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Strip Canoe Wood Alternatives (and Engineering Properties) an Essay?

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I seem to do more dreaming and designing than actual building. I have three designs that have yet to be built. At least it's cheaper. I might have some time off unemployed in the next couple months, so once again I'm thinking of a build. I'm thinking a short pack style canoe. 10 ft, but wider and higher prismatic coefficient. Basically something that can fit in the bed of my truck, small and light enough to toss in any creek or river around town, and able to drag over any downed trees and exposed rocks. I want it wide and stable enough to pole it because I'll mostly be in shallow waters. The design will come up later, once it's more complete.

This has got me thinking of the most common problem I deal with. How do I build a canoe that can handle frequent groundings on shallow rocky rivers and streams. If only it was affordable to build a giant oven to bake a polyethylene canoe. Since that isn't an option, and I don't really want to build another mold for a kevlar and glass boat, I've been thinking of a strip build. It seems to me that the advantages of a strip canoe, especially for racers, are its ease of build for one offs, relative lightness, and its stiffness. However, the stiffness of the stressed skin design with thin skins does not translate well to impact resistance. Typically constructed with a lightweight and thick core, the stressed skin design is easy punctured and the core has little compression strength once one of the skins is compromised or the impact forces the skin into the much weaker core. When grounding out with my kevlar/glass boats I can watch and feel the hull flex over the load, with little apparent ill effects as it spreads the stress out over a larger and more flexible area than a cored boat is capable of.

So, my hypothesis is, why not use a thinner yet more dense and stronger core material that acts as less of a core and more of a functional part of the laminate (I understand that cedar already does this over a foam core). Western red cedar's weakness is its lack of hardness or its susceptibility to denting upon impact. The wood species most readily available to me (Kansas/Missouri) are white oak and black walnut. White oak is about twice the price of western red cedar, walnut is much more expensive, but since I would be using much less material with a denser wood (1/8" strips vs 1/4" for the same weight) the price difference becomes much less. Kerf waste from the saw becomes the driving factor between the price points.

With a thinner core, more flexibility would be retained to spread out the impact loads similar to a non cored composite (like a Millbrook). Final weight should come out similar or lighter depending on wood species and strip thickness. The concentrated point impact resistance, such as grounding out on a rock or gravel, becomes much greater with more dense wood and should reduce delamination between skin and core.

After all that, my question for everyone is why do we build such stiff canoes when most of us are not racers, and as trippers or recreationalists (mixed water, rivers/shallows, open water secondary specific) overall durability and lightness would seem to be more important? If a non cored composite canoe is the most durable per weight on a river why do we build on such less emphasis of survivability when considering the paddling environment. Hopefully Stripperguy, our resident materials engineer, can enlighten me on the engineering details.

Not, sure if this will work, but I will attempt to add a spreadsheet of the material properties of the woods I'm discussing. Looking at the numbers it seems that these species would be suitable in thinner strips as long as the overall weight was kept the same.
SpeciesMoisture ContentSpecific GravityModulus of rupture (kPa)Modulus of elasticity (Mpa)Work to maximum load (kj m[SUP]-3[/SUP])Impact bending (mm)Compression parallel to grain (kPa)Compression perpendicular to grain (kPa)Shear parallel to grain (kPa)Tension perpendicular to grain (kPa)Side hardness (N)
Oak (Bur)12%0.6471,0007,1006874041,8008,30012,5004,7006,100
Oak (White)12%0.68105,00012,30010294051,3007,40013,8005,5006,000
Walnut (Black)12%0.55101,00011,6007486052,3007,0009,4004,8004,500
Cedar (Western Red)12%0.3251,7007,7004043031,4003,2006,8001,5001,600
 
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I can only address one item on your list, poling. From years of doing this in many different hulls I can honestly say that at 10' it will be a hard sell. In my Courier, 14' or so it doesn't handle well at all with a pole. It is nothing more then spinning. The extra few inches on my Disco makes it quite doable and has been my tripping/poling canoe for over 15 years. My longer boats do great with the pole. Hell, I had a Riverjammer 12' that I tried to pole and it was a miserable affair. Just my 02 on the thought. You may have more luck then I have if you build it.

dougd
 
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Dougd,

Definitely something I'll consider. I have only poled my 15 ft Mattawa and only a couple times. Seems from what you say a good tracking boat is important. That definitely goes against the short boat I had in mind for the little creeks.
 
