Composite pocket boom: sharing calculations and asking for help

[TANSL]

However you cannot use the classic beam bending formulae because you don't have a constant stiffness value for all the parts of your section.

I probably don't quite understand what you mean because I believe that the classical formulas for a beam subjected to bending are totally independent of the shapes of the section of the beam or of the stiffens of the parts of the section.
With composite materials, there are always different stiffeners in each of the parts that form a section and the classical theories of elasticity are fully applicable to them. What would apply if not?

Finally, for an open section like this you need to consider the buckling modes.

Although there are usually no significant compressive stresses, it is true that local deformations (called local buckling) can occur due to tangential stresses. But the same can happen in a closed section beam.

 

[rxcomposite]

Not as easy as I thought. It looks like a cantelever with a diminishing load but there is also torque (rotation) due to force on the sail at a distance away from the mast. This makes it a compound load (combined load formula). There is tensile, compression, and shear.

The shape of the boom is definitely tapered and open on one side but has a fixed cap on both ends to hold the furling mechanism. To idealize the section and find N.A., this needs integration of areas.

Further, the boom is supported by a vang, making the effective length smaller. So diminishing load + support. This requires superposition method to calculate strength and flex.

 

[rxcomposite]

On second thought, the open end of the boom will close when the bottom is subjected to tensile. Am I correct?

 

[gonzo]

Probably the easiest way to analyze it is with FEA.

 

[TANSL]

Probably the easiest way to analyze it is with FEA.

Absolutely not, it could be the most exact (if the model is created well) but it can be crazy if you don't have enough knowledge. It is not the "easiest way" and not within the reach of most yacht designers. The FEA does not guarantee anything if the approach and the model are not correct. The FEA does nothing more than analyze what you ask about the elements that you define and if what you ask for is wrong or the elements are poorly modeled, the FEA cannot perform miracles.

 

[TANSL]

On second thought, the open end of the boom will close when the bottom is subjected to tensile. Am I correct?

I think you're right. That is yet another argument in favor of making a closed section but with wings (although there may be other solutions).

 

[RMA]

Section modulus of a symmetrical set of several rectangles is easy to determine! You will need to freshen up your integration from high school.

However you cannot use the classic beam bending formulae because you don't have a constant stiffness value for all the parts of your section. Stress in your material will not vary linearly with distance from the neutral axis as per the beam bending equation. There are formulae for composite sections (generally stiff outer members with a less stiff core i.e. a sandwich beam) which you could probably generalise. You would treat the core, the 45 degree and 0 degree laminates as having different stiffnesses.

Finally, for an open section like this you need to consider the buckling modes. In horizontal bending your side walls will deflect and change the section, which will allow for buckling at lower load than if the section stayed constant. If have always used standard sections which have lookup tables for buckling, or gone straight to FEA, so I don't know whether it is possible to find these results through hand calculations for arbitrary sections.

Alternatively, you could approximate the section as a rectangular U section (take two extreme cases at the maximum and minimum widths of the section), and show that those easier to calculate sections have massive margins of safety on all failure modes.

Thank you for better articulating my concerns around the bending formulae and section modulus. I think this is the concern I was trying to describe. I like the idea of approximating with rectangular U sections and ensuring massive safety factors.

 

[RMA]

Not as easy as I thought. It looks like a cantelever with a diminishing load but there is also torque (rotation) due to force on the sail at a distance away from the mast. This makes it a compound load (combined load formula). There is tensile, compression, and shear.

The shape of the boom is definitely tapered and open on one side but has a fixed cap on both ends to hold the furling mechanism. To idealize the section and find N.A., this needs integration of areas.

Further, the boom is supported by a vang, making the effective length smaller. So diminishing load + support. This requires superposition method to calculate strength and flex.

This is very useful. Thank you for the force analysis. I think we all gained a better understanding of the forces on the boom by seeing this. Personally, I have never seen anything like this and find it extremely informative.

 

[catahoula]

Can't comment on the engineering, but as far as fabrication, it wouldn't be too complex to first mold the U-shape boom, then add an inner layer to create the partial box. You could fill the cavity with an extremely light foam, or bond in a rigid piece and vac bag more fabric over the joint

 

[RMA]

@RMA , take a look at what others are doing. Perhaps they adopt a "less fun" shape of the cross-section of the boom, but for some reason, perhaps not fun at all, they decide that the resistant section should be closed, even if they incorporate some wings forming a box, with an open side, containing the furled main sail.

