Frames: Sawn or Bent?
Locked
 
|
Sawn-frames and bent-frames both achieve the same structural purpose, which is to increase hull rigidity. But their “aesthetics” are different. Sawn-frames are “humble” and offer simplicity and sturdiness. Bent-frames are “elegant” and offer slender, light-weight strength. By convention, bent-frames rank higher on the aesthetics scale than sawn-frames. But the craftsmanship needed to build either well can be as demanding, and the strength offered by either can be equivalent. So a choice between the two --and the various sub-styles within each type-- comes down to “aesthetics”, as does nearly aspect of boat-building. What “look” are you trying to achieve consistent with the boat's intended use?
My current build is an 8', double-knuckled, keel-planked pram, which doesn't say anything about whether it is “high-end” or “low-end”. But if 'work-boat' use is intended --e.g., if the boat is to be a casting platform for fly-fishing lakes--, then striving for 'elegance' is a bit silly, because the fish don't care. OTOH, if the boat's chief function is to flaunt craftsmanship, as seems to be the only purpose of the boats displayed at shows, then a different build would be required. My preference is 'work-boats'. I like their honesty, simplicity, directness. So my choice for this boat will be sawn-frames, because I want their “visual sturdiness”, which brings me to the real point of this post: How should those frames actually be built? -One piece from solid stock, even though there's a double angle? -Laminated from multiple, full-sized, single pieces of plywood? -Multi-piece from solid stock and edged-joined in a single plane by either gusseting or splining? -Multi-piece from solid stock and overlapped? Lots of choices. Lots of looks. But, frankly, I don't want the “fussiness” of gussets, nor the work of splining or laminating, nor the "bulk" of overlaps. That leaves me no choice but --once again-- to cut multi-angled, one-piece frames, which, actually, do a pretty good job of offering strength if grain is parallel to the mid-strake.  |
|
Good topic! In traditional boatbuilding, sawn frames were made from curved members of trees. Often large branches harvested specifically for the purpose. That way the grain would 'turn the corner' of the frame. These days we can't (or won't) spend the time to select the branches, harvest them from the forest, re-saw into our frame blanks and dry them. That process will take eighteen months or so. A long time!
Today, we visit the lumberyard and choose the planks for species, thickness and grain. Often grain is that last choice. If we make it the first choice, sometimes we can find that plank where one end will 'go around the corner' to make a beautiful sawn frame. Then maybe there will be enough of those planks to make all of our frames. Hopefully we can use the rest of the planks for something else on the boat - or the next one. But the idea is: the grain turns the corner that we need it to turn. IF we can get that, then our frame can be lighter, stronger and a more elegant part. If the grain doesn't turn the corner, then the only way to structure our sawn frame is to oversize it. It needs lots of mass as the grain direction compromises strength. Your boat just got heavier. BUT: you can make this piece lighter and stronger by wrapping a lighter sawn frame in fiberglass and epoxy. This uses the fiberglass to maximum advantage. The composite structure becomes a very strong, very lightweight structure. This is an excellent way to frame with Western Red Cedar for small boats. The cedar is the core, the Glass is the structure. I know you don't like this approach from a fabrication approach Charlie, but this will make a very strong frame. And I respect your hesitation - we all have our own. Plywood frames are strong. Aesthetics aside, plywood frames are very strong. Especially if reinforced with fiberglass cloth and epoxy as described above. Splined frames are strong as well. And lapped frames work well with modern epoxy adhesives. I'm partial to bridle joints, actually. Lots of glue area and a nice joint mechanically. All of these are a bit fussier than sawn. But stronger because the grain runs parallel to the hull panel. Especially if a fiberglass & epoxy reinforcement is added. I hate gussets, too. A third option is laminated frames. They are very light, very strong and repeatable. For frames that you need a lot of and require higher strength it's a good option. A good thing to add here is that these frame techniques are all applicable to breasthooks and knees. These are required to build most boats. In every boat, there are stresses that go around corners. That's always a difficult thing to structure. Usually the most challenging structural elements in the boat. That's what we're talking about here. There are lots of ways to do this. Which one do we choose? That's up to the boatbuilder and what they have available to them. There are a lot of correct answers. How do you want to apply it to your boat? |
|
|
This post was updated on .
