stringer/Sheer etc).
• Floors and bulkheads are not shown.
• The bulkheads support the longitudinal members.
• Floors spread keel loads and/or high bottom (slamming) loads.
• In determining the thickness of the planking, the wood strength and the fame spacing are the key determinants.

The thickness of the planking can be (that is, there are other ways) determined using:

• t=s*sqrt(P/S/2)

where:

• t is the hull thickness
• s is the frame spacing
• P is the design pressure
• S is the allowable stress of the hull plating

Now imagine that every frame is missing except for the overlap with a longitudinal members, and the planking was actually plywood.

In this case the structural system would be called All Longitudinal (with bulkheads).

The thickness of the planking can be determined using:

• t=c*sqrt(P/S/2)

where:

• t is the hull thickness
• c is the longitudinal member spacing
• P is the design pressure
• S is the allowable stress

As "c" is greater than "s" the hull thickness "t" will be higher.

Should we consider the existing planking in the thickness calculation?
No as it is not plywood and all the frames are assumed to be missing.
The planking cannot support a moment in transverse direction.

Regards AlanX

 

"Design Pressure"?
"Allowable Stress"?
Much easier to get "real world" numbers/actual dimensions from "Skene's Elements of Yacht Design"
And, you have your choice of three, Lloyds, Nevins, Herreshoff.
Lloyds = heavier construction, we might call "semi-workboat", somewhat bulky.
Nevins = moderate construction, typical "well found" yacht, sail around the world.
Herreshoff = lighter weight construction, "summertime" yacht, built for the "carriage trade", if you will.
The lighter one wishes to go the more important the quality of materials and skill required.

 

@rangebowdrie,
Its an engineer thing.
Regards AlanX

 

I will get these measurements together. And follow up when I have them. Thanks

 

I showed an extensive photo survey of the boat to a naval architect/ I knew from a while back. He said it would be more expensive to fix everything than to build another boat, and that glassing might be the only option.

Building a new boat is out of the question, as it would cost at least a couple million dollars and that is well outside my budget.

And there is a boat there, with working systems, and for the most part, good wood, and a hull that is still fair and true.

It's a very special boat to me. There is only one in the world, and I have it. I would like to find a solution to being able to use it for a decade if there is the possibility. I don't care if sheathing it reduces its value, as it has no real value now. If I had to spend 100K to make the boat good for another 10-20 years, I would

Whether or not I have the needed money depends on how much it would cost to solve all the problems. If for example I could solve the structural and planking issues + plus any need for future refastening, for say 50K, by sheathing it, then I would need to determine what is involved time and cost-wise for other, things, such as ensuring a water tight topsides (the deck seems to be watertight except for the hatches, the pilot house, cap rail, and the cockpit area. The deck itself is a true laid deck, with another 1/2" laid over top of it, added many years later. The bungs in a number of places have fallen out and been epoxied over. The deck is probably a little thin, but seems like it still has some life left in it. Perhaps if those missing bungs were countersunk a little deeper, they could be re-bunged...)

But it seems first it must be determined that I have a viable solution for the hull and structural issues.

 

Deck is solid laid teak. It had another 1/2" laid on top at some point later, I think I remember hearing something like in the 80's. The bungs don't have a lot of meat, but they have some life yet it looks. The beams are all good except for one spot where the deck is leaking, or the corner of the pilothouse is leaking and has rotted out one 1/2' section. This area of the deck would need to be reefed and re caulked, but it is a small area, and I can see the failure on the deck, in the

Intent is to just poke around Caribbean, and live on it at anchor mostly. 15 years would be great. Worth nothing at the end is fine. I am not a very experienced wood worker - Though I did build a 21' laminate surfboat with some guidance about the process apprenticing at a wooden boat museum about 15 years ago... I also volunteered in a wooden boat shop for part of a summer around the same time, so I have some sense of it. I do not have an existing machine park.

Deck is solid laid teak. It had another 1/2" laid on top at some point later, I think I remember hearing something like in the
80's. The bungs don't have a lot of meat, but they have some life yet it looks. The beams are all good except for one spot
where the deck is leaking, or the corner of the pilothouse is leaking and has rotted out one 1/2' section of a beam. This area of the deck would need to be reefed and re caulked, but it is a small area, and I can see the failure on the teak deck systems.
It seems the issue with the topsides is mainly everything but the deck - cabin tops, pilot house meeting the deck at all 4 corners leak, and the cockpit will need a bit of attention...

