Minimum bend radius / Dinghy Boom

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#1

Hi there,

I’m thinking of building a carbon boom for a dinghy.

I’m planning to build it with a rectangular section, making the most of it by adding UD on top to carry bending and DB on sides for shear.

I’m thinking ok using an aluminium section as mandrel due to it’s high CTE but the question is. Does anyone has any clue about which should be the minimum bending radius in the corners of the section (ie. in between sides and top/bottom of the boom).

I remember reading about minimum radii of fibers but I can not find anything right know.

I’ll really appreciate your thoughts about this.

#2

Tight corners are stress points! I broke a rectangular boom once, switched to an elliptical shape and never heard of one breaking since. Radii can be very sharp(I have made testpieces with a radius under 1 mm), but more blunt radii are easier to do, and distribute stresses better

#3

Not sure what this is, buuuuuut. If you are meaning a square tube with 90deg bends, and you are pulling at those bends. No, there is not really a min. bend radius, but yes, you will have ALOT of stress at that area. You can maybe reenforce that 90deg bed with a stiffener (think of a flat plate welded on the inside of a corner)
Or add a more flowing design.
or non-square tubing
or add A TON of extra material at the high stress areas to over compensate.

#4

This is especially true of composites which can’t handle the bending stresses nearly a well as isotropic materials can

#5

If you want to persue with the square design, I suggest a radius of 3mm (1/8") minimum. This should be enough to relieve some of the stress encountered in the corners.

Make sure you use a tapered reinforcing stack, depending on the maximum local loads. Vang attachment and sheet are the major stress points, where you need to beef things up.
I suggest attaching these with stainless hoops (I guess Sea-Sure has the right ones) and GLUEING these on, to spread the load. Glue should be a stiff methacrylate, like Plexus MA300 or MA310 (suggesting MA310 for longer working time). Minimum glue thickness is 1mm, keep that in mind. This will spread the load evenly over the complete profile, not concentrated on just a couple of bolts / rivets.
If the boom has no track, you can attach the vang and sheet with woven band, for the sheet rivited to the top of the profile, for the vang crossed over the top, then rivited to the bottom. Topper Boss style.

I sailed one of those, and these had round carbon booms, utterly light. Breaking a boom was rare, though.

Another factor that you will encounter is that if you use vacuum or any other compression method, you will see that the fibers will “crumble up” in the corners, compromising strength. Perhaps tightly compressing the complete stack with a UD at 90 degrees every couple of layers will prevent this.
Also make sure your UDs are encapsulated in biax completely, or they will break (buckle) in compression. Preferably keep the 90degree fiber away from the UD as much as possible.

So basicly you are looking at the following schedule: (from inside out)

-woven 200gr/m2

perhaps multiple series (2 or 3) of:
-biax 45
-UD top and bottom
-biax 45
-90 UD for compression
-biax 45
-UD top and bottom

and finish with:
-biax 45
-90 UD for compression
-woven 200 gr/m2

Of course all depends on the weight of the materials used, Perhaps you can make everything from UD fibers. Quite some work to layup (if layers are different angle left/right the thing will twist during cure)

woven 200 gr is a nice finish, and will not see protruding fibers. you could also use a woven glass, which will show the UD behind it.

Heat as much as possible during cure (80 degrees or even more would be nice) and use vacuum, and you will end up with a nice lightweight boom. From experience I know that hitting a lightweight carbon boom with the head is less painful than an aluminium one. Perhaps the round section had something to do with it as well.

#6

Thanks to everybody for the replies, specially to herman as this is what I was looking for.

Might need a little bit of help with the constructive process, would be helpful to hear your thoughts as well. Initially I’m planning to build just one but numbers can go up to 3-5 if thing go wright. What I’m considering is:

Option A)
Buy a aluminium rectangular box profile of the shelve, 10 swg wall should allow my to file corners to make a small radius (3-5 mm). Wrap carbon fiber tapered plies, shrink tape and heat while curing

Option B)
Get a styrodur or any other similar type of foam and cut the section. Sand with a generous radius the corners. In this option from sheet attachment to the clew the boom can be tapered keeping therefore all the layup constant along the boom.