Mark,
You're right. Wrapping frames with epoxy-soaked 'glass is an approach to gaining strength I hadn't considered, because that isn't how I want to build boats. I have no fear of epoxy. But I limit its use to making joints. Or as Tom Hill has quipped, “I don't glue my boats with paint, and I don't paint my boats with glue.” As for fiberglass, I hate working with it and would willingly use it only to build masts, where high, strength-to weight ratios do need to be achieved. But frames? If more strength is needed from a frame, I'd increase its scantlings, or resort to gusseting or overlapping, and/or select the wood species more carefully, with laminating or steam-bending being a last resort, because my preference is to build 'easy', which means eliminating from the build every bit of complication I can. I've never had to replace a frame that broke, though I've had corner knees fail, because they had been improperly built, which is why I've eliminated them in favor of other methods of tying sides/gunnels to transoms. It occurs to me, also, the reason I prefer 'simplicity', even though simple methods, such as using sawn frames, might not offer the highest "aesthetic appeal", is that I'm always building 'one-off'. My boats might all look the same to an uncareful eye, because themes are repeated. But I've never built two boats the same even when I was working from the same Table of Offsets, as I did for a pair of prams I donated to a summer camp. Therefore, I have to keep templates and jigs to a minimum, because they'll never be used again, which is the opposite of a production shop where no piece of wood cannot be shaped from a pre-made pattern, or else labor costs trash potential profits. OTOH, my time isn't "free". Every hour spent doing one job is an hour that can't be spent elsewhere. Therefore, I'm always trying to build as fast as is consistent with an acceptable result and always trying to select from the construction methods available to me which I prefer doing. Wrapping boat parts in epoxy-soaked fiberglass has zero appeal for me, besides being outrageously expensive. Also, I think 'frame strength' is overrated, especially if the hull stresses can be predicted to be 'moderate' such as is the case with a single-user, recreational rowboat. For a sailboat, OTOH, I'd build "beefier". In fact, after I finish my current build, I'll be re-building a vee-bottomed double-knuckled hull originally intended as a rowboat to be a sailboat. The only way that 38-pound hull will withstand Force Five conditions is with a lot of bulkheading. Charlie |
|
|
Just a couple of comments:
You made this point, I thought I'd clarify: "Frame strength" is more and more important as the size of the structure is increased. Doesn't matter what you build, the larger it is, the more critical the structural system strength. Most frames in small boats ares stronger than they need to be just so they 'look right'. The stresses of a 30' hull are many times greater than for a similarly shaped 18' hull. So as hull size and displacement goes up, the frames, stem, knees & other structural reinforcements are more and more critical. "Epoxy" I will qualify this as my opinion for the way I work. Your mileage may vary. At first I hated epoxy. Now I look at it as this marvelous material that will stick together many things, given the proper fillers; but is also a terrific coating material. Whenever I need to seal a wood surface from moisture, it's my go-to. As a matter of fact, I often use it as my first coats of "varnish" as it builds a film much more quickly than varnish. Since we like to use Okume ply and many other not very durable materials in small boats (often in the search for light weight), a good seal from the water is crucial to longevity. Epoxy is your friend. It has its downsides, to be sure. But id is a material that you can learn to use and will do a lot of different things for you. Good topic. Good discussion. Mark |
|
|
"Most frames in small boats ares stronger than they need to be just so they 'look right'."