Intent is to just poke around Caribbean, and live on it at anchor mostly. 15 years would be great. Worth nothing at the end is
fine. I am not a very experienced wood worker - Though I did build a 21' laminate surfboat with some guidance about the
process apprenticing at a wooden boat museum about 15 years ago... I also volunteered in a wooden boat shop for part of a
summer around the same time, so I have some sense of it. I do not have an existing machine park.

 

Alan,


I'm not sure I understand this: “Now imagine that every frame is missing except for the overlap with a longitudinal members, and the planking was actually plywood.”

I’m trying to make sense of it. I would like to visualize it. But I’m not sure what you mean “except for the overlap”, unless you mean except tying the edges of each of the pieces of plywood together, but not forming a length of frame, of rigidity.

But the trouble I’m having with the visualization as a whole, is that it is not only bulkheads in my case. It also has frames. Most of which are fine. Some of which are cracked slightly, beginning to show stress, and the ones that are broken, are still tied together the rest of the length. It has one weak point on a frame. And so there are some weak points on the hull in a handful of places.

Now, if I imagine this covered on the outside with an inch thick of rigid laminated wood, or fiberglass, and I don't see it moving as much. And I also imagine the laminated layer being much stronger than if it was existing on its own with a few bulkheads inside.

The deck beams and deck are there holding the tops of the hull apart. The only weak points would be a few slightly weaker points.
Planking does bear the load, and spreads it out in the spaces between the planks.
It would seem that by thickening the hull you would reduce your need for frames, you would further distribute the load.
I guess its all a matter of how thick given the faults in the ribs...?
Say it were to fail, what would a failure look like - a dent in the hull? Like an area that starts to delaminate?


I'm imagining the idea is that the hull would collapse inward from the forces of the water pressure exerted upon it.

 

How would I find "P" and "S" for this calculation?

 

You need a survey from someone willing to give you quote on how much the different repair options will cost in your location.

The deck needs fixing, when the bungs start comming out, it's time to replace it. The current top layer was probably put on with 5200, so the fix will be to cut the existing deck between beams, then replace it with fiberglassed plywood. The pilothouse leaks are probably from blocking the waterways when the second layer was added to the deck. Depending on the condition of the carlins and the construction details the pilothouse may need to come off for the repair. All said, the deck is likely to cost at least as much as the hull.
Without fixing the deck the boat won't last 10 years in the tropics. Even if it leaks only "a little" where she is now, the climate change will be severe enough to make it a sieve. As good as 5200 is, by now it's probably hard and brittle, once the wood starts to move it will rupture.

If you have the time, doing a lot of the work yourself is feasible. You do have an idea of laminating wood, so a few basic tools will see you trough. You need a jointer/planer combination, a tablesaw and powered handtools. The big jobs, like cutting all veneers is best farmed out to someone with a resaw.

Just to confuse you more, there are other options for repair, like strip planking and longitudinal double planking. Like cold molding this depends on the main framing beeing sound, but by doing it you necessarily open up the boat so small repairs are relatively easy. If the planking is sound, much of it can be reused.
The methods work like this: you remove a plank at a time, starting at the garboard. Then you clean all surfaces and then you have two options: cut the plank into strips and reinstall with epoxy or split the plank in the middle and install with staggered seams (you need a stealer on one layer). It's also possible to cut the strips on the boat directly (same as splining). All rotten wood gets replaced by scarfing.
Repairing the cracked frames is done in situ from the outside, you either cut and scarf in, or laminate a short sister alongside.
Just to be clear, cold molding, strip planking and double longitudinal are only economically feasible if you do it yourself, because they are very labour intensive. The good thing is that you only need to handle light pieces of wood, and it's a piecemeal affair, so you can do it by yourself. If I had to choose I would probably strip the boat.

 

This approach is most sensible, imo. Reusing all the old planking save a lot of $$. Can small cracks in floors (&frames) be repaired with epoxy, if done well and not broke through?

How are the strips fastened with this method? And since the inside is not accessible, the strips are all epoxy puttied outside the boat on a table? And any putty would be mostly epoxy and not thickened much, if at all?

Decks and hull...rather daunting..this approach I like because you can create access to the damaged framing for sistering or laminations.

 

Hi @WhiteRabbet,

While I don't disagree with @Rumars, I am putting up a how I would calculate the required thickness for the cold-moulding.

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As I don't know which ribs are broken I have assumed they are all broken!
So my design is a worst case scenario.

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P is the design pressure for the hull, ISO 12215-5 goes through the process but in this case I will use Arthur Edmunds, "Designing Power and Sail" estimate.
He uses:

P (kPa) = 0.03*V^2+0.25*Disp^0.33+1.3*LWL​

Where:

V is the maximum speed in km/hr
Disp is the (maximum) displacement in kg
LWL is the water line length in meters​

S is the allowable (safe) working stress of the material we want to use (i.e. plywood), ISO 12215-5 can be used to make an estimate but I will just use following table:

In Australia, F17 is the top of the range (AS/NZS 2272) Hoop Pine marine plywood, F14 is pretty good and F8 is the light and flexible (Oukume?) stuff. I would assume F14 (14 MPa bending strength) to start.

