# Carbon sleeve reinforced DF

Discussion in 'Boat Design' started by abosely, Jul 1, 2023.

1. Joined: Mar 2015
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### aboselySenior Member

Not asking this for specific components, wondering whether a combination of carbon fiber and wood can increase bending of non critical component.

For example, a floor bearer 1”x3” spanning 24” that has a lift off floor panel laying on it,
could be reduced from 1”x3” down to around 1”x1-½” or even 1” by epoxying a carbon fiber sleeve over it?

Reason for reducing depth of bearer is to gain some clearance under the bearer for a water tank.
Not trying to reduce weight or increase strength (bending) over the 1”x3” but to maintain at least same bending strength allowing less than 3” depth bearer.

Didn’t know it combining carbon fiber & wood is practical because wood & carbon fiber stiffness are so different.

Again, not asking about critical structural components, in worse case if bearer broke it would let lift off floor panel sag & bearer sit on top of tank.

Edit: What about putting a strip of CF tape on bottom of bearer or on top & bottom?
Would having layer on sides add to bending strength/stiffness?

Wondering about practical feasibility of this.
Cheers, Allen

2. Joined: Jan 2006
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The "bearer" is to be treated as a beam. The strength of the beam is much influenced by the depth of the beam. The equation for finding the relative strength is..... I = ( bd^3)/12. Where b is the width and d is the depth. If we do the arithmetic, the original 1 x 3 beam has an I of 2.25. Reducing the depth to 1.5 makes I = 0.281. The 3 inch beam then is about 8 times stiffer that the 1.5 inch beam.

How the ends of the beam are supported also plays into the adequacy of the beam. In any case, depth is the more influential of the variables.

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3. Joined: Mar 2015
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### aboselySenior Member

Is it realistically practical to increase the stiffness by 8 using carbon fiber over 1.5 inch deep bearer?

Would it be a matter of laminating layer one unidirectional CF on top & bottom that will increase stiffness 8 times?

Just choosing thickness of CF layer to increase stiffness and if CF layers are little thicker than required for stiffness, wouldn’t matter.

It’s not a critical component that needs to be engineered to lightest possible weight.

4. Joined: Jan 2006
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The bottom fibers of the beam, when in bending mode, will be in tension. The top in compression. Concentrate on strengthening the bottom of the beam for best result. In this particular case, I do not suspect that you could make the 1.5 inch beam 8 times stronger if you use wood.. The I value is only part of the deal. The strength of the material used for the beam is also part of the puzzle.. Every material has a strength factor called modulus of elasticity. Engineering types use the letter e to indicate the level of elasticity. Every material will bend when loaded. That seems like a reckless statement but it is true. Even stone will bend some amount before it breaks.

You could use an aluminum beam which has an elastic modulus seven or eight times that of yellow pine. A quicky guess is that you could use an aluminum bar of about the same dimensions as your proposed 1.5 inch wood beam and have about the same bending strength as the 3 inch wood beam. The way the beam is supported is also an important matter. If you can secure the ends of the aluminum bar sufficiently, then you would probably (not a promise) be OK.

If you know a structural engineer, buy him/her a couple of beers and have him/her do the calculations for you.

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5. Joined: Mar 2015
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### aboselySenior Member

Probably not enough benefit for the effort to gain 1.5” extra clearance.

Benefit would be gain 1.5” clearance for tank under floorboards.
One of those things that if was fairly straightforward to do, would might be useful.

But enough gain to be worth the time & effort for such small potential benefit.