Carbon versus Ali main beams

Discussion in 'Multihulls' started by waynemarlow, Mar 23, 2015.

  1. waynemarlow
    Joined: Nov 2006
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    waynemarlow Senior Member

    With the recent advances in carbon manufacture and the now quite widely available stock carbon tubes at not that high prices, is it time we reviewed the age old connundrum of carbon v ali main tubes.

    I have seen a number of comparison forum threads before ( sadly I can't seem to now be able to access them as most are over 5 years old ) that with all the pros and cons of each type, it was an almost equal draw.

    Yes carbon is lighter but by the time you add rovings off axis to hold the tube together and put additional glass rovings as the top layer to give durability, it is not much lighter. Any mounts of such things as nets and attachment points are relatively difficult in carbon, and any damage can lead to failure.

    Ali on the other hand is tough and widely available, easy to work with, very under rated in regards to strength and easy to mount things to.

    Lets have a discussion once again on the merits of each.
     
  2. Jetboy
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    Jetboy Senior Member

    I bought aluminum beams last week.

    My boat is going to have telescoping beams. So I needed two sizes of telescopic tube. Needing a minimum of .010 inches of clearance, I found nominal 4" schedule 40 extruded pipe has an OD of 4.5" and ID of 4.026. 4" mechanical tube has an OD of 4.000. So they telescope with a nice fit. I doubt you'll find any carbon product that does this in this size range.

    Also,

    20' extruded 6061 T6 seamless schedule 40 (.223 wall thickness) pipe is just over $8 per foot. Drawn 4" 1/8 wall mechanical tubing 6061 T6 is only $6.10 per foot.

    I don't know anywhere I can buy carbon tube with comparable structural properties for close to this price. And with Aluminum I was able to drive to the distributor and pick up what I need the same day - off the shelf. No shipping costs.

    Additionally 4" .125 6061 T6 is 1.8lbs/ft. So the 30' that I'll use will weigh about 54lbs. How much weight can you realistically shave by going with carbon? 25lbs? Is that worth the extra cost? To some I'm sure it is. For a cruising type boat - I don't think so.

    If carbon were similar in price I'm all for it. I'm building the rest of the boat in cored composite, so I'm obviously not opposed to FRP. It has many advantages. I simply haven't found carbon to be cost competitive. If you have some sources of similar carbon tube and pricing I think it would be worthwhile to compare.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    There is no conundrum. Once you have your geometry you apply the loads. The response to the loads shall dictate what material to select based upon your budget, skills, availability and design criteria.

    That's it.

    If you go into any such design with a fixed or prejudiced mind set you'll only get what you initially assume. Never assume, calculate!
     
  4. Jetboy
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    Jetboy Senior Member

    I think the decision is made after you have the loads.

    Either Aluminum or carbon can be used in 99% of applications interchangeably with a weight penalty for aluminum and a cost penalty for carbon.

    I suspect the ultimate question is really how many grams do you save per dollar?

    If you're using round or square tubing, I'd guess the cost per gram of weight is high.

    If you're building any other shape of beam to optimize clearance, folding, etc, formed aluminum gets pretty expensive due to the need for special tooling. Of course you can form complex shapes and variable thickness tubing in aluminum with water forming but I suspect carbon becomes much cheaper in that scenario due to the dramatically lower cost of necessary tooling.
     
  5. Stumble
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    Stumble Senior Member

    One issue I see is that the limit is not likely strength but stiffness. Switching to carbon likely means a substantial reduction in necessary size, which means a reduction in the cost of the part. In other words it isn't fair to compare a 4" aluminium extrusion to a 4" carbon extrusion of the same wall thickness. But I am not an engineer of any stripe, so can't begin to guess the real gains here.

    But i looked up the cost of 4" uni tubing and at .125 wall thickness runs $50/ft. Ignoring the modest change in thickness, carbon is twice as stiff and 3 times as strong, so let's assume this tube could replace the aluminium tube above. So at 1.25lbs/foot versus 3.77lbs/ft for 4" Schedule 40 aluminium the carbon is 1/3 the weight.

    Online prices for 4" Schedule 40 aluminum is about $20/foot, so carbon is roughly 2.5 times more expensive at about 1/3 the weight.

    If we need 20' per crossbeam, that works out to a total of...

