Boom diameter

[DanWeiss]

[rich1]

I think Dan makes an excellent point, and this is where advanced composites really squash aluminum. It becomes very easy to vary flex characteristics simply by how the layup is accomplished.

I've recently started reading some books on advanced composites.(I figured since I've been using them for over 20 years, perhaps I should learn how to do it properly).

I haven't gotten very far yet, and already I'm beginning to see that the only significant advantage that aluminum will have over carbon, (when both are used in optimum configuration) is that the aluminum will bend before it breaks. And if you take weight out of the equation, it'll be cheaper. What is even more to the point is that the correct use of kevlar in the mix will improve it even further. (I hate working with the stuff, but it does have it's advantages)

I was poking around for a while trying to get various weights and stiffness numbers for competing brands but it proved to be a bit more elusive info than I had originally thought it would be. From the limited numbers I did find, it seemed to me that the the carbon booms were coming in around 2-3 lbs lighter on a 200cm model. While that equates to an overall weight saving of nearly 30%, I guess you have to decide if you're willing to pay $550 for that 30%. And of course that's not 30% off the overall weight of the rig, just the booms.

I'd be curious to poll everyone as to when and where various booms have broken from personal experience. I may even start an new thread.

[isobars]

[rich1]

I just did another search, and this time I found all kinds of numbers I couldn't seem to find before. And I must say the numbers don't really work well for anyone's carbon booms (at least in the popular 200cm size). The 2 to 3 lb number that I stated was the based on 7 1/2 lb aluminum HSM booms and their supermodel carbons (4lb 6 oz) But when I went back to the site I realized I was way off base! Firstly I have no idea where I came up with the 7 1/2 lb number, 'cause it's not, and secondly the supermodels are a pretty unique boom. I looked them up on the web because I recently saw them on the beach and I was curious. A fairer comparison would be the playmate compared to the alloy, and there is only 260 grams (1/2 lb!) and worse, they quote virtually identical stiffness.

All that being said, something is wrong here. Advanced composite booms should wipe the floor with aluminum. They don't even make masts with it anymore. Cheap masts are glass, not aluminum. The one thing that seems to be apparent is the grip diameter, with your pound to pound-and-a-half of weight savings you usually get a mm or two taken of your grip. And in the above HSM example that's not even true!

I'm beginning to wonder if maybe they're overbuilding the carbon booms these days so they just won't break. You break a set of aluminum booms, you get bummed, swim back, then go buy another set. You break a set of carbon booms, you freak out, and you call the shop or the manufacturer complaining.

Just a thought.

Chris

[DanWeiss]

A couple of observations: Carbon booms tubes rarely are 100% carbon. Most are a sandwich affair of carbon and fiberglass.

Carbon boom makers worked very hard from about 2000 to 2003 to address durability concerns expressed by windsurfers using really, really big sails. You might remember that just about everyone's carbon boom front end disintegrated when used with FW sails. Adding stiffness and durability to the front end means adding a lot more material and, consequently, mass. Same thing with the tail piece and especially so with the wide, racing tail pieces.

As well, there is a given amount of force transmitted through the boom at any time. Increasing the tube stiffness will cause the softer materials to deform more. It seems to me that once boom tubes became stiffer the front end and back end pieces needed to be beefed up lest they fail.

I'm not sure I would call it overbuilding but necessary durability given the lessons learned during the early FW years.

[rich1]

I guess the question is, that after working on the carbon boom concept for the last 15 years or so, has the balance point arrived where the very best booms are only about 20% lighter than their aluminum counterpart, and is this as far it can go? Far be it from me to dump on all the hard working guys at the various companies and factories but it seems that the true advantages of advanced composite construction are not being capitalized on. I think that engineering a set of booms is considerably more difficult than it appears. I've been mulling over all the stresses on a set of booms in my head and it's pretty complicated. (I think) I think I may have identified what may be the part that's missing but that could turn out to be naive to say the least. I'll be seeing a very good friend of mine who is a structural engineer in the next week or two, and I'm going to see what he thinks. He used to windsurf.

As for my original problem, I am still going to build a set of composite booms or two, but now I see it's not going to be a 2 day affair, so I've decided to buy a set of off the shelf aluminum booms for the interim. I'm not admitting defeat yet, just executing a strategic retreat Succeed or fail, I'm going to learn something, and for sure I'll share my results with the community.