Mark, Bingo! I don't build boats larger than I can pick up. Were I to build hulls that had to sustain a lot of stress, I'd worry more about strength than I do. But I'd never, ever use the 'glue' that epoxy is as a 'paint'. The lab tests I've seen report that epoxy isn't a water-proof barrier, but merely a surface film that slows down moisture penetration. Slathering boat parts with epoxy is expensive, messy, adds weight, and is mostly a poorly-engineered solution to a problem that hasn't been properly dealt with. (IMHO, 'natch.) Charlie |
|
|
Hey Charlie,
Recently I have been exploring all kinds of ways to work with epoxy as a sealer/coating material. This came about after this years' PTWBF. I had always assumed that I was doing the right thing by using 'penetrating' epoxy on end grains, screw holes, etc. While in Port Townsend I talked to the technical people at West System, System Three and MAS. To a person, they discouraged the use of 'penetrating epoxy' for this use. Seems that penetrating epoxies are just standard epoxy with alcohol or acetone or some other thinner added. This reduces the density of the epoxy and its effectiveness at preventing moisture. Live and learn! I know you don't like to "paint with glue" but I think of the material differently. This might come in handy sometime: Instead of penetrating epoxy, every tech person recommended the thinnest laminating epoxy available, something like Silvertip or equivalent, then let it soak up for as long as the piece will accept more epoxy. You have to be patient. When the epoxy starts going off - game over. Recently I've discovered a wrinkle on this approach: use a heat gun to warm the wood to be primed with epoxy and your epoxy. It thins the epoxy and the wood just sucks it up. Continue with the heat gun until the wood soaks up completely. I've tried this approach on some scraps of mahogany, then cut them apart. The epoxy penetrates along the end grain much more than with a 'cold' coating. The end grain is most effected, of course. I'm continuing to play with this technique and will let you know if I get any other revelations. I anticipate using this on the edges of my planks for the Shellback build, as well as any screw holes or penetrated members that don't get filled. When used in this way, the epoxy does penetrate the wood and it appears what's happening is you end up with this micro composite structure as a part of the structure of the plank. Of course, all epoxy must be protected from UV with paint or varnish to avoid failure. Something to know about. Cheers! Mark |
|
|
This post was updated on .
Mark,
Your "heat gun" trick is a thoughtful solution to creating protection/increasing hardness. I'd adapt it in a heartbeat if I needed to. Thanks for persisting with this. But truthfully, I'm beginning to find an appeal in the "traditional ways" of doing things, which means moving away from using epoxy at all and back toward things like nails and rivets. As 'a glue', there's nothing better than epoxy in terms of strength and forgiveness, and when combined with fiberglass, the pair facilitates mast building. But I refuse to 'glass a hull, and especially the interior of one as is done in strip-building. That might mean I build less "durable" boats. But I can build faster, cheaper, and lighter than the epoxy slatherers, which also means --on the same budget of time/materials-- I can build more boats, which means I can explore more designs over a lifetime. Trade-offs. It's all a matter of trade-offs and preferences. I taped seams on a couple of boats and hated it. Once I figured out how to do S&G without them (and without its wires), I never looked back. Part of the "design, aesthetics, and ethics problem" the slather advocates are avoiding is the question of "plasticizing wood". At what point does the use of epoxy "cross the line" to become no longer 'wooden' boat-building, but something else? For sure, adhering to "old ways" for the sake of nostalgia, sentimentality, or whatever can be justified purely on those grounds alone. I've got no problem with that. What I do question is abandoning literally millenniums of boat-building craft and wisdom to chase "the new". If the Vikings could put together sea-worthy craft with not much more than an ax and access to straight-grained trees, why can me with a shop full of tools put together a (nearly) all wood boat? Again, to return to a role model who inspired me, Bill Grunwald's shop in Davenport was about as close to boat shops of a century ago as it is possible to be without going full Monty and doing a "Roy Underhill", not that that doesn't have an appeal as well. But I love the ease with which gunnels can be knocked down in mere minutes with a power plane too much to do that work by hand, as also I love the ease with which I can do most of the tasks of boat-building easier and faster with power tools than hand ones. But I can't argue the result is better. In fact, probably the opposite is true. 'Hand-worked' produces a human-scale integrity and coherence that 'power-worked' cannot. But plasticizing wood is where I draw the line. If I want "plastic", I'll build with plastic, which enables hull shapes nearly impossible --or prohibitively expensive-- in wood. Charlie |
|
|
Charlie,
Good points all. I've come to think of S & G a little bit differently than 'traditional' boatbuilding. S & G is really composite boat building. You construct a wooden 'core' then use it to construct a wood/fiberglass/epoxy composite structure. The core is as light as you can make it, which is the reason for using Okume as plywood. Okume is lightweight, strong enough and easily worked. Solid wood parts are often used to reinforce this structure in a 'traditional' way as knees, breasthooks, gunnels, etc. When properly cared for, this boat will have similar maintenance characteristics to a fiberglass hull, but will be stronger and lighter in most cases. It's true that the Vikings could construct a seaworthy hull with minimal tools, but those hulls took only the form that the planks allowed - nothing more. Luckily, that form results in a hull that is easily driven by sail or oar and is relatively seaworthy. Variations on that hull form are seen all over the world. If you want to build a Faering, this is the way to do it. Most of us 21st century types want to push beyond this simple form as both of us have done. We all find a balance between 'traditional' building techniques and current construction materials and techniques. Everyone finds their own balance point. Each of us experiments with different techniques at different times for different boats. We all learn something with each build and we all find a balance point that feels right to us at the time. My belief is that the balance point continues to change as we do. We all continue to learn and figure out what gives us the most enjoyment. When I have a more complete exploration of the 'heat gun technique' above, I'll start a separate thread. I cut into the first piece of scrap Okume treated this way yesterday and it seems to be an improvement over a straight 'cold' epoxy treatment. The Okume was much more effected by this technique than solid mahogany. More to do! Cheers! Mark |
|
|
This post was updated on .
