Roa 6.4, An Exercise in Nothingness
Part One: How Big?
I have, now that I’ve gotten something of a feel for some marine software, been able to quickly estimate displacements in ways that have taken my boat designs past artistic mockups and into the realms of the possible, but in this case, maybe, not…
Before I begin with my first notes on this most recent design, I need to say that I’ve gone metric. It’s an elegant, efficient system. Since I’m not making plans for others to use, I don’t need to concern myself with the sorts of rulers another builder might have in his or her shop. For the following, the metric measurements are exact while the Imperial units are approximate.
Second, I decided to break the design notes into chunks. This post is basically an argument in favor of a 6.4m/21ft boat that displaces no more than 275kg/600lbs all up. It’s a more-than one-man boat, but not much.
This boat is a later step on the journey, and the journey involves: 1, a model (or models); 2, an entirely different boat to learn some new construction techniques; 3, this boat; 4, a genuine voyager.
Michael suggested a one-man test platform, but I can’t get enthused about that because I just can’t see myself building a boat that I can’t take other people out in. Even though it’s experimental, it’s still a boat, and a boat just has to be able to do something more than be my personal, solo toy. So I decided to figure out just what displacement would handle a crew of two with minimal gear, or one person with extra gear, food, and water for some careful cruising. However big that boat is, it’s got to be able to handle a 180kg/400lbs load. For this design, I’m working right at that bleeding edge.
But note well, I do not want, in Sven’s words, “an under built freakshow.” (I think that’s a correct quote.)
Still, I believe I really can build an a really, really light boat that I can trust. I have been in boats that have taken green water across them. I’ve been rolled in surf. I’ve scraped coral more times than I ever wished to. I’ve slammed boats right into docks and ground them hard up on beaches too many times. A trustworthy boat has to be able to take some brutality, for the sea is brutal, and I have more than a little fear of that.
So all the while I’m pushing the weight edge, I’m also imagining the worst case scenarios. Yet by concentrating the tough parts where the boat has to be tough, I think I can plane away in many other locations.
Also, It seems to me that it is displacement, and not length, beam, or depth that really determines the cost of a boat, for this reflects the amount of materials involved in the construction. This is my “Pile of sticks theorem.” If I can build a 21 foot boat for the same cost as a 16 footer, my go smaller? Why a short, stubby boat when I could have a long, sleek boat for just about the same cost?
I do realize that it does not quite work out as simply as I’ve stated. There is the “scale effect.” E.g., a 2m beam will tend to weigh 1/4 that of a 4m beam.
So I know there are limits to this way of thinking. As one scales up in any direction, loads will tend to increase in squares rather than linearly.
Even so, I’ve seen some pretty robust craft in the 6-7m/20-23ft size with main hulls that weigh 22-34kg/50-75lbs. There are good, strong masts for that size that are 4.5kg/10lbs. So with great care, it IS possible to build very light indeed. You may think I’m crazy, but I’m convinced I could build this boat to weigh 90kg/200lbs with all the rigging.
Maybe I am crazy, but I’m also convinced that skin on frame construction is an under-exploited technique. I really want to see how far it can be scaled up. I’m actually buying a kit and plans for a skin on frame Whitehall style rowboat so I can get a feel for what looks like a really brilliant method. Yeah, it’s not a proa, but I do love to row. Plus, no problem re-selling a neat boat like that. I need a boat to use for taking my model out anyway, and wouldn’t you know, it weighs 28lbs and can handle 350lbs. Not bad! I can play with my model and take the kids out too. Fun! This boat has a plywood bottom, by the way. Skin on frame can handle tremendous loads, but point loads (i.e. sharp objects) are its nemesis.
What do you think? Am I just being unreal?
Cheers,
Rick
Shoot. I forgot to post a pic. Here ya go.
Also, “Roa” is a name I got from a great book by Konrad Lorenz, the animal behaviorist. The book is called “King Solomon’s Ring.” In it, he describes a jackdaw who was, to his knowledge, the only animal who both knew his name and could say it, knowing it was his name: “Roah.”
Rick, I was perusing the Geodesic site last week myself! I love the combination of aeronautical construction with boats - it seems like a perfect combo for light weight small craft - including proas. I agree with you that in the size you are contemplating a ply bottom would be best for beaching - since the boat isn’t in the “pick it up and carry” category.
