Catamaran forebeam shape

For equal surface area can anyone tell me which is the best structural shape out of the common forebeam sections.

Round
Eliptical
Eliptical with a web
D section
Triangular

I am building a 11.9m catamaran and although a round aluminium tube appears to be the cheapest, least amount of work and lightest other than carbon I am still leaning towards building a composite beam.
The reason for this is that I want to glass the tube in but I only have 220mm of bury and was concerned that this may be insuficent to bond in aluminium.
Can anyone with this experiance advise on this.

Cheers
Andrew

 

Andrew

Why don't you specify a little more clearly your project. 11.9 meter catamaran made out of what? What is the beam on the boat. Is this cruising, racing, both? The more information that you can give the forum, the more input you will recieve. As to initial shape, I lean towards a d section.

Tom

 

Alternatively, you could take a belt-and-suspenders approach and use a single layer of carbon over an aluminum tube, molded into the shape you're after.

 

Tom, My catamaran is for cruising with a full bridgedeck built in foam, e-glass and epoxy.

LOA 11.9m
beam 6.4m
hull beam max 1.5m
displacement 4.5t

I have built the hulls in a female stringer mould split down the centre line, hand laid the internall laminates and infused the outer. Also going to infuse the underwing and the bridgehouse.

I require a 5.2m forebeam and my material cost (AU$) and weight estimates are for the beam only:-
Solid Carbon; 20kg & $2500
Aluminium round tube:- 32kg & $400
Glass timber or foam composite; 45kg $1200

Based on this aluminium seems to be the best choice, but because I want to glass in the tubes rather than bolt on to the hulls my concerns with this option is electrolosis and relatively small bury.
Although my bury is only 220mm I estimate that I can extend the glass slieve around the tube to approx 400mm to improve the bond.
But is this suficient?
And what about long term issues with electrolosis, although I have not seen forebeams with this problem, I have seen glass split by electrolosis pressure in other situations.

Are my concerns valid? can anyone with long term experiance with glassed in alominium tubes advise on this please?

So if I dont go with the aluminium tube and make my own composite beam what is the best shape.
Tom my hunch also was the D section, but was hoping that someone with the appropriate software was able to work out actual relative strength/stiffness for the mentioned cross sectional shapes.

thanks
Andrew

 

eponodyne, I thought carbon being conductive would be even bigger problem with aluminium and electrolysis. Is this not the case?

thanks
Andrew

 

Hell, man, I don't know, I'm just a shade-tree engineer with limited experience in these matters. Although thinking about it, it would certainly seem to be an issue.

I do notice that people tend to jump toward carbon fiber when there are other materials available that are *almost* as light and would serve kind of just as well: viz., Kevlar; S-glass. I don't reckon that anything's carved in aramid that says the only base matrix fabric for epoxy resin has to be some hyperadvanced man-made fiber. I just finished building up a kiteboard for myself out of doorskins and Styrofoam, and I used common cotton cloth on the bottom because my sister had some and I liked the pattern printed on it. Had it out yesterday with my cheapo 8-sq-m kite, and it worked just fine, and I saved myself $40 US and probably a pound of weight.

So: In this situation: 220 mm of bury might be enough. Why not then make up a foil-section mold form of very thin cardboard, cut some 3 Oz canvas duck to shape, and glass that in? Certainly would increase the bonding area available, though it might absorb water over time. Cheap. light. Probably strong enough.

 

Bond area not a large concern

Hello Andrew

I would go with the alloy beam. Composite beams look good but I think that after you fair a strip composite beam and add all the uni reinforcing you may find the weight gains less than expected. This is often the case with carbon cruising masts.

Many cats make a fibreglass sleeve that is bolted on to the hull. I didn't do this on my 38ft Chamberlin but sisterships did. I would not bury alloy into the hull. This is where compsosite shines. If you want to make the beam integrate into the hulls then make a composite beam. I would go strip cedar or Kiri as it is very good at handling compression loads. I don't really see the point of having a compression composite built by amateurs that doesn't use a load bearing core.

I think your concerns about bury will not prove problematic. The major load condition will be in the load of the forestay. Each beam end will have to take half of the forestay loading. A composite forestay chainplate would be much narrower than 200mm. If you follow the ideas of composite chainplates you should be fine. This is only the case if you assume that the bridgedeck takes all the wracking loads and the only load condition is forestay loading. Most modern cats seem to be this stiff.

As an aside - make sure you do include composite chainplates in your build. They are so much better than metal ones - no leaks, no discontinuous materials and lovely stress distribution.

cheers

Phil Thompson

 

Didn't read question

Sorry Andrew, I didn't read the question properly. I would not bond the alloy beam in like a composite one. You may need to remove it and this may be impossible.

If you go composite I don't think you will find a best shape. If you assume the seagull striker converts all the bending loads into compression then the forebeam will be a compression column.

Round is probably the best shape fore a compression column. You also have other loadings like that from the nets and the walkway. As the seagull striker takes the vertical loads you may want an ellipse to help in two ways.