    .............aluminium ....... Uni
    Cost .....$800 ............. $2,000
    Weight ..150lbs .......... 50lbs

    Is shaving 100lbs from the weight of the boat worth $1200? Who knows. For a performance boat it will increase speed, reduce wetted surface, ect. For a cruising boat it adds back 100lbs in load carrying capability. The trade off value of course can't be decided by the numbers but by the designer and buyer.
     
  6. Jetboy
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    Jetboy Senior Member

    The only issue I see with the calculation is that - at least in the USA 4" schedule 40 aluminum is not $20/ft. I bought some on Friday of last week - as a cash customer for $8/ft for 4" schedule 40 seamless 6061 T6 - picked up for free. How much is shipping for a 20' carbon tube? I'd guess conservatively $500.

    I literally paid $185 out the door for that piece of tube.

    So the comparison at least for me - is $370 vs $2,000.

    Also FWIW, I'm using 4" DOM structural tube for my primary beams. The schedule 40 4" nominal pipe is comparable to 4.5" structural tube size. It's actually 4.5" OD.

    4" DOM 6061 T6 is $6.10/ft. Or $122 for 20 foot piece. It weighs 1.8lb/ft, so for a 20' beam it's about 120lbs. I'm using 16 foot sections, so I'm at 57.8lbs for both beams.

    Even if carbon weighed 0 lbs, is 60lbs of weight worth ($50-$6.10)*(32) + $500 = $1905?. For me the answer was no. I'd rather have new set of custom sails than 60lbs less weight.


    Also: where are you finding carbon tube for $50/ft in 4" .125 wall? That seems like a very good price. Much better than I've seen anywhere I've found it available. Most I see in that size are more like $500/ft.
     
  7. redreuben
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    redreuben redreuben

    Another thing to consider is water stays, can they be done away with if you go carbon ?
     
  8. Stumble
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    Stumble Senior Member

  9. waynemarlow
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    waynemarlow Senior Member

    Cost is only one of the factors, others are what are the intended uses and the benefits of light v heavy materials in those uses. Sometimes having very light beams will mean less inertia and roll, would that better sea motion mean that your wife would come sailing with you, if so the extra $1000 would be worth it.

    Unfortunately by losing the seastays you have to beef up the hull structure so much to take the extra cantilevered loads, that any weight loss from the lack of stays is equally made up by having heavier beams and hull laminates.

    Carbon is dropping in price and I think we really need to perhaps consider like for like. Comparing 4" Ali tube to 4 " carbon tube is a tad unfair. My guess is that approx a 63mm OD roll wrap would be in the same ball park. I can buy that for about £ 50.00 per metre here in the UK. The equivalent Ali 4" is £ 15.00. But its nearly twice the diameter and considerably heavier.
     
  10. Jetboy
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    Jetboy Senior Member

    I think we must be looking at different parts of the website. I see 4" .127 wall tube for $559 per foot. Or $22,360 for two 20' pieces. For the right boat - that's probably a reasonable cost. For an 18' day sailer, it's pretty hard to justify.
     
  11. Jetboy
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    Jetboy Senior Member

    To equal the same rigidity (beam deflection) of a 4" .125 wall aluminum tube, you'll need a 4" .055 wall carbon tube.

    For 63mm OD - you'll need over .25 wall thickness to equal a 4" .125 wall aluminum tube.
     
  12. Stumble
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    Stumble Senior Member

    Jet boy,
    those prices are per piece not per foot.
     
  13. Jetboy
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    Jetboy Senior Member

    " MOST TUBES LISTED AND SOLD BY THE FOOT

    Please read:

    If you want 6 ft., Add 6 to cart
    If you want 2 pieces @ 3 ft., Add 6 to cart + 2 cut @ 3 ft."

    If you look in the far left column they will also say "priced per foot".
     
  14. bruceb
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    bruceb Senior Member

    Prices

    I have read the same thing Jet. Carbon is really expensive. Also, "onlinemetals" is about twice the price of my local small lot supplier here in Atlanta.
    I decided to buy new outer alloy tubes for my Buc 33, (about $800 total), carbon would have been thousands more.
    B
     

  15. Stumble
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    Stumble Senior Member

    So they have a bad website. Try dragonplate 4" carbon fiber tube $82/ft.
     
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