# calculating torsional stiffness hull

Discussion in 'Boat Design' started by Pammie, Mar 17, 2018.

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1. Joined: Oct 2008
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Hi Pammie

Ok...found some free time. So we first have to look at the basic's, before you do anything "detailed". What are your assumptions?

Well, let's first look at a simple transverse bending moment case. If you assume one hull to be on a wave and the other in the air, the distance between the 2 centrelines is roughly 4.80m. With the weight of (1250 - one hull) = 1030kg. Assume worst case 2g , therefore your transverse bending moment is 1030x9.81x2x4.8 = 97kNm.

Your 2 beams, from a previous post you have around 320+600+820 gm of laminate which is roughly about 2mm thick.
Given the dimensions you have provided, the aft beam if made from 2mm thick laminate has a modulus of roughly 92cm^3 and fwd beam of roughly 237cm^3 or a total of 329cm^3.

From the bending moment and assuming an allowable stress limit of say 200MPs, the required modulus is 485cm^3.
So on first inspection it is shy in stiffness.

So, lets look at the deflection.

Given the beams and establishing their Inertias, with the hull hanging off, as in the load case noted above the deflection assuming an E of 20GPa, gives a deflection of 674mm.

Thus, you need to first of all design the layup to give the stiffness you need with the right materials.

Forget torsional stiffness for now, you need to prove the simple case first.

So you need to get known hard data of the E and yield stress of the layup you want to use, or in the absence of coupon testing to prove the mechanical properties, use the minium values from data sheets. Then you can design the beam to provide the required stiffness for the basic transverse bending load case. Once you have satisfied this part, we can move onto the other cases.

How does that sound?

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### PammieJunior Member

OK, I compared my mast calculation with Skene's. What I did find strange in the first calculation method (proposed in Rxcomposites earlier posted pdf) is that it uses breaking load to calculate halyard loads? In my calculations halyard loads are more than backstay load? So I skipped the first calculation as there is little difference with Skene's method in the example.
I now see that I also calcalated wind speed limits according to ISO12217-2. For main+jib this is 27 knots (apparent, 19 knots true). At this sailload my mast compression would be 32 kN. Best use this number for beam calculations I think.
Any suggestions on what safety factor to use for mast itself if I want a light mast that is not going to break ?

@rxcomposite: I understand your point. Will post a new load plan in a few days.

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### rxcompositeSenior Member

Pammie- Thank you for the link on the simplified method of calculation. That is quite a lot of mast load you have there. I made my own spreadsheet and will have to revisit tonight and feed your boat's particulars to see if I come up with the same result.

A quick check on the spreadsheet showed it inputs nominal Breaking Load and uses 1/5 of BL.

The way I remember it, there were 3 methods to use. I think, the third one, Gerritsma was so far out I did not use it.

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### PammieJunior Member

Yes it does, but I have some comments on the numbers. In calculating bending moment not all the rest of the weight is at 4,8 mm. 220 kg was empty hulls. Loaded hulls are 2 x 287 kg, which leaves 676 kg for the middle hull. But then things get more complicated. Great idea to introduce higher g forces, I didn't think of that.
Deflection would be less as the beams are higher in the middle (because of the spine). I think I made a mistake in thinking that L^3 = 3*(L/3)^3 which is ofcourse not true.
Beams certainly need thicker laminate.
I allready tried to find more info on the glass/ carbon but didn't get anymore than what's in it, not what it does or is able to. But will ask again. Coupontesting: could this be done by bendingtests (from laminatestrips)? Not as good as breaking tests of course

Last edited: Apr 15, 2018
5. Joined: Oct 2008
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Perhaps ask RX, as the detailed layups and known strengths are more his bailiwick than mine.
He can give you a good layup that he knows what the design strength values to sue should be and will work...since I always sue coupon testing.

Once the 2 of you have decided best layup...we can review further.

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### rxcompositeSenior Member

Pammie. I checked my spreadsheet. I deleted the halyard load as you feel it is not important. Mast load is 31.2 kN. Almost same as ISO. Use whatever is comfortable with you.

As for the material properties, LR and ISO furnishes formula for computing properties. I will use that as a guide instead of using published data. There is a lot of variation in published data because of materials used. I just won't be using Eglass/carbon fiber cloth weave combination. Not a good practice.

I am making a new spreadsheet as the old one, I have to use three load models for computing. Maybe I can just simplify to one.

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### PammieJunior Member

@rxcomposite: Good that numbers on mast load are confirmed. I noticed there is a lot of different information about materal properties. As I understood it strength and modulus of Eglass is quite standard, but those of carbon depend much on its special target: high modulus and/or high strength. But I suppose the real difference is in the combination of fibres and matrix? In your experience can a calculation of such a combination predict reality?

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### rxcompositeSenior Member

LR and ISO predicts the property by calculations by using using a specific range of properties. It is reliable as they have countless data to back it up. Carbon has three types, the low modulus, medium, and high modulus and will accept only epoxy resin.

Fibers also are problematic due to weave patterns. I stick with the standards. WR, Uni, Biax and stay away from combination weave. It is not as they claim it is.

Combining with a different type of resin, especially epoxy, will produce a different result as there are many types of epoxy formulation. Class approved polyester and Vinyl seems to be consistent but there are other types of resin they do not approve. DNV approves the type but requires coupon test.

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### PammieJunior Member

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### rxcompositeSenior Member

That will work. I do crude testing myself. Carbon is around 600 to 1,200 N/mm2 tensile strength depending on the variety of carbon and epoxy used. UD Glass/epoxy is lower at around 200 N/mm2. The simplest one I saw is a pyramid type steel tube structure with a screw type tensioner pulling the specimen.

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### rxcompositeSenior Member

Have you an English version of the epoxy material properties? need to know tensile strength, modulus, elongation. If possible the "neat resin" stress strain curve. From there I can do strength prediction.

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### PammieJunior Member

From the epoxy I have this information: But it is with other hardeners. https://www.carbonwinkel.nl/nl/attachment.php?id_attachment=772
I have asked for technical info on the specific combination, and will report as soon as I got this.

Pyramid with screw tensioner: But it will be more difficult to measure what the actual force is?

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### rxcompositeSenior Member

A tensiometer is attached. It is a DIY test jig and appeared in PBB magazine a long time ago.

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### rxcompositeSenior Member

Looking at the info sheet, it is there, at the very last page. The info on "neat resin" or cured unreinforced resin specs and the typical strength when laminated with a fiber cloth.

We have to do some test. The specs is below LR standards. The specific gravity is 1.01 to 1.02 (lighter than a polyester (1.2)? Epoxy has 1.3 to 1.32) and the modulus is 2800 while the min required is 3500.

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### PammieJunior Member

This is the spec with the right harder. I see that GL uses >2700 MP modulus. L+GL1 reaches almost 3500 (but cures quite fast), L+GL2 s around 3050 MPamodulus.

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