SOF Stress skin panel frames

Submitted byAppleHillBoatyard onWed, 12/16/2020 - 10:12

Total newbie; Planning on building a SOF using the "fuselage" frame method. An out of the box idea occurred to me.

Instead of 3/4 plywood frames, would a 1/8" ply, 1/2 solid foam insulation, 1/8 ply sandwich appropriately sealed on the edges provide the strength needed in addition to being lighter and providing more flotation?

I have not seen this anywhere so there is probably a good reason this would not work, but I am just experimenting. 

Thanks in advance. 

in the fuselage approach, the frame is primarily under compression.

there are manufacturers out there that are making frames out of carbon fibre and probably have foam cores or are otherwise hollow.

https://www.trakkayaks.com/

i think in order to get the strength you want with a foam core you would actually need to skin all sides with 1/8 ply and glass them.... so you get a true box beam effect.  if the ends are open foam i don't think it will resist compression properly 

you can make a sample of what you want to do and crush it (and compare it to a 3/4 ply piece) to see how it performs.

h

daviddewitt

Thu, 12/17/2020 - 20:04

There is no need to use anything thicker than 1/2” plywood.  3/4” is overkill and does nothing but add weight.  However, you should definitely use some variety of marine plywood.   

JayBabina

Mon, 12/21/2020 - 15:18

I built one kayak and used 1/2 foam panel for the bulkheads. I epoxied both sides (no glass). I could not believe how strong it got. Very light too. You can experiment with some pieces and just use epoxy coatings on all sides. I just used that Home Depot blue stuff.

I’m in the same boat (har har), experimenting with thin plywood along with carbon fibre and fibreglass laminations to come up with a lighter way to build a fuselage frame kayak. Relative to 3/8 plywood, the results so far with 1/4 and 1/8 inch Baltic Birch are not stupendous in terms of weight savings vs strength, but there is some potential there. I have some data from a few tests I just completed that I can share if there is any interest.
 

 Jay, on the all foam bulkheads, were they for the actual frames on a fuselage frame build or do you mean bulkheads put in for hatches in something like a strip build? Thanks

JayBabina

Thu, 12/24/2020 - 07:45

Traveler: My bulkheads were for a stripper. I could easily snap a piece of 1/2" foam piece over my knee. But once epoxied on both sides, forget it. I could not believe the strength. I suppose I could snap it but no comparison in the stiffness and strength.

This would be of most interest to anyone thinking about building a fuselage frame style skin on frame canoe or other paddle craft, but I think there might be some general utility for anyone either building or repairing other styles of boats as well. It’s a pretty crude approach to stress testing, and the material sizes here are pretty specific, but it’s all I got - take it for what it is worth.

Basically, for my project I wanted to find out what combination of wood with fibreglass or carbon fibre laminations would get me more or less equivalent strength, at less weight, than some selected baseline, wood only scantlings. I was not looking to determine the absolute strength of any test pieces, but only their relative strength within the confines of the primitive testing mechanisms I was able to put together - e.g. if subject to the same stress, would a .5 x .75 piece of carbon-sleeve wrapped cedar be as strong but weigh significantly less than a “baseline” .75 x .75 piece of bare cedar?

My test subjects were 1/8 and 1/4 inch thick Baltic Birch plywood relative to 3/8 inch thick Baltic Birch, and .5x.5 and .5x.75 inch cedar relative to .75x.75 cedar. I used 6 ounce fibreglass and carbon fibre cloth, and 8.9 ounce carbon sleeve, as noted in the table below. For curiosity sake I also tried a very limited test of of Corecell A500 foam, and some 4 ounce s glass (which I just realized has some wonky results on the table which I think is a data entry error of some kind), also as noted.

For the plywood pieces, I used a gym weight tree with one end positioned on the plywood as it was suspended across a pair of dumbbells, and just added weights to the tree in 2.5 pound increments until the piece broke. The weight tree weights 22 pounds, but because one end is resting on the ground the “baseline” weight actually applied to the test piece was only 12 pounds before adding any additional weights.

Image removed.

For the cedar scantlings, in order to let me add the necessary weight I had to suspend the weight tree from the test pieces, with the whole thing hanging about 6 inches from the floor, and added weights accordingly (again in 2.5 pound increments). Baseline weight here is the full 22 pounds of the weight tree.

All laminations were done by hand in a wet layup - for the plywood I used peel ply and paper towels under wax paper to absorb excess epoxy while pressed between two boards under 80 pounds of weights until cured. For the carbon sleeve I used a gloved hand to compress and “squeegee” the sleeve once epoxied, and then just left hanging to cure. All pieces were cured at least 72 hours at about 65-70 degrees F before testing.

