Building Method & Materials Advice - Power Outrigger
In the design spiral for my power outrigger design, it’s time to think more about the building method and materials, and I would enjoy your advice.
There are some drawings and discussion of the design here: http://proafile.com/forums/viewthread/200/
As a refresher, the boat will be around 32’, lightweight cabin cruiser for the inside passage up in Washington, Canada, Alaska. Driven by a 30hp outboard. Goal is cruising speeds in the 10-12 knot range with higher speeds possible. Based on similar boats in different configurations I think it is reasonable to expect the boat to weight in at less than 2000lbs. For example Paul’s 32’ proa is expected to be around 1700 lbs. http://biekerboats.blogspot.com/2012/12/an-update-to-our-32-foot-proa-design-in.html
I would like to keep the building methods low tech, quick to build, and low waste (dreaming I know). I would prefer not to vacuum bag. My skill experience is with marine plywood and fiberglass cloth and epoxy. I will be building in Arizona, outside (with a roof). Dry and hot most of the time.
Unfortunately the shape of the boat is fairly complex. I want to us the Malcolm Tennant CS underwater hull shape, that has been proven for displacement powercats. It has a canoe stern underwater, but a wider transom closer to the waterline. Above the water the hull needs to flare dramatically to allow for the accommodations, very much like a Farrier trimaran.
Originally I liked the idea of foam with glass skins, much like the Farrier trimaran home builds. But I don’t think it is a good idea. I am not an engineer and composites like that require more knowledge to get them right. Wood is just so much stronger than foam that it is more forgiving to my lack of engineering skills. The glass skins on the foam could easily be too thick or thin, heavy or weak. Plus you have to reinforce every area that takes a beating with a denser core. And it’s expensive.
So what building method & materials?
Cold Molded With Plywood
For rounder shapes, and a strong boat, the cold molded method has come up as an option, with two layers of diagonal plywood strips, glass outside. I have no experience with this, but it seems doable. It’s how Russell builds a lot of his boats.
Cedar Strip Plank
To get those round shapes I could also strip it in cedar. That would yield a beautiful strong boat, with no need for stringers or as many frames, but it seems like such a difficult way to build, having to fit and epoxy each and every strip. And there is a ton of fairing. I doubt it would be as light as plywood. But the pretty factor is high.
Hard Chine Plywood Panels
Long plywood panels and hard chines over frames is an enticing option, like the way the Kendrick Scarab trimarans are built, and the way Paul’s proa is designed. http://www.teamscarab.com.au/ Sure, the design would lack the nice reverse curves, and may be a little boxier. But for a motorboat, this may not be too bad. And it seems like a fast way to build, with not as much fairing?
That is where I am leaning. Hard chines, and long panels. What I really want is the pretty curves though.
How would you build it, given my requirements and skills, and the basic design idea, above and below the water?
Thanks for your thoughts.
Galen,
I can recommend Dave Gerr’s book “The Elements of Boat Strength*
You can calculate your own scantlings fairly easily for different methods of construction and estimate and compare approximate weights and costs.
Basically, the bottom line is that the lightest timber construction for equivalent strength is the Lord method,which is plywood panel, glassed each side (as is Madness) then cold moulding (as is Jzerro) and very little further behind is strip planking. Ply panel will obviously be cheaper as well, with not much in it between the other two again in terms of timber cost..
It seems to be general consensus that for complicated curved shapes strip plank is easiest and fastest.
Rob
I have been learning how to weld, and I have come to the conclusion that welding aluminum seems to be quite doable without expensive equipment (except a TIG-welder and some argon-gas) or much previous experience. I think aluminum has a lot of advantages over plywood or cederstrip, especially in speed of construction and impact-resistance. Aluminum is easy to cut and shape with normal wood-working tools (carbide tipped), light-weight and strong.
Fast sailboat built out of aluminum:
Beowulf sailboat - 27 knots peak - Youtube
Cheers,
Johannes
http://setsail.com/fpb-78-plating-thickness-factors-of-safety-and-emotional-comfort/
http://proafile.com/forums/viewthread/87/
http://www.tadroberts.ca/pics/penduickIV.jpg
This old aluminum trimaran is laying in Gothenburg… It is really cool, even though it looks a little sad at the moment.
