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Redimensioning Gunwales,Stringers, and Keel for other type of wood


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Hello from Europe!

 

I am planning to build the curlew kayak from the book. As can not find Western Red Cedar in my region, so I am planning to use fir instead.

Since it is heavier (26 instead of 23 lbs/ft3), but also stronger, I was wondering if I reduce the weight a little bit by making the keel, stringers, and gunwales a little bit smaller in height and width.

 

I did this the following way: comparing the Modulus of Rupture and Crushing Strength tells me that western red cedar has only 80% of the strength of fir.


Thus if I scale the cross section area to be 80% of the numbers from the book, it should give me the same strength.

Does this sound reasonable, or are there other things that have to be considered? How are stringers dimensioned, or in other words: how did the dimensions in the book were determined?

 

Thanks :)

 

Some more details are attached:

Modulus of Rupture: Modulus of Rupture, frequently abbreviated as MOR, (sometimes referred to as bending strength), is a measure of a specimen’s strength before rupture. It can be used to determine a wood species’ overall strength; unlike the modulus of elasticity, which measures the wood’s deflection, but not its ultimate strength. (That is to say, some species of wood will bow under stress, but not easily break.)
 

Crushing Strength: Sometimes known as compression strength parallel to the grain, this is a measurement of the wood’s maximum crushing strength when weight is applied to the ends of the wood (compression is parallel to the grain).

 

Western Red Cedar:
    Modulus of Rupture: 7,500 lbf/in2 (51.7 MPa)
    Crushing Strength: 4,560 lbf/in2 (31.4 MPa)

Fir:
    Modulus of Rupture: 9,590 lbf/in2 (66.1 MPa)
    Crushing Strength: 5,950 lbf/in2 (41.0 MPa)

Ratio for rupture: 78%
Ratio for crushing: 77%

Numbers taken from here:
    https://www.wood-database.com/western-red-cedar/
    https://www.wood-database.com/european-silver-fir/

 

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Hello Europe!

It's been many decades since I studied strength of materials, but I don't think your calculations capture the whole picture. You're ignoring the modulus of elasticity, which has to do with how the wood bends. Based on ratio of the average densities of the two types of wood (26:23), you could probably save only about 2 pounds. We must also keep in mind that there is significant variability among trees of the same species, among sticks from the same tree, and among how the sticks are sawn. We're not dealing with a more uniform material like steel. Look at the grain of each stick (how much runout is there? How tight is the grain?). Flex  each stick. And then decide the scantlings that give you a good feeling. With nice, straight-grained fir, you could probably go 1/16" smaller, but I'm not sure that it's worth it.

Have fun!

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HI kayak_building_with_fir,

 

i build last Winter a Vardo in Europe using spruce,

Gunwales: 15x38mm

Stingers: 15x25mmmm

Frames: 12mm Plywood DIN BFU / AW 100

weight of the ready made boat: ~19Kg

including Heavy fabric, Heavy Orbix hatches and many  coating layers

I Think with light fabric and without hatches the boat has had neary 15Kg (30 lbs)

 

the Boat was used nearly 200km (120 Miles) this summer on lakes and rivers .

 

greatings ..

 

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  • 1 month later...

Thank you both, andy00 and lattenkracher for you responses. I was on holiday, so my response is a bit late.

 

Anyway, I have decided that I will built the curlew and went wood shopping. They did not have fir, so I bought a nice thick plank of spruce without knots, which I will break down for stringers and gunwales in the dimensions that lattenkracher suggested (25 x 15mm; 38 x 15 mm)

 

I'll post some picutres in a separate build-log.

 

cheers!

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Quote

purchaseing spruce without knots is the real competition :)

Yes indeed! What I had to ask for in the end was "Wertholz Tischlerware" (in German).

 

Yesterday I finished the frames and oiled them with tung oil, turned out quite nicely. But I was lazy and applied a thick layer, I figured it is easier to saturate once and then wait a bit longer for it to dry.

 

BTW, do you have any tips on to find waxed sinew in the German speaking world, and also for the paint used for the skin ? (the skin you have used I saw in your buildlog)

 

Thanks :)

 

PS: attached are the oiled frames, and a picture of the plank I'll cut into the stringers. Quality is bad, but it's 4m of what can be seen on the picture.

IMG_20201002_222315521 (copy).jpg

IMG_20201003_205239111.jpg

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Hi

 

i have tried out different  types auf senew, finaly this one was good:

https://www.lederwerkzeugladen-chiemgau.de/1001-00-Wachsgarn-Kunstsehne-braun-teilbar

I think i have used two or three reels.

 

The paint i have used was "Wilckens Boot & Yachlack" but it is no as durable as i hoped.

 

I think i try finding some Ureatan/Coelan Coating next time.

 

 

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Today I cut apart the board(picture from last post) I had into stringers and gunwales.

 

I cut away one side to I have a straight edge, and all the others can i cut with the small tablesaw I have. I had help from a second person. Was quite easy once you figure it out, one has to push and the other has to pull, and then I ended up with these beautiful 4.2m long stringers and gunwales, which I then scarfed together with one extra meter.

 

Really happy how it turned out, as there are no knots, defects, or branches. Also, the grain is quite straight over pretty much the whole length, so it is as good as it gets. The board I bought at a local lumberyard, where I explained what I wanted and they gave me this for 22€.

 

Now it looks like this and the joints are drying:

 

IMG_20201004_201426354.thumb.jpg.d787eaa6a5b39a57029c0057c08d40d8.jpg

 

Next I will give it a quick sanding with the beltsander and a 120 sandpaper, and oil it with tungoil as well.

 

Next week will be quite busy at work, so I guess I will only start assembly next weekend.

 

What I still don't know is which lashing/fabric/paint to use, and of course in which colors to paint it ?

 

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