Carbon Boom Design

[russell_miller]

Probably depends on the type of material your track is. Most replies will probably be for Plexus or Proset. Be sure to fillet around the track as well. You could also wrap the boom and track in a light layer of fiberglass or carbon and then route out where the cloth covers the track. That would be much more structural. Looking forward to seeing how it turns out...

 

[petereng]

Probably depends on the type of material your track is. Most replies will probably be for Plexus or Proset. Be sure to fillet around the track as well. You could also wrap the boom and track in a light layer of fiberglass or carbon and then route out where the cloth covers the track. That would be much more structural. Looking forward to seeing how it turns out...

To respond to your original question Russell:

1) As you are an engineering student or similiar have a go at building the boom. Yes it is a simple beam and it can be analysed and built in a garage. The vertical loads can be estimated by the maximum loads that can be applied by hand to the vang and b then x these by the purchase of the vang. Its the lateral loads from the sail that are the problem. These loads are a result of the membrane action of the sail and are considerably more than the direct "aero" loads of the sail. You can place a stiff spring at the clew and go sailing and see how far it stretches or compresses. Then use the spring stiffness to figure the load. Or if you have some contacts use a load cell much better.

2) Or you can back calculate the strength and stiffness of an existing successful boom. Most would do this.

3) Since I do a lot of FEA work if you have access to FEA you can model the sail and calculate its "edge loads" then use these to design your boom. If you have a friendly sailmaker they can get someone in their company to calculate sail edge loads for you.

4) It will be the building of the thing that is difficult not the design. To produce a high quality laminate that matches your laminate design is the trick. If you infuse the boom you will achieve better strength/stiffness than a prepreg part. Not quite as good as a wound part but very good in the scheme of things. If you hand laminate and do a vacuum bag I think you may be wasting your time. These things are compression critical and making laminates good for compression is a big learning curve.

5) Build two and statically test the first one to destruction so you know you acheived the design values. Service failures will then teach you a lot.

6) I'd suggest that you try to do it in glass. It won'lt be as stiff as carbon but I've found booms that a are a bit flexible perform betterthan ones that are super stiff. Just like automatic flexy mast tops. Infused glass is super strong, nearly as strong as carbon and at least 4x stronger than aluminium.

So go on get it done! Peter (Composite Mast Engineer)

 

[Bill E Goat]

On a 12ft skiff we were doing maximum luff tension of about 75kgs. One method is to set your rig with the sail on and pull it on until you achieve your maximum mast bend. Then remove the sail and put a load cell or even fishing scales and pull it on until you get the same amount of bend. That will tell you how much load there is at the clew.

Its not that hard to build, mine lasted fine and that was in a 12 which was pretty loaded up.

 

[petereng]

was talking to a boatbuilder the other day who suggested wrapping the mandrel in paper so that the paper is about 1 or 2 mm thick then wrapping in plastic film. The idea being that when the carbon shrinks it crushes the paper. Pulling the mandrel out at about the 24 hr mark. as the mandrel is pulled out the paper unravels and is then pulled out after.

Any thoughts/has anyone tried this

The best release for a round mandrel is to use mylar. Get some thats 0.2mm or .25mm thick. Nearly any plastic that is stiff and is not attacked by the resin is OK. Its the stiffness that counts. If the tape or paper is capable of being crushed in your hand then its too soft. It may work but if it folds over when you extract the mandrel it will ball up and jam and its then curtains for the part. You will not get it off. If you can;t crush the plastic by hand (something like a overhead projector sheet stiffness) then its unlikely that it will fail in this way when you extract the part. But if you are going to this level why not use the tube or boom or mast section make a glass female mould (some chop strand matt and cheap resin) then build the part inside this. Only takes a day or two extra. Use infusion so its one piece and all of those problems go away. Take this one step further since you are now going to make a mould. Shape up some timber or mdf into an interesting shape one that is more optimised for the application and take the mould etc. Much more interesting. Round tubes are boring! If you want a round tube you may as well buy one. Peter

 

[petereng]

So I liked the idea of molding a separate plate and then bonding it to the top of the section. JB's boom appeared to fail by a combination of crushing(vang strop tightening around the section) as well as bending/compression failure at the top of the section. With bonding a reinforcement plate on as a separate piece, I can make the part look a bit more professional as opposed to trying to bond the carbon directly onto the tube. Will bonding a separate piece have a significant strength impact or is it a novelty in comparison to adding several full wraps around?

Using JB's broken tube as a mandrel, I wrapped it with packing tape and coated that with mclube as a mold release. I laid up a 4 layer laminate(8 oz carbon-5 oz kevlar-9 oz glass-8 oz carbon) and vaccum bagged it to consolidate. Everything un-molded nicely and I trimmed and sanded the part to shape. I think I should have added a few more layers, but I am not sure. The part is pretty stiff in bending but since it is not a full round it is not fully rigid(can be twisted).

What do you guys thing about sanding the mating surfaces and bonding this to the boom as shown? I was thinking of using epoxy and filler, but perhaps something like plexus(hate the mess) is a better move? Please let me know what you guys think. I am also going to make a new boom strop that doesn't tighten around the boom when loaded. This should relieve the "crushing" component that appeared to play a role in JB's failure.

Great job on the patch. Epoxy or PMMA (Plexus or equivalent is fine, just follow the makers directions) The reason this type of falure occurs on these booms is that the laminate is optimised for bending not shear. At the vang there is high shear loading as well as bending load. These tubes usually wound have really good tension and compression properties. But shear is another question. So the patch you built is the best "outside" way to go. Trying to attach vangs without holes is a tricky one. Back to the shear thing. This type of laminate has around 20% off axis fibre in it so its hoop strength is limited. A wrap of hoop carbon about the same thickness of the tube will put back the required strength. This acheives too things. Improves hoop stiffness and improves hoop strength. Bending tests on these types of tubes usually fail by hoop buckling not compression or tension failure. The "inside" way to fix it is to put a larger hole right though it vertically and connect the two sides of the tube so it can't buckle. I have a philosophical aversion to holes so use this last but often it is lighter then overwrapping. There are technical reasons why not to wrap as well (but its easy) so take your pick! Hope this helps. Peter

 

[olivers]

A good way to make tubing, selective reinforcements if cut down, and even certain repairs, is to use carbon fibre and/or glass braid/sleeving.

I've made 6' tiller extensions for 29erXX sailing that are only 10% over the weight of CST's professionally made ones for the same length with 6 layers of braid.

I use http://sollercomposites.com/ to source my materials. Their speed and service has been impeccable.

You can find mylar film of various thicknesses at TAP Plastics.

The keys are mylar film to wrap and subsequently help release the tube from its mandrel, the biaxial braid that contracts when stretched like a Chinese finger puzzle, and coated heat shrink tubing of the appropriate size.

You can mix and match the materials into layers however you want (glass for toughness, carbon for strength and kevlar for snap resistance ....)

The process is very easy. You can lay up two layers at a time and it wets out the braiding perfectly.

This technique is used by the model rocket community for their casings.

Good luck!