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I'll be interested to see what you come up with as I've been contemplating something similar. I want to build another composite canoe next winter but would like to do away with the mold and I'd also like a core to hopefully cut down some of the weight and expense from all that cloth. So I was thinking of building a stripper with ~1/8" strips and laminating it with carbon and fiberglass on the outside and Kevlar on the inside.

I hadn't thought of using something as heavy as oak but had thought of going with something like poplar for a little added strength but not much extra weight. One of the things I've been wondering is how much strength I'd gain from a stronger core (hardwood) in such a thin lamination and if the thin lamination would be more prone to causing problems due to flexing. Also how much extra stiffness/strength will the carbon and kevlar add over just fiberglass?

One thing I try to keep in mind with these mental exercises is how well the standard stripper layup of 1/4" strips with a single layer of 6oz. glass on either side holds up. Lots of people have used them pretty hard without failures. No matter the outcome it's always a fun problem to think about.

Alan
 
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For some reason, I always had the impression that one part of the equation regarding strength/rigidity, was the distance between the inside and outside layers of composites. And that is why they where using "foam" core that is quite thick maybe like 1/4 inches..
 
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Using a hardwood for strips will become a sanding nightmare. I have used poplar for accent strips, white pine for the entire hull, as well as white and red cedar. I haven't noticed a difference in strength. Dragging your boat around or having impacts will have minimal effect on a stripper if you build it for that purpose. Brass stem bands will add years to your bow and stern. Put a double layer of six ounce cloth on the football area, and if you are really concerned, a double layer on the inside football as well.

I have only punctured my canoes to the wood twice, and I use them hard. The first time I foolishly ran what turned out to be around a class 4 rapid with extremely strong current. Canoe got broached on a big rock midstream, almost wrapped, and then popped off. Somewhere in the midst of the wipeout, it got a two inch long slice down to the wood on the outside of the hull. Duct tape covered it for the rest of the trip, with no other adverse effects. The other time I was on the Steel, jumped about 15 feet vertically down into my canoe, which was sitting in the water on top of a sharp branch. Made it back to civilization with duct tape. Both of these canoes only had a single layer of six ounce inside and out. I have never punctured a double layer.

So the solution is right in front of you; a standard build with double layer outside for abrasion resistance, and yearly maintenance.
 
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Using a hardwood for strips will become a sanding nightmare. I have used poplar for accent strips, white pine for the entire hull, as well as white and red cedar. I haven't noticed a difference in strength. Dragging your boat around or having impacts will have minimal effect on a stripper if you build it for that purpose. Brass stem bands will add years to your bow and stern. Put a double layer of six ounce cloth on the football area, and if you are really concerned, a double layer on the inside football as well.

I have only punctured my canoes to the wood twice, and I use them hard. The first time I foolishly ran what turned out to be around a class 4 rapid with extremely strong current. Canoe got broached on a big rock midstream, almost wrapped, and then popped off. Somewhere in the midst of the wipeout, it got a two inch long slice down to the wood on the outside of the hull. Duct tape covered it for the rest of the trip, with no other adverse effects. The other time I was on the Steel, jumped about 15 feet vertically down into my canoe, which was sitting in the water on top of a sharp branch. Made it back to civilization with duct tape. Both of these canoes only had a single layer of six ounce inside and out. I have never punctured a double layer.

So the solution is right in front of you; a standard build with double layer outside for abrasion resistance, and yearly maintenance.

I think of you and your strippers often when I'm contemplating all these other building methods and it helps to keep me grounded. But unfortunately I think Muskrat suffers from the same affliction I do in that he just can't bring himself to do something the same way everyone else does. I think he has it a little worse than I do though. ;)

"If I tried this or that could I make it a little lighter and stronger? How much can I get away with?"

