Aluminum Cross Beams on a Wooden Catamaran - What Think Ye?

[Gone2long]

Ahoy Threadmates,

I am considering buying a cat which has been designed by it's owner with some unusual features which I'd like to "float" by everyone.

Firstly, the boat is a 35 foot expoxy over wood build, but it uses aluminum cross beams. Wondering if anyone has run across a design using such a construction method, and, if so, what they thought of it?

Secondly, one of the aluminum beams is bent, in order to compensate for various forces that would come to bear on the structure. My son, who is an engineer says this makes sense to him, but such stuff is beyond my understanding at the moment. Has anyone run into such an anomolie?

I know, that the common knowledge would dictate staying away from something like this, so let's talk merely about the design and materials.

Any help that can be offered would be greatly appreciated.
Regards,

G2L

[SailFastTri]

Bent or curved? There's a difference. Can you post some pictures? I see no issue with the choice of materials, but it's a question of implementation.

Also, there's the question of how well proved this design is. If it's owner-designed and only sailed in sheltered waters, that's different than if the prior owner has sailed it around the world or if it was built from plans sold by a known designer and there are many others out there.

[DaveOnCudjoe]

Wood/Epoxy with Al beams is common in smaller tri's but I am not aware of any larger tri's, say greater than 25 ft in that method. The Brown A-frames used an Al. truss. Bent is bad,curved is OK. Being aware of the loads on my 31 I can't, though not an engineer, imagine an Al section I would feel good about given the weight savings of composite box beams. Dave

[DaveOnCudjoe]

Gone2long,
Sorry, I didn't catch the Cat referance. Much the same applies but in a cat, the beam is easily reinforced with a"Dolphin Striker". Cat's don't interest me much but it seems several examples of foam sandwich with Al beams exist and wood epoxy wouldn't bother me too much noting the comments in my earlier post. I once looked at a McGreagor 36 but found considerable corrosion in the Al tubes. Dave

[Gone2long]

Originally Posted by SailFastTri

Bent or curved? There's a difference. Can you post some pictures? I see no issue with the choice of materials, but it's a question of implementation.

Also, there's the question of how well proved this design is. If it's owner-designed and only sailed in sheltered waters, that's different than if the prior owner has sailed it around the world or if it was built from plans sold by a known designer and there are many others out there.

Thanks for your input,

No pics available at the moment, however the cross beams are not bent but "curved upwards" as in a slight arch with the top of the arch being halfway between the two hulls.

As per the design, the owner calls it an FB35. Have not been able to find a reference to this design on the internet.

Any help would be greatly appreciated.

Best regards,

G2L

[Gone2long]

Originally Posted by DaveOnCudjoe

Gone2long,
Sorry, I didn't catch the Cat referance. Much the same applies but in a cat, the beam is easily reinforced with a"Dolphin Striker". Cat's don't interest me much but it seems several examples of foam sandwich with Al beams exist and wood epoxy wouldn't bother me too much noting the comments in my earlier post. I once looked at a McGreagor 36 but found considerable corrosion in the Al tubes. Dave

Thanks Dave,

Gives me something to look out for, but I am wondering how you found the corrosion INSIDE the tubes. I'm a relative novice at construction materials and trouble shooting them, so give me a clue. Looks like I am going to have to do my own survey so I can use all the help I can get.

See my earlier response to our other thread mate for more.

Thanks again,

G2L

[Gone2long]

Originally Posted by SailFastTri

Bent or curved? There's a difference. Can you post some pictures? I see no issue with the choice of materials, but it's a question of implementation.

Also, there's the question of how well proved this design is. If it's owner-designed and only sailed in sheltered waters, that's different than if the prior owner has sailed it around the world or if it was built from plans sold by a known designer and there are many others out there.

Also, the owner shows documentation that he has sailed it some 3,000 nm. All of this was done in the S. China Sea harbor hopping most of the way but with some significantly long (4 day) passages over relatively open and shallow seas which have been known to create some uncomfortable swells.