Mark,
I'd bet that occume is more porous that mahogany, not that either label describes a single species, but rather families of species, all of which get marketed under single labels. Think about the diff between 'white oak' and 'red oak'. One wicks water. One doesn't. In fact, to tell the diff between two samples, a common test is to cut a stick of each and stand them in a glass of alcohol. Red oak is a straw. I'd disagree that conventional S&G is 'composite construction' in the way that 'cold-molding' (in its several variants) can definitely be argued is composite construction. S&G is just a sh*tty way to build a boat whose only merit is it enables the less skilled to create better floating boxes than they would otherwise. One could go all elistist and say S&G isn't "real" boat-building. But I'd definitely say it is boat-building with the training wheels left on, perhaps a place to begin, but nothing one can be proud of when compared with the building that once was done in this country. I've done eight hulls so far S&G, with more intended. So it's not if as I'm not engaging S&G in daily practice and turning out decent work. But it can't be called skilled boat-building, and it certainly isn't 'composite'. It's just fake 'plank-on-frame' in which epoxy fillets are substituted for frames and chine logs, so cutting rolling bevels and fitting complex joints can be avoided. On a different note, I spent most of yet another evening trying to find a substitute for epoxy that would be 'traditional', 'waterproof', and 'forgiving of gaps and light clamping pressures'. But there just doesn't seem to be such a thing. So I'm stuck with using the very modern glue that epoxy is for any boat built other than carvel and then caulked and meant to be hauled out between use, which isn't a bad thing for epoxy being cheap, safe, and fail-proof. Getting rid of building with plywood, a very non-traditional material, and moving back to using only solid lumber would be easy. But not the glue. Probably the compromise I'm going to have to accept is tightening up my below-the-waterline fits, so that fillets are no longer needed and making greater use of traditional mechanical fasteners above the waterline. Not wholly 'traditional', but an acceptable, practical compromise that lets me keep building lighter and cheaper than nearly anyone else except the SOF crowd (whose work is actually very traditional and antedates POF). Charlie |
|
|
Hi Charlie,
I think you are right about the Okume being more porous. My initial experiments would tend to bear that out. That's the primary reason I don't like to use Okume without epoxy as a sealer. Nick the paint or varnish and it sucks up water! Not the goal of a boat. By the way, I plan to get some hydrotek scraps to compare with the Okume I have been using as well. It's heavier but might do better as a boatbuilding material for long term use. My reference to S&G being composite construction is from the generic term of a 'composite' being any structural system involving multiple dissimilar materials. We tend to think of 'composites' as exotic structures these days, but in fact common materials can form the structure while still being referred to as composite. In building construction, steel reinforced concrete meets the definition of a composite structure. In a S&G boat, fiberglass cloth and epoxy is used to sheath a plywood core. Exotic? No. Composite? Yes. The structure is the glass & epoxy skins and the plywood basically holds the space between the skins, which makes the structure stronger and lighter than an equivalent thickness solid fiberglass hull. I'm not going to make a value judgement on S&G. It works. Some of it is extremely crude, as with all construction types. Some of it is very sophisticated. Anyway, that's my take on it. And of course, your mileage may vary. Clinker plywood is a very similar technique and uses most of the same basic materials. It can produce a very elegant result and practiced skills are required to do it well. I like that the plank overlaps give you a reinforcement at that point and not as much structure needs to be in the boat. I don't like that fasteners are used to secure the plank ends at transoms and stems. Have you experimented with using chine logs, fasteners (screws or ring shank nails) and bedding compound to make those joints? It would probably make the boat heavier, as it would need more structure to support it. (Not exactly your end goal!) But the epoxy could be eliminated. A good bedding compound like Dolphinite will last for 20-30 years without losing its seal and can be renewed. Just a thought. Cheers! Mark |
|
|
This post was updated on .