Most of the time it's more money and more work for something that might not be any better than "normal" but in the end it's what makes us happy. And sometimes you really do find something that works.

Alan
 
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You've definitely got me pegged Alan. I'm a scientist by nature and trade. By doing things the same way as the standard all the time I'm not advancing our collective knowledge (or learning from failure). Closest thing to religion or purpose in life that I have.
 
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For the sake of my ongoing exercise, I made a sortable spreadsheet relating wood hardness to density. There are separate tabs for hardwoods and softwoods, but apparently I can't attach an excel document. So, sorry I have to do it this way, but I'm just going to paste in the table at the end of my post. Western red cedar does not stack up well here, even when compared to softwoods only. All the data presented so far comes from the US Forest Service "Wood Handbook, Wood as an Engineering Material." It is a great resource and is available free online here:

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

Their definition of hardness: Generally defined as resistance to indentation using a modified Janka hardness test, measured by the load required to embed a 11.28 mm (0.444-in.) ball to one-half its diameter. Values presented are the average of the radial and tangential penetrations.

I also came across this table in the book comparing hardwoods to softwoods relative to density. Interesting numbers, with trade offs for each side, but that hardwood advantage in side hardness has got me thinking there might be some merit.

Hardwood v Softwood.jpg

SpeciesSpecific GravitySide Hardness (N) Hardness/ Density
honeylocust 7,000#DIV/0!
tanoak #DIV/0!
hickory true pignut0.75 9,500 12,667
hickory pecan pecan0.66 8,100 12,273
hickory true mockernut0.72 8,800 12,222
hickory true shellbark0.69 8,100 11,739
hickory true shagbark0.72 8,400 11,667
locust black0.69 7,600 11,014
oak red pin0.63 6,700 10,635
maple sugar0.63 6,400 10,159
birch sweet0.65 6,500 10,000
oak white swamp white0.72 7,200 10,000
ash white0.6 5,900 9,833
oak red cherrybark0.68 6,600 9,706
oak white bur0.64 6,100 9,531
ash green0.56 5,300 9,464
ash oregon0.55 5,200 9,455
oak red willow0.69 6,500 9,420
oak red scarlet0.67 6,200 9,254
maple black0.57 5,200 9,123
beech american0.64 5,800 9,063
oak red northern red0.63 5,700 9,048
birch yellow0.62 5,600 9,032
magnolia southern0.5 4,500 9,000
oak white post0.67 6,000 8,955
oak red black0.61 5,400 8,852
oak white white0.68 6,000 8,824
oak red luarel0.63 5,400 8,571
cedar eastern redcedar0.47 4,000 8,511
oak red water0.63 5,300 8,413
oak white overcup0.63 5,300 8,413
cherry black0.5 4,200 8,400
oak white swamp chestnut0.67 5,500 8,209
walnut black0.55 4,500 8,182
oak red southern red0.59 4,700 7,966
maple bigleaf0.48 3,800 7,917
tupelo water0.5 3,900 7,800
maple red0.54 4,200 7,778
ash black0.49 3,800 7,755
oak white chetnut0.66 5,000 7,576
elm american0.5 3,700 7,400
hackberry0.53 3,900 7,358
sweetgum0.52 3,800 7,308
birch paper0.55 4,000 7,273
tupelo black0.5 3,600 7,200
elm slippery0.53 3,800 7,170
larch western0.52 3,700 7,115
sycamore american0.49 3,400 6,939
pine virginia0.48 3,300 6,875
douglas-fir coast0.48 3,200 6,667
hemlock mountain0.45 3,000 6,667
pine longleaf0.59 3,900 6,610
maple silver0.47 3,100 6,596
pine spruce0.44 2,900 6,591
cedar port-orfird0.43 2,800 6,512
magnolia cucumber tree0.48 3,100 6,458
alder red0.41 2,600 6,341
pine loblolly0.51 3,100 6,078
pine shortleaf0.51 3,100 6,078
fir grand0.37 2,200 5,946
cedar yellow0.44 2,600 5,909
pine jack0.43 2,500 5,814
douglas-fir interior west0.5 2,900 5,800
birch butternut0.38 2,200 5,789
fir california red0.38 2,200 5,789
spruce sitka0.4 2,300 5,750
yellow-poplar0.42 2,400 5,714
spruce black0.42 2,400 5,714
cedar incense0.37 2,100 5,676
douglas-fir interior north0.48 2,700 5,625
chestnut, american0.43 2,400 5,581
hemlock eastern0.4 2,200 5,500
spruce red0.4 2,200 5,500
pine red0.46 2,500 5,435
pine western white0.35 1,900 5,429
redwood young growth0.35 1,900 5,429
fir white0.39 2,100 5,385
hemlock western0.45 2,400 5,333
redwood old growth0.4 2,100 5,250
pine lodgepole0.41 2,100 5,122
baldcypress0.46 2,300 5,000
cedar atlantic white0.32 1,600 5,000
cedar western redcedar0.32 1,600 5,000
douglas-fir interior south0.46 2,300 5,000
fir subalpine0.32 1,600 5,000
pine ponderosa0.4 2,000 5,000
spruce engelmann0.35 1,750 5,000
spruce white0.36 1,800 5,000
tamarak0.53 2,600 4,906
basswood american0.37 1,800 4,865
fir balsam0.35 1,700 4,857
pine eastern white0.35 1,700 4,857
cottonwood eastern0.4 1,900 4,750
pine sugar0.36 1,700 4,722
fir noble0.39 1,800 4,615
cottonwood black0.35 1,600 4,571
cedar northern white0.31 1,400 4,516
fir pacific silver0.43 1,900 4,419
aspen quaking0.38 1,600 4,211
ash blue0.58 -
aspen bigtooth0.39 -
cottonwood balsam poplar0.34 -
elm rock0.63 -
hickory pecan bitternut0.66 -
hickory pecan nutmeg0.6 -
hickory pecan water0.62 -
oak white live0.88 -
sassafras0.46 -
willow black0.39 -
pine pitch0.52 -
pine pond0.56 -
pine sand0.48 -
pine slash0.59 -
 