The data are below. As this post is getting a bit long I won’t get into any detail. There are a few combinations that have some potential, but the upshot for me is that for any combination with strength approaching the baseline, the weight savings are pretty small or non-existent. Within the wood sizes and budget I am looking at here, it just doesn’t seem worth it to go through the trouble and cost of laminations for the marginal weight savings over baseline I would get. Given the importance of core thickness and length for lamination strength, I think it might be a different story with beefier pieces. I will be looking at this when I start on seats, thwarts, etc for my canoe build, but in the meantime if there are any comments or observations on this set of tests ....

thanks all

Tony



 

Wood core and laminate stress tests

 

 

Wood core and laminate stress tests

 

 

 

 

 

 

 

 

Weight (grams)

Base pounds of testing device

Additional pounds

Total pounds to failure

Total pounds to failure relative to baseline

Weight relative to baseline

 

Plywood Stress Test *

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Baseline - 3/8 Baltic Birch, plain

42

12.0

122.5

134.5

100.0%

100.0%

 

 

 

 

 

 

 

 

 

Samples

 

 

 

 

 

 

 

1/8 plain

15

12.0

0.0

12.0

8.9%

35.7%

 

1/8 1 layer e glass

22

12.0

15.0

27.0

20.1%

52.4%

 

1/8 2 layers e glass

26

12.0

20.0

32.0

23.8%

61.9%

 

1/8 1 layer carbon

21

12.0

30.0

42.0

31.2%

50.0%

 

1/8 1 layer carbon, 1 layer e glass

29

12.0

55.0

67.0

49.8%

69.0%

 

1/8 2 layer carbon

27

12.0

70.0

82.0

61.0%

64.3%

 

 

 

 

 

 

 

 

 

1/4 plain

28

12.0

57.5

69.5

51.7%

66.7%

 

1/4 1 layer e glass

34

12.0

75.0

87.0

64.7%

81.0%

 

1/4 2 layers 4 ounce s glass

35

12.0

50.0

62.0

46.1%

83.3%

 

1/4 1 layer carbon

36

12.0

107.5

119.5

88.8%

85.7%

 

1/4 1 layer carbon,  1 layer e glass

40

12.0

135.0

147.0

109.3%

95.2%

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stringer Stress Test **

Weight (grams)

Base pounds of testing device

Additional pounds

Total pounds to failure

Total pounds to failure relative to baseline

Weight relative to baseline

 

 

 

 

 

 

 

 

 

Baseline - 0.75 x 0.75 plain cedar

48

22.0

390

412.0

100.0%

100%

 

 

 

 

 

 

 

 

 

Samples

 

 

 

 

 

 

 

0.5 x 0.5 plain cedar

22

22.0

145

167.0

41.5%

45.8%

 

0.5 x 0.5 cedar 2 layers of carbon sleeve

43

22.0

250

272.0

67.7%

89.6%

 

0.5 x 0.75 plain cedar

32

22.0

215

237.0

59.0%

66.7%

 

0.5 x 0.75 cedar 1 layer of carbon sleeve

46

22.0

300

322.0

80.1%

95.8%

 

0.5 x 0.6 Corecell A500 foam

9

22.0

 

22.0

5.5%

18.8%

 

0.5 x 0.6 Corecell  A500 2 layers of carbon sleeve

33

22.0

75

97.0

24.1%

68.8%

 

 

 

 

 

 

 

 

 

 

* Baltic Birch (1/8 and 1/4 inch thickness, 1x10 inch samples) with selected carbon fibre and fibreglass laminations (6 ounce/sq. yd. carbon and e glass cloth,  laminated top and bottom, with exception of one test of 4 ounce s glass as noted for 1/4 inch plywood ), test weight applied on mid point of flat face of samples, across 8 inch span

 

** Stringer stress test - 12 inch long samples using 8.9 ounce biaxial carbon fibre tubular sleeve (0.75” diameter) as indicated, test weight applied at mid point on edge of widest dimension, across 6 inch span

 

 

 

 

I actually think it’s the forum software - can’t seem to load any photos or screen shots whatsoever. Oh well - it’s a bit tedious, but you can still read the data table by mentally lining up the numbers for each test piece with the sequencing of column headers that show at the beginning (weight, base pounds of testing device, additional pounds, etc.). 

traveler

Wed, 01/13/2021 - 09:59

In reply to by JohnAbercrombie

Tried the slideshow feature as well, and again no joy. Too bad - hopefully it’s temporary, as photos of others’ work make a build  forum much more interesting.