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Frequently Asked Questions

Planking Method Comparisons

What are the differences, and advantages and disadvantages between different traditional and glued construction methods? (Carvel, Clinker, Cold Molded, Strip Planked)

Traditional Methods: Carvel

Carvel is edge to edge smooth skin with caulked seams. All planks are individually shaped to follow the curvature of the hull. They vary in width along their length in order to compensate for the girth difference between the middle of the hull and the ends, and they are also shaped in their thickness to fit the curvature of the sections. That is, they are hollowed out on the back and rounded on the outside. Carvel works best when planking thickness is 3/4" or greater. It is possible to go down to 1/2" but the caulking has to be done very gently.

Traditional Methods: Clinker (Lapstrake)

Below 1/2" plank thickness clinker comes into its own. Here the planks overlap and there is no caulking. Watertightness comes from the fit of the plank lands and close spaced riveted fastenings. Clinker is generally quicker than carvel as the planks are not shaped in their thickness, only in their width. Clinker is a great way to build small traditional boats.

Both carvel and clinker are done over bent frames in small boats, in larger sizes carvel may be over sawn or laminated frames.

The advantages of the traditional methods are that they are usually the quickest way to build a one off boat. They are to a large degree self fairing, that is because of the natural spring in the wood of both planking and framing, unfairnesses that exist in the temporary molds will show up and can be corrected, or will be self correcting when the molds are taken out. Also it is pleasant work, no glue or fibreglass means no latex gloves or respirators and most of the work can be done with hand tools.

The disadvantages of the traditional methods are that the boat relies on the swelling of the planking to stay tight and if the boat is kept out of the water on a trailer, it will likely leak a little when it is launched until the planking swells again. In larger sizes the wood to build traditionally is getting harder to come by.

Glued Methods: Cold Molding

Cold molding is distinguished from its predecessor hot molding by the fact that the glue will cure without high temperature. This technology came out of World War II when airplane fuselages were made from multiple diagonal layers of veneer laid up over a jig and then baked in an oven. It wasn't until after the war that cold setting glues came along and cold molding was born.

In cold molding most of the planking goes diagonally, in alternate directions with the outer layer running fore and aft to avoid diagonal plank lines showing through. The planks are individually shaped to compensate for girth changes but are thin enough to bend without steaming in both directions. Essentially in cold molding we are building a piece of plywood that conforms to the compound curvature of the hull. Because the planking layers are thin and glued to each other into a monocoqe structure, the finished hull will retain any unfairness that exists in the jig over which it is built. There is no self fairing as in the case of traditional methods and therefore the standard for lofting and jig making must be higher if a quality hull is to result.

There is no question in my mind that of the glued methods, cold molding is the superior system. It is utterly stable, has maximum strength and minimum weight.

The only disadvantages are that there are more hours in a cold molded hull than a conventionally planked one and on aesthetic grounds it is not always the best fit with the character of the boat. The lack of framing and the flawless finish tend to give cold molded boats a too perfect, 'plastic' finish that often makes them indistinguishable from glass hulls. This is often not what we want in a wooden boat and we have to work hard to get some texture back into the thing.

Glued Methods: Strip planking

Strip planking is a glued construction method very popular with amateur boatbuilders because it appears to be a quick method that avoids complex temporary jig work and does not require shaping of the planks. My diatribe on the method farther on in this section of the web site gives my views on this. See Paul's opinion below.

Hybrid systems

Now we get to the hybrid systems, those which combine features of both traditional and glued systems. This is often the answer when we are looking for a boat with a traditional air but would like some of the advantages of the glued methods.

This was the case with the Tomales Bay One-Design "Jessie". The customer wanted a traditional wooden boat, if I remember rightly he first approached us about a clinker boat. However he was in California, we are in British Columbia and the boat was to live out of the water on a trailer. I was concerned about the potential for drying and leaking if we shipped a conventional clinker boat down to him from here. Instead we planked her over bent frames with two skins of red cedar, one diagonal and one fore and aft fully glued with epoxy. Apart from the run of the inner skin visible in the varnished interior, it looks like a traditional carvel planked hull but we don't ever have to worry about her opening up.