I’ll stick with my opinion that a 16’ version would be a good step, as well as making a wonderful little boat - the “Rob Roy” of proas, and you could still daysail with two. An outrigger canoe built like this would be really beautiful, with the play of ribs, plywood, struts and fabric - somewhat like an early 20th century airplane.
BTW, Is there any way to turn down the metallic surface reflectiveness in your models? It’s hard to see what’s actually going on.
Hey Michael,
That’s my thinking too: aircraft technology.
The thing about a 16 footer is that I can’t seem to come up with one that has lines I like. But it’s something I keep coming back to. I think it’s a good idea. I haven’t dropped it, but it’s real challenge for me. Also, building a boat fore a few hundred dollars has a very strong appeal. There must be a way to satisfy all my requirements, but I haven’t figured it out.
And funny, but I started using that mirror finish because it was easier to see what was going on! I’ve been experimenting with Blender for the rendering. It’s not Freeship or Delftship’s thing, really. But Blender, it seems, has a pretty steep learning curve. Yikes!
Best,
Rick
In the boat building industry, designers often use the mirror surfaces and high-contrast reflections to make any “unfairness” in a hull model stand out as it is moved… but it doesn’t often make for a useful still-shot rendering because our brain doesn’t have any frame of reference to build upon: it just tries to see the zebra stripes or the sunset as the case may be. It’s a great tool for the designers to use on their computer screen, but it’s a headache for anyone else who is trying to make something out of it in their garage.
Aside from that, I like the stick-and-membrane idea for a smaller concept craft; we use it for spars and sails all the time, so why not for a hull and deck as well, right?
I’m digging the V-ed decks. It makes a nice detail where the akas cut through them.
I also like the skin and frame direction. Have folks done skin on frame designs where the skin was wetted out glass instead of that membrane that’s commonly used on these boats. Its usually polyester, right?
The aesthetic possibilities, from antique aviation to eskimo kayak, are certainly interesting to think about.
Best,
Chris
Thanks for that info on the mirror finish, MTP. I just thought it looked better than the standard shading.
And Chris, I wouldn’t even say there is a standard skin, but heat shrink Dacron is probably the closest thing to a standard. This is often followed by an epoxy finish—that combination has a strength weight ratio that is much superior to glass and epoxy, and I have never heard of anyone using glass that way, but there are a LOT of people doing a LOT of different things. Skin on frame is definitely in a development phase. I’ve been interested in doing a double skin of Dacron followed by Polyester using epoxy, with carbon powder added to the epoxy below the waterline, and even there, the skin would be going over ply.
Understand, the only skin on frame boat I’ve actually built was to re-skin an old kayak my grandfather gave me, and I used an old canvas tent and varnish, so I’ve learned most of the new stuff I know from the internet. I would say that there is one particularly influential book, and that is Building Skin-On-Frame Boats by Robert Morris. He’s the one that popularized the Dacron, if I’ve gleaned the source of gold upstream, as it were. I don’t have the book. It’s out of print, and the prices on Amazon are insane.
Glad you like the vee decks. That came out of a kayak kinda hull, which are semi-submersible and need to shed water fast. They make for precarious footing, however. If all goes well, this boat will only run a foresail in the calmest conditions, and I have left a walking strip. I figure if the sailors on clipper ships could stand on a rope, eighty feet above deck, in a storm, to take in a reef, I can bloody well walk on a four inch strip of teak veneer eighteen inches above the water.
Cheers,
Rick
I was planning on following the foregoing with an expansion on some of the design concepts, but the more I thought about it, the more sensible a smaller boat seemed, so the the answer to my first question, “How big?” became “smaller than this.”
So I spent the past couple weeks of evenings working out a five meter design. That’s a touch more than 16 feet, but it’s the metric equivalent of a “nice” number.
I should be posting the latest incarnation of the sail test boat in a day or so.
What do you think? Am I just being unreal?
Cheers,
Rick
Nope. Check out two other sources for SOF ideas as well. One is Dave Gentry’s Ruth and the other is the SOF outrigger canoe built by the Cape Falcon guy at http://www.capefalconkayak.com/outriggersailingcanoes.html
http://www.youtube.com/watch?v=VgzhImuHWfc for some more inspiration
Dan
Rick, I was perusing the Geodesic site last week myself! I love the combination of aeronautical construction with boats - it seems like a perfect combo for light weight small craft - including proas.
I have built in this method—not hulls, but rather wingsails. Same method and materials, just a different shape. Was shocked by the light weight and small costs. I’m happy to answer any questions.
Dave