Cheers,
Johannes
Galen,
The book I recommended also covers aluminium, steel, solid fibreglass, fibreglass sandwich and traditional timber construction as well…....
You can determine fairly easily the scantlings for any construction methods that might interest you and compare the results.
Rob
Thanks for the book suggestion. I will check it out.
I do like the idea of strip planking with cedar (or maybe Kiri). Because I could design exactly the shape I would like. But I have heard some reports that with larger boats (not canoes) over time the strips will print through the glass, showing lines, and that even if it is completely sealed in epoxy inside and out that sometimes strip planking can split because all the grain runs in the same direction. Does anyone have experience with this size boat and strip planking?
Thanks Johannes for the Aluminum suggestion. But I have also been teaching myself to weld, and have learned I really don’t enjoy metal working. So I can’t see building a boat out of metal.
One of the reasons I was initially considering foam core is because of the very dry heat in Arizona. I won’t be building the boat quickly, and it will be sitting outside (although in the shade), so I am concerned about the wood drying out too much and warping as I build. Someone in Phoenix said that it is a nightmare building boats with wood in this dry environment. It will go from nearly zero humidity most of the year, and then when the summer monsoons hit, it can suddenly rocket up to 90 percent. Foam seems like it would be more stable in that situation. Anyone with experience or thoughts about that?
Thanks again.
You can glass the inside of strip planking, which by Gerr’s scantling rules allows you to reduce strip planking thickness.
The Gougeon Brothers book recommends pre-sealing wood with epoxy wherever possible, so if you pre-seal all your strip planking all in one go, before you assemble it, you should be able to stabilise the moisture content sufficiently.
Thanks Rob.
What does everyone here think about foam core?
Yes, I do want to open that can of beans.
Foam core is stiff, strong and lightly, but needs a Hard Chine design for easy building. I recommend vacuum glassed flat panels. The 39’ proa P12 was built in that way (http://www.multihull.de/Proa12/). Hulls with 15 mm foam core, bulkheads 20 mm.
Interesting. Here is a link to some build picture of the P12 proa. http://www.multihull.de/Proa12/seiten/bilder.html
I am liking the idea of strip planking for the freedom of shapes.
But foam once again is a consideration for a hard chine, panel type of build, because of it’s moisture stability while building. Someone told me they had some beautiful marine ply shipped to Arizona and it all warped horribly when it dried out. I am going to have to try and find some boat builders in Arizona to learn more about this.
With foam I am sure I can figure out the skin thicknesses and vacuum bag it all on a table. In some ways this boat is much simpler to design than a sailing Proa since I don’t have the rig forces to figure out.
Another reason foam may be a good ideas is because the boat will be sitting out of the water in Washington most of the year, unattended, and only used during the summer for maybe a month. So the boat will get some neglect. I have heard that foam is more forgiving of less upkeep.
This is the way I figure things out, going around and around, until it feels right.
Cedar Strip Plank
To get those round shapes I could also strip it in cedar. That would yield a beautiful strong boat, with no need for stringers or as many frames, but it seems like such a difficult way to build, having to fit and epoxy each and every strip. And there is a ton of fairing. I doubt it would be as light as plywood. But the pretty factor is high.
In my direct experience cedar stripping you do NOT epoxy each strip - the time would be astronomical and the curing time delays impractical. I used a waterproof glue (Titebond III) to glue the strips to one another in the bead/cove. I wiped the excess off before the Titebond III cured, and was able to proceed with strips continuously. Each strip was “fixed” as necessary with a removable staple, elastic or plastic staple as required to hold it secure until the Titebond cured. Once the Titebond cured, the temporary fasteners were removed (or cut at surface height for plastic staples). The outer surface was then faired - first with a small block plane, then with longboard and 50 grit. Once basic fairness was achieved, I coated the outside of the hull with a penetrating/thin coat of epoxy, filling in gaps with mixed epoxy/microballoons/colloidal silica mix, being careful to not leave excess.