There are many other variations along this line. The boat we built for our own use is also double planked over bent frames, with both layers running fore and aft. (See 22 ft Cutter "Surprise") In larger boats one could also go two diagonal and one fore and aft for an exceptionally strong hull with a traditional interior. That is one I would like to try one day.

So there you have the range of possibilities. Each has its up side and reverse. I would just finish by reminding you that the fun is in the traditional methods. The glued systems all involve using copious amounts of toxic sticky substances, none of which are very good for you. It therefore entails working in gloves or using barrier creams and/or respirators at various stages in the process. This seriously detracts from the experience, believe me.

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

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.

Clinker (Lapstrake) Construction Techniques

Is it up to the builder to decide the placement of the planks in clinker boatbuilding?

The plans do not give the positions of the individual planks. Lining out is the job of the builder and can only really be done in three dimensions. In clinker construction where the plank lines show, it is obviously very important that this be done well and the nice even progression of widths and line is maintained. The only sure fire way of achieving this that I know of is to line out with a series of light battens tacked over the molds to simulate the plank edges. I usually make these battens the width of the plank land (3/4" in the 7 ft Clinker Pram). These are then adjusted by trial and error until a pleasing run of plank lines is obtained and the widths are within permissible limits. Planks in the bottom and topsides can obviously be wider than those in the turn of the bilge because it is important not to go down to a feather edge when dressing off for the next plank. Making an even transition of widths so that it looks right to the eye is the secret and I suppose the art of it. In the 7 ft clinker pram design (Design #80) I have given widths for the keel plank to get you started, and I will usually note the expected number of strakes per side.

One other thing I should mention on the lining out. The type of gunwale has a bearing on the width of the sheerstrake. If there is an outside gunwale at the sheer, the width of this has to be added to the width of the sheerstrake. Otherwise the sheerstrake will look narrow in relation to the planks below. If, on the other hand, there is no outer gunwale, but a rubrail under the land of the sheerstrake, then the reverse is true. The sheerstrake has to be made narrower by the depth of the rubrail in order for harmony to exist in the land. This type of gunwale is common British practice and can be seen in design #86, the 13ft- 6in rowboat. All this is easier to do than describe, but needs to be thought about at the lining out stage. It is the kind of detail that makes all the difference.

Continuing with the 7ft clinker pram, are the transoms to be cut to the inner edges of the planks or to the continuous curves as lofted and shown on the lines?

Now what happens at the transoms? There is no difficulty with the bottom planks or those on the topsides, the problem is how to deal with the planking which lands on the curved surfaces of the transoms. Actually the problem is only really with three or four strakes at the turn of the bilge. Planking in the bottom and topsides will sit comfortably on these relatively flat sections. For the bilge planks there are three possibilities:

  1. The transom can be trimmed with a sharp chisel to make a flat for each plank to land on. This can only be done progressively as the planks are laid.
  2. The planks can be bent to fit the curve before being hung.
  3. A third method is to hollow and round these planks in the manner of carvel.

I prefer the second method, as the result is much better looking and in my view a better demonstration of the boatbuilder's art. We use red cedar for planking here and have no difficulty putting an edge bend in the last few inches of a plank by soaking with a kettle of boiling water and then clamping between curved blocks of wood. This is best done overnight so the thing is dry to fit the next day.

If you were planking with something like mahogany this is not going to be so easy and I would experiment first with scrap wood before deciding on the best approach. If I remember rightly in this little boat it is only the plank lands on the stern transom that require this treatment. The lands at the bow are so much narrower they don't need it.

I have also hollowed and rounded planks in the manner of carvel. To do that you have to leave the plank end thick by backing off the thickness planer gradually as it approaches the end of the plank, and then finish it by hand with a round bottom plane.

I hope this helps. There is plenty of real boatbuilding even in a little boat like this, but at least on this scale if things don't work out right first time it is not all the world to cut a new plank or two.

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Last modified 2007 06 28