Climate Conditions Vs Wooden Model Ships

[tmj]

Has anyone ever experienced issues with wood swelling and cracking via changes in weather and humidity, over time, where lots of 'deadwood', etc. is used in the stem and stern of your models? Expansion, contraction and eventual cracking. If so, how do you counter this problem and prevent it from happening? 

 

 

Edited October 18, 2023 by tmj

[allanyed]

Somewhere in the past I saw then current photos of the Victory model by Longridge and the planking had a lot of cracks and/or lifting if I remember correctly.  Hopefully someone knows where to find the photos and can post them.  I seached the net and specifically RMG with no luck.

I have not personally seen it on "modern" models going back to 1978, but you may want to check with a museum expert such as Grant Walker at Preble Hall at the USNA in Annapolis to get their input as they have a vast collection.

 

Allan

 

 

[Keith_W]

These massive yawning gaps appeared on my HMS Bounty, completed in 2008. This is a single planked model. Back then, I was less experienced with wooden ship model making. My method at the time was to soak the planks in my bathtub, bend it with a steam iron, then install it. It appears that the wood was not dry enough when it was installed, which has two effects - (1) the wood will shrink as it dries, and (2) the PVA glue will not penetrate wood which is saturated with water, it needs to be dry - it will thus form only a surface coating resulting in a weaker glue bond. The combined effect of both - stress pulling the joint apart + weaker glue bond, resulted in the gaps. You can also see that different wood species react differently to being installed wet. The walnut (darker coloured wood below the wales) remains nearly perfect. The limewood (yellow coloured wood below the painted blue band) has suffered the most. My theory is that the limewood is more porous and swells more than walnut. 

 

To my mind, the solution would be: 

 

1. Construct your models properly. Don't install wet wood, make sure the planks are glued together and not only to the frame. 

2. Keep models in a humidified condition, or conditions similar to when it was constructed 

3. Treat the wood with swelling agents like polyethylene glycol to keep the fibres saturated and fat. This is what is currently being done to the Mary Rose in Portsmouth, and has been done to the Vasa and Batavia in Fremantle, Australia. 

4. Choice of varnish and sealant will affect the rate of moisture migration in wood, but won't prevent it entirely. 

[mtaylor]

To add to what Keith and others have said.   Read the planking tutorials about "prebending" the planks and using a quick soak and heat.  The heat will soften the inner wood and when cool the wood will stay bent. 

[Bob Cleek]

I have encountered the sort of issue pictured above with planking, but I've never ever seen any checking in major timbers like keels and deadwoods on a model. I have restored models of varying ages (all over 50 years old, at least) which did show cracking and splitting of larger thin solid sheets of wood such as were used as decks and cabin tops. In that instance, it appeared that the better course would have been to plank the surfaces individually, although the wood sheets in every instance I've encountered were solid wood and not thin laminated plywood. I would expect if a larger flat sheet of wood were required modernly, thin modeling plywood would be the preferred material.

 

There seems to continue to be a lot of misunderstanding and consequent misinformation persisting in the modeling community about bending wood, probably because the process is so often referred to as "steam bending." This has been discussed in great detail in previous posts. It's a matter of well-settled scientific fact and not opinion. In an attempt to set the record straight, 

 

1)   Wood doesn't need to be wet to bend.. In fact, it is better that it is not wet. Wetting the wood, if anything, causes it to absorb moisture and swell. To the extent it does that, fitted joints will open when the wood shrinks upon recovering equilibrium with the ambient humidity.

 

2)   The amount of movement dependent upon the wood's moisture content is a relative constant percentage depending upon the species of the wood and the direction of the grain. Wood swells more across the grain direction, and less, if at all, with the grain direction. Some species move more than others.

 

3)   Given these movement properties and the small size of the wood pieces used in ship modeling, under just about any circumstance, the amount of movement is so small as to be unnoticeable. That said, the cracking of the large thin wood sheets I've encountered before was not a result of the flat sheet shrinking or swelling alone, as the structure should shrink and swell at the same rate, or close to it in cases where different species are adjacent to one another, but rather more likely due to the movement of the entire hull structure which could not be incrementally accommodated by the single fixed flat sheet that cracked under the strain. Again, this should not be encountered with plywood, a much stronger engineered material.