First it takes the horizontal loads from the nets and also gives an easier build to make holes for said nets. Schionnings use elliptical beams with a central shear web. Spirited cats use a half round with a flat on top that is easier to build and makes net fitting quicker - have a look and see what will fit.

There will not be one special shape - as always there will be many different approaches.

cheers

Phil Thompson

 

Phil (Catscatcher)
The bury is not a concern to me for a composite beam.
I have some reservations with aluminium but not against it, in a way I was hoping to be told that the bury or long term swelling of the aluminium is not an issue as it is the cheapest, easiest and lighter than composites bar carbon.

If I go with the timber composite then I think the Spirited example is a very good one.

I will be making composite chain plates, I was hoping to use S glass for this but it appears that it is not available in a uni directional form in Aus.
So I think I will end up using E glass rather than carbon.

Cheers
Andrew

 

Eponodyne
What is a cnvas duck?

 

AndrewK, since the forebeam is in compression, a circular section would make the most efficient use of material.

Regarding your concerns over the bond area - if you used a 150mm diameter tube, and glassed 400mm at each end you would have a bonded area of @ 3.14 x 150 x 400 mm =188400 mm2 - or 1884 cm2. At each end. That ought to be more than adequate.

There will be no dissimilar metals in the glassed part of the beam will there? Electrolysis shouldn't be an issue then. Epoxy bonds to aluminium well enough to be used in aircraft construction - there are epoxies available that are formulated for aluminium, if you are concerned. If you paint the beam corrosion will be effectively eliminated as well.

I would certainly use the aluminium. Your own figures suggest it's the best option, and you have not included the additional hours needed to build a composite beam. Spending $2100 and who knows how many hours to save 12 kg doesn't seem sensible for a cruising boat. You should be able to get the compression figures for the section you use from the manufacturer. If you build your own you will never be as sure - especially seeing that you haven't even decided on what shape section to use yet.

 

I would agree that an aluminium tube is the best option, and that 220mm of bury is enough, assuming that the beam is not taking any of the hull wracking loads.

I would suggest an oval mast section. I have found that the round tubes tend to bend aft and to twist in the hull, even if epoxy/glassed. Also there will be a built in track to take netting slides.

Hope this helps

Richard Woods

sailingcatamarans.com

 

G'day,

There are plenty of alloy forebeam cats out there, so it is not a bad option. However, plenty of these have corrosion, electrolysis and fatigue problems, so maybe it is not the best option.

Building a carbon tube is not terribly difficult, particularly if you have built the rest of the boat. Using a cheap mould and vacuuming it, your materials costs for a 20 kg tube break down to roughly 8 kgs of carbon at $45/kg, 2 kgs of glass for the off axis loads @ $14/kg and 10 kgs of epoxy at $16/kg, all aussie dollars. Add in the mould and the paint and the consumables and it is still more expensive than the alloy, but will last forever and is lighter, particularly after the weight of fastening the alloy tube is included. Add in the labour and it is a no brainer.

It is also easier and cheaper to make composite fittings and bond them on rather than paying for stainless ones and their fastenings. You also have much more choice regarding shape. For example a flat top is much easier and safer to walk on than a round tube.

regards,

Rob

 

ha ha so funny, it is to laugh, with this joke of humor you have making!

Looking at the post now, I see I may have been optimistic in my materials specifications.

But even though this is not the thread, or probably even the sub-forum for it, my statement still stands: Why are so many people so quick to jump to carbon? And who says the matrix fabric for the layup has to be manmade? Or that the glue has to be epoxy or poly resin? Any of you materials science types have links for charts showing relative strengths/properties for various layups? Why NOT cotton in resorcinol?

Sometimes I think we let the tail wag the dog on this. For the average boatman, a couple pounds extra weight is probably a decent tradeoff for increased longevity due to stouter construction, and/or lowered toxicity of materials. Look, for example, at the Tornado catamaran fleet. Put the top crew on a well-constructed wood boat, and I'd lay odds of 4:3 that they'll be competitive in open competition.

 

G'day,

No reason at all why you can't use resorcinal and cotton. It is called tufnol and was one of the early composites. Very good for bearings and similar uses. Weighs less than carbon/epoxy, but is nowhere near as stiff. Requires large amounts of heat and pressure to produce it, so is probably just as environmentally unsound as carbon/epoxy.

There are no natural glues with anywhere near the strength, toughness and waterproofing ability of epoxy.

I jump to carbon for jobs where stiffness and/or strength for light weight are important, such as unstayed masts. No other material comes close. Using tow and low tech methods, it need not be very expensive.

The average boatowner is quite happy in a polyester/chopped strand mat boat with average performance. However, someone who is prepared to vacuum infuse a foam/epoxy hull is a fair bit above average, so spending money to save a few kgs in the bow, and to eliminate the alloy problems may make sense.

If top sailors want to win at the highest level, they need the same equipment as the other top guys. Tortured ply and cedar strip tornados are almost as stiff as vac bagged carbon ones, but not quite. Autoclaved ones are stiffer again. The rules are the problem, not the competitors.


regards,

rob