Because the foam is not gonna stand the exotherm: Lay up the first layer of 0/90 carbon, wrap with peel ply tape and let it cure. Lightly sand and add the rest of layup, shrink tape and heat.

I guess option 1 is the best but carving the boom out of foam will allow me to add an increased radius to avoid any stress raisers and to taper the boom along its length.

#7

Option A will be possibly very bad. The carbon and aluminum will act as a electrical circuit and corrode VERY quickly, especially since it’s water based product. I would suggest a thin layer of glass first, THEN the carbon.
Option B should work well, especially if you add more radius/material in the corners, or bond in a bracket later. Not sure hot much heat you are creating, but if your foam melts during cure due to heat, you might light the resin on fire!!! If you are REALLY worried, switch resins, do a SLOW cure, or do a few layers with peelply outside surface, then add more later.

#8

I’m planning to get rid of the mandrels on both options so corrosion shouldn’t be a problem.
Using aluminum which has a high CTE will help me to pull the mandrel/tube out specially if a heat during curing and cool down while pulling to extract it.
If I decide to go for option b, just pouring acetone into the inner foam will melt it and make it disappear.

#9

For straightness I would go for option A.

Buy or borrow a router to make the radius, you can use woodworking router bits for this without a problem. Wear eye protection.

You could have an aluminium shop cut the section and weld it back together to make a taper.

#10

Thanks again to you all for the help!

Just as a last question, assuming I stick with the aluminum option what should I use as release agent? Can I use just simple wax or should I think about something else.

I’m expecting to need some help of a manual winch and maybe cool the mandrel to extract it but i don’t really want it to get stuck.

#11

I would recommend a very high slip semi permanent release thats designed for mandrels, also use a sealer

#12

Cooling should not be neccesary, but heating during cure is the key. Make sure you have a heatgun blowing through the profile during the complete cure cycle, and wrap the thing in insulation. (can be simple bleeder, couple of layers)

Indeed high-slip semi perm and a good sealer are the key. For instance the sealer GP from Zyvax. For the heck of it, but I do not remember which of their products produces the highest slip. Consult Zyvax for that. (or any other semi perm manufacturer.)

Indeed a winch can come in handy. Make sure you are able to connect the winch to the aluminium. A bit of overlength, and a 12mm / 1/2" hole should be enough, combined with a bolt.

#13

assisted with building sportboat masts during my intership, aluminium mandrel, prepreg carbon layup. No issues with release because of the crimp in the alu. A winch isn’t necessary, but something to get tension on the mandrel and part is :wink:

#14

A guy I know made carbon masts for Europe sailboats. He pulled the mandrel with his car, and someone holding the edge of the carbon against the doorframe. Worked perfectly, that is, untill he pulled the doorframe out…

#15

shakes head Oh gezz.
as for mandrel pulling, many places have a shaped end of the mandrel, and that gets clamped into a pulling device (mechanical, hydraulic ram, etc) and the other end is somehow clamped on…either extra material put on either end, and then it too, is clamped. Once you break the adhesion, as long as you have a taper, it just slides right off.
No taper, might mean you need a long travel winch/ram. I personally have done straight tubes from 1-3" about 30" long, just by pulling. Used Frekot 55 and 700
As for a bent mandrel (can you get pics??) Might be able to “fit” together pins and holes on the parts so they latch together for curing, and then you pull, each piece will come out separately.

#16

haha, ropes around the top end of the mast, lashing straps around the mandrell, both attached to the building.
One time someone came across when my chef was releasing the mast, loads of tension,my chef tired, and the thing wouldn’t came loose. He attended my chef he was pulling the mast on to the mandrell, the wrong way, hahaha. wish I was there at that moment, but it still was a good laugh later on.