As the plywood will be encapsulated in epoxy then I would assume the moisture will stay at 15% but if you want to use a wet number (say 25%) then derate the strength 40% (8.4 MPa = 14 MPa *60%).

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First let us calculate the hull thickness assuming the frames are good:

The thickness of the planking can be (that is, there are other ways) determined using:

• t=s*sqrt(P/S/2)

where:

• t is the hull thickness
• s is the frame spacing
• P is the design pressure
• S is the allowable stress of the hull plating

​

So what was the spacing (s) between frames (amidships)?

---

Now let us assume that a frame is broken.
The nearest structural support is likely to be the longitudinal members (if not then break some more adjacent frames!).
Note: the longitudinal members do not need to be in contact with the planking.
What is the spacing (c) of the longitudinal members (amidships and near the keel)?

Now you can estimate the plywood thickness for the cold-moulded sheathing.

It the thickness is too thick for your liking, then you could consider external longitudinal members/stringers.

AlanX

 

I remember reading bout the extra skin added to Curlew and I also remember that other similar projects were mentioned,all being classed as successful.As has been mentioned,you need to eradicate any rot in the structure and a survey would be sensible..Having gone to that amount of trouble to prepare for the project,all that stands in the way is any surface coating that would inhibit a good bond.So old paint and varnish would have to go and the planking would need to dry out to a suitable moisture content.Then it becomes a matter of determining the correct amount of material needed and begin applying it.It would be a tedious job and done overhead it would be physically demanding.Done well,a decade should easily be surpassed.Were it my boat,it would be done with wood rather than glass.

 

Some cracks can be repaired, but it's not worth it. You can remove every other plank, this leaves you with a stringered form onto wich you can laminate (or steam bend) new sections of frame by clamping them to the remaining planks. Floors are similar, you laminate new ones, replacing grown or plank floors.
Fastening is by screws and thickened epoxy, after you repair the existing holes in the frames.
The planks are resawn into strips, but they (and the frames) must be cleaned on the inside face, wich is often hollowed, and you have to sand it by hand instead of running it trough a planer. Glueing is as usual, butter with thickened epoxy, fix in place. After two or three strips you can coat the inside with neat epoxy and then paint using a bent brush.
The whole process is very time consuming, a commercial outfit will be cheaper if they simply plank with all new material. The amateur has the luxury of time, and can save a buck in material by doing this.

 

Alan, I think if we want to use your plywood analogy we must determine the theoretical skin thickness based on bulkhead spacing alone, then take this figure and half it. The resulting number can then be compared with the existing hull thickness to see if there is a need for additional longitudinal layers.

If you want a better model you have to analyze the skin like a monolithic glass one, considering each wood layer as unidirectional fiberglass and using the specific values of the wood species in question.

 

Hi @Rumars,

You get the idea, whatever supports the ribs/frames can support the new shell.
At the moment we have no information on the longitudinal members.
At this point I would not assume any contribution from the ribs/frames or the hull planking.
Particularly as the longitudinal members are parallel to the planking
(i.e. the planking has no bending strength across the planking direction).

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Sure we can calculate a thickness based on a shell formula.
For a single curve (amidship cross-section) the buckling pressure is:

P=0.16*E*(t/R)^2​

You would need to use a safety factor, say 40% (consistent with ISO 12215-5).
Therefore:

t=R*sqrt(P/.16/.4/E)​

Where:

t is the hull plating thickness
R is the (local) radius, I would use R = (4*DWL^2+BWL^2)/DWL/8
​

ISO 12215-5 does consider "natural" stiffeners where the whole hull section acts as a stiffened arch.
This allows plate thickness reductions via design pressure reductions. But I have not studied it.

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I my mind, if the cold-molded thickness is too thick, then just assume the old hull frames still provides some "percentage" of support.
Or we could add external structural "spray" chines to reduce the longitudinal members span and therefore the plating thickness.

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Thinking about the shell buckling pressure calculation, I think it would be reasonable to include the hull planking in thickness estimate as it supports the cold-molded skin (from buckling) and we have used a 40% safety factor.

We should probably include a hoop stress check of the skin as well:

Sc = P*R/ts​

Where:

ts is the skin thickness
Sc is the skin (plywood) working compressive strength (from Table 3.1)​

Regards AlanX

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