My new boat

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Shu
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Post by Shu » Sat Dec 13, 2008 4:46 pm

Since I've already blown 20% of the budget, I just may have to do the beg and borrow bit.

Finally, here's the promised pic of the centerboard case. The big fat diagonal piece is a temporary brace to keep the case plumb and square until I glue on the bottom skin.

Also, here's a pic of the stem, with the rough fairing done to the starboard side. You can see the stringers need to be trimmed to fit in (actually, the whole starboard stem and bow area is all faired in and ready to be glued in place and skinned, but I'm always behind in the pics).
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Post by Shu » Fri Dec 19, 2008 2:01 am

Picked up a pair of used Holt autoratchets for the spinnaker sheets. Only $35 for the pair. I already have the mainsheet ratchet block.

Running total is now $140.68
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Post by Shu » Sat Dec 20, 2008 3:09 am

picked up another handful of micro blocks: $48.22

Running total now $198.90

These tiny blocks are going to kill me.
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Post by JSW225 » Sun Dec 21, 2008 4:08 am

Just tossing this out there, I know very little about boat building (would love to learn someday)...


But couldn't you use the wood structure you had assembled for you as a male mould for carbon struts, then fill with something like dense foam? Or would the carbon sheets need to be too thin (weight concerns) and have buckling problems from the foam being too weak or not fully adhere to the carbon?

Just tossing some numbers and ideas out there not knowing what kind of wood you're dealing with. From my quick calculations, Carbon and epoxy weighs half of an oak wood, and is at least 100x stronger. Even going to fiberglass, it seems to be half the weight of some woods and at least 50x stronger.


Just reread that you're using plywood, and probably losing even more strength.

Sorry if this is completely off the chart, my engineering mind catches an idea and goes off on its own without any control. If this doesn't make any sense, you should try to understand my notes sheet.

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Post by Shu » Mon Dec 22, 2008 3:52 am

JSW225 wrote:Just tossing this out there, I know very little about boat building (would love to learn someday)...


But couldn't you use the wood structure you had assembled for you as a male mould for carbon struts, then fill with something like dense foam? Or would the carbon sheets need to be too thin (weight concerns) and have buckling problems from the foam being too weak or not fully adhere to the carbon? I'm not quite certain what you're proposing, Maybe carbon framework supporting foam core with carbon skins? That could work very well if done right.

Just tossing some numbers and ideas out there not knowing what kind of wood you're dealing with Okume plywood, about 30 lbs/cuft; and Port Orford cedar lumber, about 27 lbs/cu ft. From my quick calculations, Carbon and epoxy weighs half of an oak wood, and is at least 100x stronger. Even going to fiberglass, it seems to be half the weight of some woods and at least 50x stronger. For bending strength and stiffnes, which are the chief concerns for the hull skin, wood has a better strength to weight ratio than fiberglass/epoxy or even carbon epoxy. Now if you go to a foam sandwich construction, carbon/epoxy/foam is considerably better than wood. I am using wood, becuase it's easier for a one-off construction, and it should be stiff enough for the general hull. I am adding carbon where I need the extra strength and stiffness.

Just reread that you're using plywood, and probably losing even more strength. Yes, if I could guarantee that all the loads would be oriented along the grain of the wood, plywood has less strength than regular lumber. However, loads on the hull skin are usually multi-directional. Plywood takes care of this and has more consistent bond strength between layers than cold-molded wood. That being said, The curved part of the hull bottom is going to be cold-molded of two layers of 1/8" plywood. Majority of the hull skin will be made from sheets of 1/4 plywood
Sorry if this is completely off the chart, my engineering mind catches an idea and goes off on its own without any control. If this doesn't make any sense, you should try to understand my notes sheet.
Great questions. You're basically right. I just want to build it in wood, cause it's fun to work with, it looks sexy, and I think I can produce a competitive boat if I engineer it carefully.
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Post by JSW225 » Mon Dec 22, 2008 5:11 pm

Where are you getting that wood has a better strength to weight ratio? All the information I could find showed that this wasn't true at all, however I did have to estimate the weight of epoxy. I'll post my numbers later tonight.

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Post by Shu » Mon Dec 22, 2008 6:50 pm

For simplicity sake, I will take the case of a linear, rectangular cross-section beam with all the carbon or wood fibers oriented along the longitudinal axis of the beam.

Equation for bending stress is

fB = M/S, with:
M = bending moment
S = section modulus = (b(h)^2)/6. simplifying for unit width b, S = (h^2)/6.
substituting for S, fB = M/((h^2)/6), and solving for M,

M = 1/6(fB)(h^2)

for Port Orford cedar, modulus of rupture for bending is 12,700 psi, density is 27 lb/cuft

for unidirectional carbon laminate, in close-molded resin-infused (best possible process) flexural strength (comparable to modulus of rupture for wood) is a whopping 129,000 psi, density is 100 lb/cuft.

for ease of calculation, lets assume we have 1" thick carbon laminate. An equal weight of port orford cedar would be (100/27) x 1" = 3.7" thick

for the carbon, The max moment at failure would be 1/6(129,000)(1^2) = 21,500 lb-in

for the wood, max moment at failure would be 1/6(12,700)(3.7^2) = 28,977.

So for the same weight, wood gives better strength in bending than a solid carbon laminate.

The other important issue for the hull skin is resistance to deflection (stiffness). The equation is Deflection = wL^4/EI where the variables between carbon and wood are the terms E and I

E for port orford = 1,700,000 psi
E for carbon = 16,800,000 psi,
about a 1:10 ratio in favor of carbon, but

I is a function of the cube of the thickness or h^3.
1^3 = 1
3.7^3 = 51
about a 1:50 ratio in favor of wood. Again, wood wins over solid carbon laminate.