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For some reason, I always had the impression that one part of the equation regarding strength/rigidity, was the distance between the inside and outside layers of composites. And that is why they where using "foam" core that is quite thick maybe like 1/4 inches..

Strength is a vague word in engineering terms. The distance between the outer layers increases stiffness and resistance to flexing, but not impact/puncture resistance. The core separation actually magnifies the compression and tension forces on the outer layers, as far as I understand it.
 
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Using a hardwood for strips will become a sanding nightmare. I have used poplar for accent strips, white pine for the entire hull, as well as white and red cedar. I haven't noticed a difference in strength. Dragging your boat around or having impacts will have minimal effect on a stripper if you build it for that purpose. Brass stem bands will add years to your bow and stern. Put a double layer of six ounce cloth on the football area, and if you are really concerned, a double layer on the inside football as well.

I have only punctured my canoes to the wood twice, and I use them hard. The first time I foolishly ran what turned out to be around a class 4 rapid with extremely strong current. Canoe got broached on a big rock midstream, almost wrapped, and then popped off. Somewhere in the midst of the wipeout, it got a two inch long slice down to the wood on the outside of the hull. Duct tape covered it for the rest of the trip, with no other adverse effects. The other time I was on the Steel, jumped about 15 feet vertically down into my canoe, which was sitting in the water on top of a sharp branch. Made it back to civilization with duct tape. Both of these canoes only had a single layer of six ounce inside and out. I have never punctured a double layer.

So the solution is right in front of you; a standard build with double layer outside for abrasion resistance, and yearly maintenance.

Always good points Mem. It would definitely be in my best interest to heed your advice, but on the other hand you're still up there chowing down on canned meats. ;) I'm just saying there's some things we can't give up. Cockamamie schemes being mine. Well, that and tobacco.
 
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The other time I was on the Steel, jumped about 15 feet vertically down into my canoe, which was sitting in the water on top of a sharp branch.