Mark,
If you need some scraps of Hydrotek (which is Merenti), I've got plenty. Swing by my house sometime when you're on your way to Rivers West. (I'll send contact info by email.) As I've said before, I've weighed sheets of it, and they were 21-22 lb, which is exactly what my sheets of Okume weighed. But my sense of Merenti from working both with planes is that it's “denser” than Okume. Your complaint about Okume sucking up water when a surface coat of paint or varnish is nicked helps me understand why you're obsessed with coating it with epoxy. I'd never choose to build with Okume. It's expensive and all but ugly for being so bland and blond. So I haven't had to manage a wicking problem. Your definition of 'composite' as 'construction involving dissimilar materials' is far too general. By that definition, nearly anything other than dugout canoes is 'composite construction', which really is the case. What I'd called 'composite' is boat construction in which glue is used more extensively than to make joints and becomes a major structural element. What 'composite construction' in that sense does allow is more complex hull forms than 'traditional' methods, as well as stronger hulls. I think those experiments need to be run. E.g., America's Cup boats. But I'm not interested in exploring that path toward boats, because I'm not encountering the problems that composite construction could solve for me. My self-set design problem is how to build 'easy', 'low-cost, and 'light-weight' for single users with oar, paddle, or sail, but never motors. That's my interest. That's my niche, and I'm wrapping up my fourth boat for this year. You're right that the quality of work done under the label 'S&G' varies widely and doesn't deserve a blanket condemnation. Also, pitting the method against traditional ones creates more conflicts than it solves if the goal of either is to put people on the water, which both can do. As a specific example, my current build is a flat-bottomed, double-knuckled pram, which could have been built 'glued lapstrake' (instead of 'filleted lapstrake', as I'm doing) or even 'strip-planked', but with far more work. There's a half dozen methods that can produce the exact same hull in terms of performance on the water and even a very similar look. But them that build boats know which took more 'skill'. So, is the purpose of building to flaunt 'skill'? Or to put a good-enough boat on the water? I despise what is displayed at boat shows. That stuff is so !$*# elitist I refuse pay homage. Right now, in this country, “authentic” Adirondack guide boats are still being built. But at $14k-$16k per hull --most of which is merely a fair wage rate for labor-- who can afford them but the super rich? But why shouldn't the “average guy" have access to what that hull form can offer in terms of performance on the water? 14'-16' is longer than I'm set up to build (except during the summer), nor am I willing to spend the three months it would take to build one. But if someone wants a car-topper for fly fishing, and doesn't want to pay Spring Creek $1,300 for their ugly, heavy, 8' pram, I can point to ways of building under 50 pounds and under $150 bucks that never fails to attract admiring comment. Right now, I'm having to build such boats S&G. But I'd like to move back toward more traditional methods that even a first-time builder could use. Not sure exactly how. But it's a problem I'm working on. A large probem with most designs is they attempt too much and fail at nearly everything. A boat that rows easily and well isn't a boat that you'd want to be bashing into waves under sail in Force Five conditions, green water coming over the bow. Nor is a boat that would motor well a boat that would sail or row easily and well. Most designs don't lend themselves to being built easily, nor are they properly detailed. Nor are users all one size, and a 90lb, 5' foot, ten-year old needs a different fitting out than a 6'2', 200 pounder. If bike shops can fit bikes to their users, boat shops ought to be able to do the same. But I don't know of anyone who attempts it. There are something like 500 plans currently avaible for skiffs/dinghies under 16', many of them free or low-cost. But how many of them are "good boats"? I don't know. I really, honestly don't know. I do know most of them have problems not worth fixing compared with the ease with which decent plans can be drawn that match a specific user to specific water. But again, I know of no one pursing that path for low-budget users but me, and even I can't find "clients". Of the probably one hundred persons who've seen my boats, taken pictures, asked questions, expressed an interest in building one, not a one has ever followed up. So I