Stripping actually goes pretty fast, as the cure times are pretty quick. Cedar stripping in my experience produces a lighter hull than plywood - as the compound curves present less surface area than flat panels and chines. Good WRC (western red cedar) is light as a feather and really strong.
I bought good cedar fencing and ripped my own strips and then milled my own bead and cove with a router table. This was just an afternoon’s prep work to end up with good strips. You get to scarf your own long strips and eliminate knots in the process.
My choice would be a combination of methods - using ply panels where possible, and cedar stripping where the plywood can’t do what you want. My last boat built this way had plywood everywhere above the waterline and cedar everywhere below.
Your mileage may vary, but I would not dismiss cedar because you misunderstood the build process. I’m sure people with abundant time may use epoxy between every cedar strip, but I sure don’t have enough spare time for this. You’d be really well advised to read Canoecraft by Ted Moores and Nick Schade’s The Strip Built Kayak to get a handle on the process and techniques. Everything used in a Canoe build applies directly to other boat builds.
You could not go wrong to attend a canoe build class locally. I’ve also recommended people build a plywood Puddle Duck Racer (PDR) as their first build - all the things learned apply, and you get a dirt cheap tender out of the deal. Your time learning techniques on the PDR will actually make your “real” boat build much more efficient so you will actually save time and money in the long run (and get the free tender).
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Bill in Ottawa
You could not go wrong to attend a canoe build class locally. I’ve also recommended people build a plywood Puddle Duck Racer (PDR) as their first build - all the things learned apply, and you get a dirt cheap tender out of the deal. Your time learning techniques on the PDR will actually make your “real” boat build much more efficient so you will actually save time and money in the long run (and get the free tender).
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Bill in Ottawa
Thanks Bill. Real world advice. I will checkout the books you recommended. I wondered about epoxy between each strip. It seemed a little crazy. I am perfectly happy with using Titebond III in the core, and I can see that you could strip a hull quickly that way. Thats a great tip on the cedar fencing idea. Well worth considering, and a good way to get rid of knots.
I also like your idea of cedar in the corners, plywood in the areas that it can do. How do you deal with the difference in thickness between the ply and cedar? Seems like the ply would have to be thinner to save weight since it is likely stronger and heavier than the cedar. What type and size of boat did you build like this?
I actually have built a 24’ trimaran main hull in plywood and glass, so I do have some experience with that build method. But I do plan on first building a nesting dingy that I will use with this boat, just so I have something to row and sail in the lakes in the area while I build this boat.
Darn it. In Port Townsend I could have got any of these books you have all recommended at the library. But in the middle of the desert in Tucson, even with a massive library system, no such luck. I guess I should not be surprised. Not a lot of water around here. Not that I mind buying them.
Darn it. In Port Townsend I could have got any of these books you have all recommended at the library. But in the middle of the desert in Tucson, even with a massive library system, no such luck. I guess I should not be surprised. Not a lot of water around here. Not that I mind buying them.
Either book is great - they cover the same ground. I’m sure the Ted Moores book is available - Woodenboat sells it and I think Amazon does. You don’t need both.
Here’s the boat I built attached. I matched size of the ply to the cedar. My boat weighs about 140 pounds all up and ready to sail. I’m dealing with early onset arthritis which is why I’m building an adult boat now.
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Bill in Ottawa
Here’s the boat I built attached. I matched size of the ply to the cedar. My boat weighs about 140 pounds all up and ready to sail. I’m dealing with early onset arthritis which is why I’m building an adult boat now.
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Bill in Ottawa
Nice boat and picture.
Just found a very interesting new building method I have not seen before. The Wavelength 780 trimaran. It uses thin pieces of plywood that conform to round shapes, with cedar stringers inside, glassed inside and out. Very interesting.
http://smalltrimarans.com/blog/?p=3211
http://wavelengthmultihulls.com/building-the-wavelength-780/