I would have liked to have seen this. On remote trips, I try to not jump into my canoe from any higher than 12 feet.

Quinn
 
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Muskrat,
You might be on to something...it all comes down to modulus and geometry, right? Well, density too, if you're rotating.
The thought of a thinner, higher modulus hardwood with greater impact resistance certainly has merit. It's very difficult to model the composite stiffness of a cored structure, even though the stiff membranes (no puns here, please) are well characterized. What most folks in the field end up doing is to build a sample structure and test it. Once you have a composite modulus, you can design to suit your expected loads. The hertzian contacts and deformations are, again, really difficult to accurately model for a composite.

I'm with mem on this one, I think. Double layer on the outside is tough to beat. If you want high strength, the inside should be kevlar or carbon to take advantage of the greater tensile strength. Of course, if you have a shear failure in the core, all the stiff membranes (there it is again!) won't help. For abrasion resistance and stem protection either Dynel or a metal stem band. Ultimately, there are only so many ways to reinvent the wheel.
And lastly, I'll offer this anecdotal reference:
I was just paddling with a couple guys with fancy boats...a Kestrel and a Rapidfire, both really well made, high performance (and pricey), modern hulls. We passed over many beaver dams. I would shift weight aft, float my bow up on the beaver dam (upstream travel) and then walk forward in my simply supported hull to the dam, hop out and do my thing. When the guy in the Kestrel broached the beaver dam (downstream, he didn't dare bridge on the upstream travel), we heard a sickening Rice Krispie cereal thing...you know, snap, crackle, pop!! So let's see...my cedar cored strip built hull simply supported, not a problem. Fancy boat crossing over a beaver dam, kinda dicey.
 
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Looking through the numbers, sugar maple looks like it would be tough to beat for a thin core. I'm on my phone now but I'll post the numbers tomorrow. No way I'll get any of that around here. Folks up in the northeast might have good luck. From what I have seen sugar maples are pretty upright, so finding straight grain shouldn't be an isssue. Sugar maple looks very hard for its density and also has some great properties allowing for a lot of flex before rupture or deformation.

Stripperguy, your beaver damn anecdote would definitely support a cored hull as the best design for the situation. Essentially you're creating a case where a beam is the ideal structural member. You have a long unsupported span relying on the stiffness of the beam to support the whole load. The kestrel (hemlock right?) isn't cored so spanned out across two points with a load in the middle would certainly put more stress than the thin "beam" could handle. In my proposed thin core design I think the traditional cedar construction would win out. In a shorter span situation I think it would be the opposite, of course that's just my hunch so far. The difference between standing on a hull or bashing it with a peen hammer.
 
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Quinn;n47382]

I try to not jump into my canoe from any higher than 12 feet.

Quinn[/QUOTE]

I'm sort of surprised that you would jump into anything from 12 feet... I'd think the landing would be dicey, even if you weren't worrying about your canoe.


Muskrat Sugar maple is hard. It's often called "Rock Maple" for very good reason. I should know, I've used it as an accent wood in my recent build. Will not be doing that again. I have seen one instance of a Strip Canoe with point deformation damage. My second canoe was thrown against the corner of a pole building. In addition to the surface scratches that it picked up on the way there, it had:

- A series of small ruptures in the inner glass, parallel to the strip lines. More delamination than break.
- The strip itself had split along the grain.
- Some delamination of the outer glass, where the split had caused a peeling force.

The damage was in a thin line, about 16" long.There was no discernible crush damage to the cedar.

If you are having difficulty visualizing the load, think of a canoe pinned in current, against a small, hard edged obstruction.
 
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Muskrat: Just curious whether or not you built it, how it's held up and what you might do differently if you had a "do-over".

I'm pretty much set on Yellow Poplar w/ Butternut trim for mine but I've yet to decide on plank thickness and glass layer(s). Kinda sounds like you treat your boat similarly to what mine can expect (not outright abuse but it won't be babied either) and I am thinking hardwood might provide better impact resistance on rocky, shallow rivers where impacts and abrasion are inevitable (and often continuous).
 
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