Chris

[dllee]

Problem seems to currently be....
Nobody makes aluminum any stiffer than what is currently availible in the US. As said, my Ergo booms are lighter AND stiffer than any carbon boom, including new HPL's, and Fibs.
Also, we insist on tiny diameters (which the Ergo is), so we need thicker walls to retain stiffness in both carbon and aluminum.;
Another thing, we also insist on stiffer heads, which always weigh more.

[shreddbob]

Hey Chris--Maybe this info will help you. I apologize for all the data, but once I got started I just got pulled in...

I've broken booms mostly right where the head joins the arms. All my 1980's aluminum Fleetwoods failed this way. This was also a major problem with the old style aluminum/plastic head 1990's Fiberspar "World Cup" carbon models--electrolysis ate away the aluminum where it contacted the carbon tube arms. It wasn't until full carbon composite heads that I had any semblance of carbon boom reliability, with my Fiberspar "All Carbon", HPL, and Maui Sails modern booms--these booms also being about 1/2 pound heavier than the 1990's Fiberspar "World Cups" that always broke. My North aluminum boom from the 1990's is my only surviving alu model, owing to the detachable head that allows rinsing and drying at this critical head to tube junction.

I've put together a table showing some weights for booms I've owned. These are actual measured weights, in pounds. I've not been using any larger slalom aluminum booms (fear of breakage) so I only have the wave aluminum for weight comparison to carbon.

NOTES FOR TABLE BELOW

* Center of Gravity point, measured as percent from the front, with boom at 4.7 sail size.
(I did not measure C.G. for the slalom booms since too much of a pain to do with harness lines and such attached now. But I can say that the HPL, with that heavy tail, sure feels sluggish compared to the Fiberspar, swinging it around and while sailing!)

** This is the North "Progression" model, that features push pin snap together assembly of head to body tubes, and tail tubes to tail end. Disassembling the head for rinsing with fresh water keeps corrosion out of the equation at this critical area. Not a bad boom, with a beefy over-designed head, my only real complaint being the grip diameter (1.35") is bigger than all my other small diameter wave booms (1,28" or 1.23"-Maui Sails).

*** This old Fiberspar World Cup is a "heavy" one for the time, with beefed up arm tubes. (I broke two before getting this "special" one under warranty.) It has been retrofitted with a modern 100% carbon head. It still retains the original tail that has carbon tubes and aluminum "U-shaped" tail end- piece. this boom is every bit as stiff as the 2003 Fiberspar All Carbon and 2008 HPL slalom booms (surprising given the alu tail-end piece and 1-1/4" extra width). I guess this boom is testament to the value of "beefed up construction", since it has lasted so long and is so stiff. The two broken ones (1994/5) this replaced were significantly lighter (4.8 lbs.) and less stiff (though even if these had beefier tubes I'm sure they'd have broken anyway due to the electrolysis issue).

SUBJECTIVE STIFFNESS

- Wave Booms: The Maui Sails are noticeably stiffer versus the other carbon waves, which are about equal to each other. (And the Maui Sails has a slightly slimmer grip diameter versus the others too (1.23" vs. 1.28" which is a noticeable difference to my hands)--these are impressive booms!) If the Maui Sails is rated at a "10" for subjective stiffness then the other two carbons are "8", and the North aluminum is "7".

- Slalom Booms: The 1995***, 2003, and 2008 all feel about the same stiffness, which is interesting since the noticeably slimmer HPL has a 32.5 mm (1.28") grip diameter, versus the Fiberspars at 34.4 mm (1.35"). The 1994/5 Fiberspar World Cups are long gone (broken) but I know they were not as stiff as the 1995 beefed up Fiberspar.

TAIL PIECE EVOLUTION, HPL

The 2008 HPL's , both wave and slalom, stand out in that their carbon tail pieces have gotten significantly beefier/heavier versus the 2003 wave (also used for slalom in that era) HPL carbon tail. The extra weight is not noticeable on a little wave boom, and the seemingly bulletproof tail may be an asset in the waves. For the 2008 slalom size, however, this 1/2 pound extra tail weight (levered out at say 200 cm or more from the mast) was noticeable to me...so much so that I retrofitted an old style HPL carbon tail piece to this boom (at great effort I might add, since twin pin hole spacing/pin size and adjustment clip design were all different!). I heard that HPL started outsourcing the wave and slalom tail assemblies in later years, which accounts for the beefier/heavier tails. For the race sizes I believe those were always made in house by them.

[rich1]

[shreddbob]