TANSL,
I think you are right in that the best way to approach this is by just creating a closed section with non-structural wings forming an open cradle for the mainsail. After reading everyone's input, I agree that a fully open section in cored composite is probably too complicated, especially given my limited engineering abilities.

Also, I reached out to GMT Composites for more information. They estimate a pocket boom for my boat would weigh ~68 kg (150 lbs)! This is almost 5x heavier than my initial design, implying that the skins must be >3x thicker.

1) 68 kg is just too heavy to even consider, the current aluminum boom is ~20 kg (44 lbs).
2) Making the carbon laminate 3-4x thicker would bring the cost of carbon from ~$800 to ~$2600, plus a lot more resin ($$$).

A much more reasonable option would be to clean up the old boom and add a lightweight cradle section to the top. This wouldn't require any critical structural changes, would only add about 6-7 kg (15 lbs), and only cost about $500 in materials.

It won't look as good but it will be more functional and still give me confidence in the end product.

Thanks, everyone, for the help thus far.

I'll continue to update this thread with photos along the way.

 

[RMA]

Can't comment on the engineering, but as far as fabrication, it wouldn't be too complex to first mold the U-shape boom, then add an inner layer to create the partial box. You could fill the cavity with an extremely light foam, or bond in a rigid piece and vac bag more fabric over the joint

I agree, a closed section and open box would be relatively straightforward to build. Probably the easiest would be to shape the closed section in foam and wrap the laminate around it, then build the open section from a simple mold, bond and tape the two sections together. However, this will not be cheap, as the closed section laminate will need to be pretty thick, adding weight and material cost. It would certainly look a lot better but at the end of the day, I can't justify spending thousands on this project.

 

[Rumars]

The boom is mainly in compression since the sail is attached to it at the ends and is pulling up. The vang is pulling down on the boom while sailing, rigid vangs only support the weight of the boom and sail while the halyard is unloaded. The mainsheet is pulling sideways and down (the proportions depend on the angle). There is some torsion but that is mainly a consideration for the gooseneck.

@RMA it's not mandatory to put the "cradle" on top of the Al boom, you can just as well fix it to the bottom (or have it wrap around the bottom but fastened to the top of the boom). Just make sure that any carbon is insulated from the Al by fiberglass, and that there is appropriate drainage.

 

[rxcomposite]

I have run similar calculations before and the problem with this structure is rigidity. The problem associated with this is not strength but rigidity. To approximate the rigidness of aluminum alloy, it has to be made up of carbon fiber, prepreg, filament winding, or infusion. Hand lay up has too low modulus. Wet bagging is barely passable. Cross sectional shape also plays a part. Open channel will not work. should be rectangular or hexagonal with just a slot.

Anyway, I have been playing around with resolution of forces. Still complicated but there are several formulas that will apply and all are readily available in the net or YouTube.

 

[gonzo]

I think the force distribution should be higher at the clew. The tension on the luff should be higher than that at the center.

 

[RMA]

Updating this thread with my latest design. It keeps the original boom in place but adds a much lighter cored composite sail cradle mounted from the bottom. I like this design for two main reasons: 1) provides much better support for the sail than a sail bag and 2) hides all the loose reefing lines while sailing. I stripped off about 13 lbs of unnecessary hardware while reconditioning the boom. The cradle will weigh ~14-17lbs and then an additional 3.5 lbs of hardware will go back on. So all in, I'll only be adding 4-5 lbs.

The cradle will be made from 1/2 inch divinycell and single skins of 12 oz biaxial e glass. I haven't done much for calculations with this configuration because the original boom is still the main structural component. The cradle is only supporting the weight of the sail when lowering. This laminate schedule and core dimension should be plenty just based off of my personal experience working with these materials.

Let me know your thoughts.

Profile view of the boom pocket on my C&C38

• RMA
• Dec 16, 2022

Aft end of the boom pocket and original boom

• RMA
• Dec 16, 2022

Foreward (gooseneck) end of the boom pocket and original boom

• RMA
• Dec 16, 2022

Boom pocket dimensions

• RMA
• Dec 16, 2022

Single line reefing design on the original boom

• RMA
• Dec 16, 2022

The original boom was designed with a track for track-mounted reefing blocks on the starboard...