« More on Strip Planking: Paul's Opinion »

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.



Comments

[Christopher Burford]

I am in the early stages of an impractical flirtation with your design #102, a 27 foot cruising canoe of excellent proportions. She's also the largest design I've found on your site to be listed as "intermediate" skill level, and the estimated time to build her is a "mere" 2000 hours. A light, fast, spartan cruiser that one might conceivably actually build, if a remarkable series of miracles were to occur.

And her method of construction? Strip-planked.

Mind you, this might have added to her allure had I not read this essay of yours just before looking at her (not that I know much about any form of construction other than plywood and epoxy). But I wonder, given the concerns about strip-planking that you voice in this essay, why you designed this boat for that construction method? Is there something exceptional about design #102 that exempts her from the concerns listed in your essay? If you were building this boat for yourself, would you choose another method?

[Paul]

Chris,

First my apolgies for the long delay in replying. You are right I present a confusing picture when it comes to the choice of strip planking as a building method. I don't care for it as a method of construction for the reasons given in the FAQ piece, but I use it frequently in my own designs. How come?

Strip planking can be used successfully PROVIDED it is stabilised either by a layer of glass cloth and epoxy resin applied inside and out, or by overlaying it with a couple of layers of cold molded planking at least one of which is applied diagonally. Either method, properly done will prevent the movement that causes the problems I describe.

In small boats and those built cheaply and quickly without regard for long life, sheathing inside and out is the fastest method and it works well. I use it for small open boats all the time. Indeed I much prefer it to the alternative of plywood lapstrake on ethical grounds. High grade marine plywood uses species that are fast disappearing and which we should move away from.

I wouldn't use strip planking sheathed inside and out on a boat of high value or where long life was hoped for. The problem is if water gets into the core through fastener holes or damage, it is trapped there and the greenhouse- like environment will foster decay in short order. For long life it is much better to use the strip planking as the first layer, then stabilise it with further cold molded skins. That's the prefered method for larger boats.

Having said all that, my opinion that strip planking is the least pleasant way to build remains for the reasons outlined in the FAQ piece. As always it is a matter of compromise. Sometimes skill level combined with availability of materials will make the use of strip planking the best choice. Where the emphasis is on quality or the enjoyment of the building process it should be placed much further down the list.

I hope that helps.
Paul Gartside.