Tell me what you think,

Thanks again,

G2L

[noelex 77]

Originally Posted by Gone2long

Thanks Dave,

Gives me something to look out for, but I am wondering how you found the corrosion INSIDE the tubes. L

What grade of aluminium was used? (if the builder cannot answer that question run!) If its a marine grade aluminium corrosion inside the tubes should not occur unless there are some dissimilar metals or other cause.
Where the aluminium contacts the wood is where I would be concerned. Marine grade aluminium will not corrode in seawater, at all, but it relies on air to provide corrosion protection. Trap it against a piece of wood with some moisture and you will often get problems.
It does seem common for aluminium cross beams to be used on fiberglass cats, which would induce many of the same concerns, so in practice it can done successfully

[barnakiel]

I have seen alloy beams in a couple of composite multis. Pictures perhaps?

b.

[DaveOnCudjoe]

G2L,
In the case of the Mac 36 the ends of the tubes are open and the corrosion was evident in the bottom of the tubes. I have seen corrosion in 6061 T6 which is a recommended alloy if not annodized. Dave

[bangkaboat]

G2L,
(I'm assuming I know the one you're posting about)
I'd have greater concerns about the attachment points, than any other issues with the tubes. You could always end-cap the tubes. If the attachment points are simple through-bolts, over time the holes in the tubes will be stretched & there is a risk of cracking(tearing), but this can be re-engineered. The Mac cats were well-known for issues at the tube attachment points & were mounted much like the bow tube on the one I think you're referring to. I note that there is only one lower shroud/side directly athwartships of mast, no uppers. Personally, I'd prefer duals, slightly fore & aft, & single uppers to the present chainplates, but again, you could add that if you felt it necessary. Nice deck clearance, better than many production boats. That Stiletto is attractive, albeit smaller. I think that the tubes should all be cradled, but better to ask someone more knowledgeable of cat design.
Mike

[bangkaboat]

I see that the bow tube is capped.

[Gone2long]

Originally Posted by noelex 77

What grade of aluminium was used? (if the builder cannot answer that question run!) If its a marine grade aluminium corrosion inside the tubes should not occur unless there are some dissimilar metals or other cause.
Where the aluminium contacts the wood is where I would be concerned. Marine grade aluminium will not corrode in seawater, at all, but it relies on air to provide corrosion protection. Trap it against a piece of wood with some moisture and you will often get problems.
It does seem common for aluminium cross beams to be used on fiberglass cats, which would induce many of the same concerns, so in practice it can done successfully

Hi,

Yes the aluminum is marine grade, and I have the specs on it, though, frankly, I can't find them at the moment.

I agree, the junction between the wood and aluminum would be the critical place to search for leaks, cracks, rot, etc.

Thanks for your help,

G2L

[Gone2long]

Originally Posted by DaveOnCudjoe

G2L,
In the case of the Mac 36 the ends of the tubes are open and the corrosion was evident in the bottom of the tubes. I have seen corrosion in 6061 T6 which is a recommended alloy if not annodized. Dave

Thanks,

Appreciate the clarificatrion.

Regards.

G2L

[Gone2long]

Originally Posted by bangkaboat

G2L,
(I'm assuming I know the one you're posting about)
I'd have greater concerns about the attachment points, than any other issues with the tubes. You could always end-cap the tubes. If the attachment points are simple through-bolts, over time the holes in the tubes will be stretched & there is a risk of cracking(tearing), but this can be re-engineered. The Mac cats were well-known for issues at the tube attachment points & were mounted much like the bow tube on the one I think you're referring to. I note that there is only one lower shroud/side directly athwartships of mast, no uppers. Personally, I'd prefer duals, slightly fore & aft, & single uppers to the present chainplates, but again, you could add that if you felt it necessary. Nice deck clearance, better than many production boats. That Stiletto is attractive, albeit smaller. I think that the tubes should all be cradled, but better to ask someone more knowledgeable of cat design.
Mike

Points well taken.

PM me and we can talk more about various intricacies of this design. Just trying to get a broad overview here of what most folks might think.

Thanks for all your insights and input on the various threads posted.

Talk to you soon,

G2L