BucketEars - Plywood and Lumber For Your PDRacer Sailboat
2 sheets of 1/4" plywood - for sides and bulkheads and decks
1 sheet of 3/8" plywood - will be used for the bottom.
2x4 by 16' -- For the mast. Get one that is clear, and has little or no knots.
2x6 by 8' -- Get 6 of them, for framing lumber and various other parts
1x12 by 8'-- For rudder and leeboard
The edges of the plywood are held together with little strips of wood (called chine logs and framing lumber) that are about 3/4" square. To make these strips of wood, it really helps to have clear lumber to start with. Clear meaning not very many knots in the wood. 2x6 or 2x8 lumber tends to have less knots than 2x4 lumber. So I usually get the larger boards, and cut the chine logs (and other pieces) from them, but almost always have wasted parts of boards because of the knots.
Thickness For The Bottom Of The Hull
I prefer 3/8" thick plywood for the bottom of my hulls. The 3/8" is very durable and I have run into numerous rocks while sailing and it held up just fine. I have used thicker, and it just seems to add extra weight.
I have used thinner 1/4", but that doesn't seem to be thick enough and can crack if I suddenly put all my weight on one knee in the center. 1/4" bottoms seem to "oil can", meaning they flex a lot as you are sailing. Friends of mine have holed their 1/4" bottoms after running up on rocks and stumps.
Keep in mind, I am a big guy, and others have used 1/4" bottoms with great success. Whatever you go with, I strongly suggest using stringers along the bottom which greatly increase the strength of any thickness of bottom.
Types Of Plywood
SYP, meaning Southern Yellow Pine is the most common type of plywood used for ducks. This type is about the cheapest available plywood, and is made from a tree that grows so fast, it is considered a crop like corn or wheat. I commonly find it at my local store for about $12 per sheet. It is very soft wood, and the problem with pine is that as it dries, it tends to have portions of the skin shrink, which opens up little cracks. This effect (called checking) happens pretty bad in arid desert regions, but in humid areas it isn't that bad. Also if you sail your boat often, the plywood will pickup the moisture from the lake and it will have less of an effect.
Although it sounds horrible, plywood checking isn't that bad of a problem and it can be fixed fairly easily by filling the cracks with glue or epoxy, or sometimes just painting over those areas and the paint will fill the cracks. I have had a couple of checking cracks open up so bad that I could see daylight all the way through, but instead of fixing it, I just went sailing. The crack leaked for about 20 minutes, and then the moisture soaked into the plywood and the crack closed itself back up again.
Lauan is another common type of plywood that many boat builders use. Some builders swear by it, some swear at it. It is fairly inexpensive, usually less expensive than pine. I don't remember what the core is made from, but it has a veneer on the outside of some kind or hard wood. It is intended as a decorative plywood for interior use, and while the glue is not rated to be waterproof I have never heard of it's laminating glue failing from use in a boat. Lauan is fairly light (less dense) and so many boat builders use it for boats they want to be light. The big problem with Lauan is: if stored outside, some portions of the Lauan can easily delaminate and rot. I have built boats from it and some hold up great, while others fall apart really quickly. Other boat builders I know have experienced the same thing, and so far we have not figured a way to pick the good lauan from the bad. So my best suggestion is if you decide to build from Lauan, you should store her inside when not sailing, or in some other way that protects her from the weather.
Birch is my favorite !! One boat I built, I made from various types of plywoods and happend to make part of it with Birch. The hull lived outside in the sun for a few years and while the pine and lauan parts of her exhibited all of the described problems listed above, the portions of the hull made from Birch held up great. Almost no checking, no delamination, and still felt great. The compromise is that Birch is about 3x more expensive than pine.
Grades Of Plywood
Interior or Exterior grade These are the main two differences. Exterior grade is supposed to be made with glues that can be exposed to water and still hold together. The outsides of houses are exposed to rain and moisture, much worse continual weather than boats like ours are typically exposed to. Interior grade plywood usually isn't rated for moisture, but often uses the same glue.
Boil Testing There is a common test floating around the boat building hobby where people test their plywood to see if it can hold up to the weather by placing it in boiling water for a period of time. The concept is that if the sample falls apart, then it isn't waterproof. The problem is that the heat can melt the glue and make it fall apart, where as if it just wasn't heated up that high, it would have held together just fine. I normally do not sail in boiling water (but have sailed in 96 degree water), so personally I do not have faith in the boil test.
Grade AC or BC Plywood that is commonly available with a grade of AC or BC, meaning the A side has been sanded and is nice, and the C side is rougher. There are other letter versions of this grade, but basically this grade is just talking about how smooth the plywood is. The most important thing to do is examine the sheets of ply when you get them, and dig through the stack to get a good sheet that you like.
Marine Grade This grade of plywood is supposed to have the best waterproof glue, and the interior layers of the sheet do not have any voids in them. There are a number of other standards this type is supposed to meet. It is also the most expensive type of plywood of the ones listed here. Personally I have never used it, but I know others that have and they were all happy with it.
Indepth Explanation Of Plywood
The above explanation of plywood is my personal opinion from what I have experienced. Almost every other boat builder will also have their opinions too. We happen to have a ducker that has significant education and experience, John Bridges completed a 5 year Timber Technology Studentship based on a Plywood factory in England and learnt to use each type of machine and study each part of the process plus studied Engineering to understand how to work with it and Botany to understand the cellular structure of wood. His career consisted of working within the wood industry based in many tropical countries. In his travels around the globe, he has built wooden sail boats everywhere he went. See: Plywood For Puddle Ducks
Other Hull Materials
There are many other types of plywoods and materials you can build your puddle duck from, some ducks have been built from foam and various combination of materials. Above are the most common materials, but if you find something else and it looks like it will work, then you are welcome to give it a try. Worse case scenario, if the materials don't work out, then build another hull and transfer your number over to the new hull.
Bronze Ring Shank Nails
When attaching the chine logs or framing lumber to the plywood, if you use a regular steel nail or screw, you will get rust stains that surface through the paint or epoxy. Even if you glass your hull, moisture does get to the nail and will make it rust.
Some people will use screws to attach their bottom, and then remove the screws. Then... you can dowel the holes by drilling each screw hole just big enough so you can put a skewer stick (like for shishkabobs on the BBQ) into the hole with some glue so the hole is sealed up. The little wooden stick & glue also sort of works like a fastner.
Personally what I do is use bronze or stainless ring shank nails. They are quick to nail in, you can just leave them in and they won't make rust stains. I usually get 14 gauge x either 3/4" or 1". The only thing is they are very hard to find at a local home improvement store so you gotta order them online.
Glues & Paint
Titebond II glue - This is a yellow construction glue that is widely used amongst MANY boat builders for many years. There is a III version, and many other common construction glues that will work just as good.
Keep the dust from your power sander so you can use it as a filler. Here is how to fill the cracks. You can also mix other powders like wheat flour to make fillers.
PL-200 This is great for sealing the inside edges of your joints, such as inside the chine logs and around framing lumber. Some people build their entire duck using PL products. Phil Keck did a great experiment to test PL products.
Latex Exterior Paint My house is made from plywood just like my duck is, only my house sits outside every day and gets rained on often. Latex paint does a great job of protecting my house, so that is what I use on my boats. I have used oil based paints before, but they became hard to find and I switched over to just latex exterior paint and it has served me well.