This all changes if we separate two thin skins of carbon with a lightweight core of sufficient shear strength, like PVC foam. Then carbon is the clear winner. That's why nearly all 14s are built of carbon/foam/epoxy.

The wood also loses if we look at pure tension and compression stresses, such as those developed in the hull skin by the rig loads. That's why I'm reinforcing with carbon in critical areas.

Sorry if this got long winded. I'm also going to look at the case of the Okume plywood vs. a solid laminate using carbon cloth, but I will do that off line and not bore you with the details.
-Steve
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Post by Shu » Mon Dec 22, 2008 7:19 pm

Similar results for plywood vs. carbon cloth laminate. Plywood has better strength and stiffness to weight ratio than solid carbon laminate. Carbon with foam core is best.
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Post by JSW225 » Tue Dec 23, 2008 12:51 am

Not boring at all, I do have a bit of background in structural analysis and design (mostly buildings).



However, you are incredibly skewing your numbers by stating an equal piece of wood would be 3.7x taller than carbon. I can take almost any material known to man and have it take more of a moment than a much stronger material by making it nearly 4x taller.

The better test would be to assume equal ratio'ed surface area of squares between 1" by 1" stick of carbon and an ultimately 1.92"x1.92" stick of cedar.


Even then, due to the difficulty of equallying out moments of inertia, it may be better to get equal moments out of the two, recalculate areas and I's and see if the wood is 3.7x bigger or not.

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Post by Shu » Tue Dec 23, 2008 1:31 am

Excellent observation. Yes, we could stand a piece of 1/8" plywood on edge and claim all sorts of amazing properties for a stringer, but in the real world we have buckling, so we must keep the width to depth ratio reasonable. The proportions should be the same for both materials when looking at a stringer, and that will probably show that a solid carbon stringer will be stronger than a wood one. Stiffness may still favor wood.

In the example, I was thinking of a 1" wide section of hull plating. For the plating comparison to be valid, the width needs to be same for both materials, and only the depth vary. To be fair, it was probably confusing that I was using the cedar (stringer material) in the example for plating (or hull skin as I called it - I couldn't think of the correct term at the time).

Again, this all becomes moot if you look at cored carbon composite construction. But it does explain how popular 1960s and 1970s US dinghys with solid fiberglass construction were so much heavier than the predominately wood construction dinghys in NZ and Oz. And if you also consider the stiffness deterioration of fiberglass/polyester construction, why a 20 year old wood 505 is still competitive, and a 10 year old fiberglass one is not. (If you were allowed to build a 49er out of wood...but I won't go there :wink: )
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Post by dave » Tue Dec 23, 2008 10:17 am

some good valid points there for boat builders and buyers alike. The old ways are not dead yet and dont forget its been a long time since noah built the ark carbon boats have been around for how long! going back to my days in the grand prix 18's all boats were made from carbon and none of them are sailing around now. A boat that has passed many hands now comes to mind a flying dutchman that ben lexcen him self made that i had the pleasure of sailing still keeps up with the carbon boats so it goes to show you never can tell


its up to people much smarter than me to make something that is durable and fast
hamilton island crusing yacht club
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for henning island

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Post by JSW225 » Tue Dec 23, 2008 5:03 pm

What kind of pure strength (either tension or compression) are you getting for plywood? All the basic stuff I could find online inferred 1/4-1/3 the axial strength of lumber (though, that's in all directions where as regular lumber is only strong in one).


How heavy is PVC Foam?

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Post by Shu » Wed Dec 24, 2008 6:27 am

That's a good question. I can find no published values for plywood. I calculate it by counting the plys and using the published values found in the USDA Forest Products Library's Wood Handbook for the lumber of which it is made. Then I just use those plys that run in the direction I'm interested in. So for 6mm (1/4") plywood with 5 plys, I get 2/5 the axial strength of the lumber in one direction and 3/5 in the other. I think this is conservative for marine grade plywood, because:
1. It's compressed. Okoume plywood runs about 30 lbs/cu ft, but Okoume timber is supposed to be about 22 lbs/cu ft. That compression should mean that you have more load bearing fibers in a given cross-section.
2. Glue. how much strength does this add? (it probably accounts for some of the extra 8 lbs/cu ft)


PVC comes in varying weights. I have not worked with it enough to have the numbers off the top of my head. I know the denser commonly used PVC foams are less dense than balsa, which is much less than lighweight woods such as oukume, spruce, and cedar.

By the way, I want to get some 1/4 PVC foam for the stiffener web in my new boom, and maybe for some of my rudder gantry panels. If anyone reading this knows a good supplier in the US of small quantities, let me know.
http://www.fpl.fs.fed.us/documnts/fplgt ... gtr113.htm
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Post by Shu » Thu Jan 08, 2009 4:27 am

More new pics. This time they are current (as of this morning)
1st is the transom with all stringers glued and ready to be cut flush.
2nd is the stem trimmed and faired and ready for the plywood skin. The clamp was used to keep the stringer twisted as the epoxy cured.
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I-14 011.jpg
I-14 009.jpg
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Post by Shu » Thu Jan 08, 2009 4:54 am

Here's the head on view. Note the flare in the stem and how the 1st stringer (closest to centerline) is recessed.

Next photo shows the plywood doubler fitted to the 1st stringer. This provides a wide faying surface for the joint between the center panel and the plywood panels that will make up the curved portion of the bottom. This detail was first developed by Dudley Dix as part of his "Radiused-Chine" construction for keelboats. Note also the stringers have been trimmed and sanded flush with the transom.
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I-14 013.jpg
I-14 016.jpg
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