More on Strip Planking: Paul's Opinion
February 26, 2011
Paul in epoxy resin, rot, strip planked

This section is taken from a letter to the editor of Watercraft magazine, in response to articles about strip planking as a good method of construction for the home builder. Strip planking appears to offer an easy route to planking the hull, because it does not require lining out the planking or constructing temporary jigs.

Some Problems with Strip Planking

  1. Time consuming and hard to repair
  2. Unstable
  3. Aesthetics
Time consuming and hard to repair

Any discussion of strip planking needs to address the serious problems inherent in the method. There are two main problems with strip planking. The big one is that is is not stable. You cannot edge glue a half acre of planking—regardless of how thoroughly kiln dried and edged grained it is—and without expecting trouble as temperature and moisture levels change. In a cold molded hull the fibre orientation is different in each layer, and each layer acts to stabilise the others. With strip planking the fibre orientation is in one direction only and the expansion is cumulative. Ah hah, you say, but that was before the miracle of epoxy resin. Now there is no transfer of moisture and hence no movement. Well I am afraid that is where we run smack onto the shoals that lie between the sales literature and real life. My experience is that a strip planked hull will shrink and crack in a most distressing manner if it is hauled out in the sun for too long, and I have seen everything from broken frames to the decks pushed up off the clamp as a result of swelling. Sure, epoxy resin is a wonderful coating and applied in sufficient thickness drastically slows the changes in moisture content, but I have plenty of evidence that it cannot prevent movement taking place in the wood entirely. I would certainly not rely on it to overcome the colossal tensions this form of construction engenders.

In contemplating a strip planked project, then, it is essential at the outset to devise a strategy to stabilize the structure. In small boats that will live mostly out of the water, sheathing inside and out with epoxy impregnated glass cloth works well. This is the method pioneered by the canoe builders and in small frameless hulls will produce a durable boat with an easily cleaned interior. It is surprising how soon the plank lines will start to show through the glass skin on such hulls. But the glass seems to be able to handle the stresses in light planking, and I haven't seen outright failures with this technique if done properly.

On larger hulls I am not in favour of sheathing on both sides. That just looks like a rot trap to me. If you are an epoxy believer, you will say that since no water can get in, there is no way rot can get going. That assumes the only passage for water is through the laminate, but every time a through hull is installed or a fitting is bolted through the hull there is the potential for water to enter the core. When it does the owner will likely be oblivious until the problem is widespread. It is worth noting that ABS (American Bureau of Shipping) rules do not recognise solid wood, fully encapsulated, as structural below the waterline. This means they do not recognize the wood as contributing to the strength of the structure, because is is not likely to last. I don't see it makes any difference whether the resin is polyester or epoxy given the reasons above.

In larger hulls with internal framing the glass sheathing is often applied only on the outside. To me it seems the height of optimism to think the enormous stresses in an edge glued hull can be controlled by a skin of glass glued to one face. I suppose if you get into biaxials and lay it on heavily enough you will eventually have a glass hull with a wood lining, and lick the problem that way, but that is hardly good engineering.

There are other problems with this approach too. Traditional hull models often have large areas of solid deadwood which are difficult to sheath successfully. There is always movement here even if these members are themselves laminated. 'Zippers' in the glass are common here. And of course sheathing under the keel is very vulnerable to being torn on grounding.

I think a better method for larger hulls is to use strip planking as the first layer of planking, and then follow it up with two or three layers of cold molded planking laid on the diagonal. We have found the method reliable, although it does not make as good use of the properties of the wood as a fully molded hull. A few test panels will clearly show the greater strength and stiffness of a fully cold molded laminate. However, extra thickness can be used to provide adequate panel stiffness.

In all cases however the outer layer should run fore and aft. No matter how well the hull is sealed with epoxy, or even if there is a glass skin as well, it won't be long before plank lines begin to show. If a dark colour is used on the hull the plank lines show sooner and more clearly. Diagonal lines across the topside will neither gladden the heart nor improve the value of the investment.

Aesthetics

The second problem I have with strip planking is an aesthetic one. The majority of people who set out to build themselves a boat do so for the fun of it, because it is one of the most pleasurable things to do in life, or should be. With that in mind we should surely look for a method that will be enjoyable to do and a delight to behold throughout the process. For the long distance singlehander, this is more than mere sentiment. The pleasure that comes from opening the shop door in the morning is perhaps the single biggest factor in maintaining momentum through to the finish.

To my eye a strip planked hull is simply ugly. Because there is no compensation for girth differences, the planking cannot follow the natural lines of the hull. Instead it bunches up in the most offensive manner and on many hulls has to be cut down periodically so that a fresh start can be made. Compared to a nicely lined out carvel or cold molded hull the result is a dog's breakfast that must be concealed with paint at the first opportunity if morale is to be restored.

Conclusion

In view of the amount of time and money involved in even a modest boat building project I would caution first time builders to weigh the options carefully and be particularly skeptical of 'instant' methods. Strip planking has its place, I use it from time to time myself, but I can honestly say that I have never built a strip planked boat that I did not wish at some stage I was building by another method.



Article originally appeared on Gartside Boats (http://www.gartsideboats.com/).
See website for complete article licensing information.