build for myself. What does occur to me, as I whine about this, is it's not as if there isn't an easy solution to obtaining excellent, user-specific, water-specific designs at a reasonable effort/cost. The investment world has created "robo-advisers" who can out-perform expensive, human advisers. The same could be done for the boating world, though entrenched interests would push back hard. But I know of no checklist a would-be user can work through that then directs him/her toward a specific design (or creates one on the spot) that that would meet his budget for materials, time, and skills. Part of the problem is that plans for self-built boats don't have the market they once had. This country has grown too used to using debt to want to self-build in order to afford a boat, and recreational boating is also very much in decline compared to the 1860's-1890's. Seriously, creating cafeteria-style software that could design hulls wouldn't be hard to do, and a more ambitious project would be tied to a CNC machine and could cut key parts. "Here's what you asked for", the computer would say. "Some assembly required." Charlie |
|
|
Charlie,
I'll take you up on the offer of the Hydrotek scraps. It would be good to play with them and I will gladly share the results of my crude testing. I'll check my email and send you back my contact info. Based upon published data, Meranti IS more dense. What I want to figure out is how do I build the most durable boat for the weight and money. It's a similar, but not identical pursuit to your own. I'm doing all of this as I want the little Shellback to be as durable as possible. It has a likely future with a grandson, so it will be tested! I may also want to build another larger boat & I want to know this stuff. But that's another story... Cheers! Mark |
|
|
Mark,
If you want 'durable', then proceed as you are doing. Plasticize the crap out the planking, exterior and interior, by soaking it in warmed, runny epoxy. But also worry about managing the UV problem anywhere it gets exposed to sunlight through scuffing or nicking. That means painting the boat, inside and out, and forgetting about bright work unless you go teak or a similar oily wood. From what I've read, paint over epoxy can be tricky. That means more research and testing. Me? I'd build the kid a throw-away boat, good for a couple seasons, and then build 'em a new one. First boats, like first bikes, first cars, (first wives?) get trashed. That's just part of the learning-curve. Where I would spend an inordinate amount of care is building the sailing equipment, the rudder, daggerboard, mast, spar, blocks, etc., all of which could be moved forward to the next boat. But the hull? Get it done and get it in the water as fast as is feasible. Also, if you don't care about weight, look at Sapele plywood. Not cheap, not easy to get, but the stuff is nothing if not 'durable' in terms of surface hardness. Finished bright, it'd make a truly gorgeous boat. But probably, all things considered, you need to build with Merenti. I've bought from Crosscuts and Shurway and never had a problem with sheets from either. Charlie |
|
|
Mark,
In a boat-building context, 'durable' can means at least three things: - ability to resist impact injury. (Think 'cedar' versus 'walnut' or 'white oak'.) - ability to resist damage from bugs, bacteria, moisture, food and fuel spills, etc. - ability to be restored seasonally to OEM conditions easily, simply, cheaply. The opposite of 'durable' is 'fragile'. Unless they are built like barges, boats are inherently fragile. So it's important to identify what you want to guard against, at what cost, and what effort, which is why I'd focus on the third meaning. If the boat is going to be dragged on beaches, then runners, doubled skegs, replaceable keel shoes should be considered. If the boat is going to be run into piers, pilings, and docks, then worry about proper gunnels and rubbing strips. But if it's just algae, salt residue, muddy shoes, etc., you're worried about, then focus on easy maintenance and forget the expense of plasticizing the planking. Also, color is going to matter hugely. Sunlight on a dark hull is going to cook it. That means classic, boring white is the only viable choice unless the boat is meant as a throwaway. Then color choices become unlimited, and some really fun things could be done. Charlie |
|
|
Mark,
I stumbled across some info on penetrating epoxies. The web pages are a jumble. But the info seems trustworthy. http://www.epoxyproducts.com/penetrating4u.html Also, the presence of nonyl phenol is what to avoid if painting with enamel over epoxy. Charlie |
«
Return to General discussion
|
1 view|%1 views
| Free forum by Nabble | Edit this page |