 

4.   All that is needed to bend wood (understanding that the tolerance for bending varies from species to species,) is heat. Moisture enhances heat's ability to spread through the wood (which is why "green" higher-moisture-content wood is preferred for bending stock in full-size boatbuilding) but has nothing to do with making the wood more pliable for bending. In full-size boatbuilding, wood to be bent is steamed in a steam box not to make the wood wet, but simply because, given the available technology, the best way to transfer heat to a long plank is to stick into a box or pipe and fill the box or pipe with steam. For modeling purposes, any of the customary methods of applying heat to modeling wood without soaking it in water are just as effective, and a lot less messy, than steaming them. As noted in the previous post, wet wood does not glue as well. In fact, heat applied to dry wood glued with PVA glue with speed the drying and curing of the PVA such that spot-heated planks can actually be "tacked" in place with PVA.

 

5.   There is no reason to treat ship modeling wood with polyethylene glycol to retain moisture as is done with archaeological artifacts recovered from long periods of immersion.  This stabilizing agent is applicable to long-immersed wooden artifacts only. It wouldn't do anything for a ship model, unless, perhaps, the ship model was brought up from the bottom of the sea a few hundred years after it had been put there.

 

6.   Within normal ranges of humidity fluctuations, there is no need to artificially humidify ship models. If the model is displayed where it was built, it should be just fine indefinitely, at least from a humidity fluctuation standpoint. If one were to move a model from an extremely dry climate to an extremely humid climate or vice versa, there may be some concern regarding wood movement, but once the wood have reached a moisture content normal relative to the new humidity environment, no problems should be encountered. Obviously, displaying a ship model in a bathroom where people regularly take long hot steamy showers isn't the greatest idea. Wood reaches its moisture equilibrium at a relatively standard rate. The rule of thumb for this is that wood should after being felled and milled be allowed to dry in a covered area for a minimum of one year per inch of the stock's thinnest dimension. You can do the math, but assuming the wood was dry when milled into planking strips or other modeling wood, it is going to be so thin that it should be able to adjust to humidity fluctuations very rapidly so there's no reason to be concerned about its needing to be "acclimated" to the building environment. (Alternately, for example, hardwood flooring should be stacked and left in the area where it will be installed for a few days to ensure it has reached a moisture equilibrium equal to the room in which it will be laid.)

 

7.   No wood coating is entirely impermeable. The most effective moisture barrier is said to be shellac. In fact, it is better to allow a wooden structure to move evenly than to attempt to prevent if from moving in one place and not another.  The uniform movement of a wooden structure is always less stressful because any resulting stresses are evenly distributed.  

 

I have never encountered a sprung plank or a cracked plank seam on a model which did not appear to be the result of either poor adhesive fastening, incomplete bending before installation, or both. I see many instances of "edge setting" (bending a plank across it's wide dimension) in modeling. This practice would never occur in full-size boat and ship planking, were it even physically possible, because it builds stresses into the structure in a manner contrary to sound engineering practices. (Similarly, I see no reason why properly fitted planks should ever require any glue in their seams, another apparently common modeler's practice that seems to add a rigidity to the planking structure which prevents uniform movement, concentrates stresses, and thereby promotes structural damage.) The ability to heat-bend small dimension strip wood across its widest dimension and the availability of strong adhesives seems to make edge setting possible in scale modeling, but perhaps not entirely advisable. While the stresses are proportional to the scale, the stresses created by wood movement are relatively great. I cannot say for certain, but when I look to assign blame for cracked plank seams and started plank ends on ship models, I look to a failure of the fastening system to overcome otherwise normally-expected loads, rather than any excess of ordinary ambient humidity fluctuations, and that is generally a result of an improperly shaped and fitted plank.

 

With respect to shaping and bending planking, I would urge a close study of Chuck Passaro's planking tutorial materials. He makes it clear that a plank should be spiled and bent so that it fits perfectly in all dimensions when offered up to the frames, as is the case with full-size construction practices. He does bend planks across their wider dimension, but he does this with heat, resulting in a permanently bent curve without any spring-back. Planks should never be unduly forced into place because if they are, they will spend the rest of their days trying to find a way to return to the shape God gave them when they grew!

 

Just say'in. Your mileage may vary.