Seeking materials; long fiber Rods?

[science abuse]

I'm looking for a source of lightweight rods that I can slip down down a 3/4" aluminum tube to give it a bit more rigidity, a bit less deflection, without the added weight of a thicker wall tube. I'm hoping to find some fiberglass/carbon Fiber/Kevlar rod or tube stock, and at least 8ft long.

It's for the yard on a small lanteen sail, it bends signifigantly under a good breeze, and I end up backing off for fear of damaging it.

Thanks in advance!

[gonzo]

A thicker wall tube is the lightest structure.

[Cheesy]

Originally Posted by gonzo

A thicker wall tube is the lightest structure.

For a given mass a larger diameter has a greater stiffness

[science abuse]

Only if I can pick and chose my extrusions. Unfotunately I only get to pick between .06 (what's there already) and .115.
Going with .115 wall thks would essentially be like adding the weight of a whole additional .72 O.D. spar to the rig, which would weigh more than what I'm *hoping* to find in a rigid fiber rod.

On top of that, I can't easily find 12ft lengths of alumiunum anyways, 8ft is the best I've found.

Originally Posted by Cheesy

For a given mass a larger diameter has a greater stiffness

Also true, but on top of the supplier problem, it must fit down a sleeve meant for a 3/4" spar.

[Cheesy]

part of the problem will be that a rod to slide inside your existing tube probably wont add much to the overall stiffness of the spar, if you need stiffness/minimum deflection a carbon tube would be the way to go without the aluminium, on the other hand if deflection is alright and you just dont want to break it a pultruded fibre glass rod could work, it wont be as stiff for its mass as the aluminium tube though

[science abuse]

In a perfect world, I'd have carbon parts for both the boom and the yard, and batons for the sail area forward of the mast, etc.

But in the end, this is a boat that cost $200 brand new in 1978, I have other projects more deserving of full carbon spars. I'm not looking to lavish it too heavily, just enjoy it with impunity.

[gggGuest]

If you put carbon down there you'll be in electrochemical hell so that's best avoided, and I really am unconvinced that glass will make very much difference.

[ancient kayaker]

A bundle of rods loosely attached together is not as stiff as a single rod with the same overall diameter, so you can fill the entire tube without much improvement. Mostly they will just help to prevent the tube collapsing at one place in the event of a failure.

For best results you will need to glue the rods together to stop them sliding past each other when the tube is flexed: even better glue them to the inside of the tube. This will make a solid rod much stiffer than the original tube, but much heavier due to the resin. There are other ways, carbon cloth on the outside of the tube, or standing rigging. Alternatively, perhaps you can figure out a way to get material or thin rods glued to the inside of the tube using an inflatable rubber tube ...?

Cheesy is correct though, the best approach is probably to use a slightly larger diameter tube: stiffness is proportional to the 4th power of the diameter and strength is proportional to the cube of the diameter so it multiplies up fairly rapidly.

[science abuse]

Originally Posted by ancient kayaker

carbon cloth on the outside of the tube,

This is actually on the table, though I'm saving the cloth I have for another project and would prefer not to use it on this. Again, a larger diameter tube won't fit, otherwise that would have already been done.
Mcmaster Carr can get me 10ft lengths of fiberglass rods, solid, in the sizes I need. A 1/2" or 5/8" should hass some stiffness, but would be heavier than the .115 wall aluminum tube.

Looks like there's no perfect/easy solution, as usual.

Originally Posted by gggGuest

If you put carbon down there you'll be in electrochemical hell so that's best avoided.

That's something I'm not familiar with, what will happen? Excessive oxidation?

[ancient kayaker]

I just had another thought. Applying carbon to the outside of the tube may merely add stiffness without adding much strength.

To add strength the 2 materials need to share the load. Because the carbon is stiffer than aluminum it may reach its yield strain well before the metal, in which case it will break before the aluminum is sharing the load, allowing the full load to fall on the aluminum. This can lead to series failure of the 2 materials.

The yield strain is given by the yield stress divided by the linear modulus of elasticity, if you have those data. Ideally it should be similar for the 2 materials.

You are getting good advice above and are just going to have to bite the bullet (Tho, thicker wall does almost nothing for the weight). You could temporarily stiffen the structure by shoving a broom handle or something in there (spruce dowels?) but buying carbon tubes of the correct diameter to replace is the way to go. You are going to weigh a ton by the time you shove glass rods in there and they won't much help.

[science abuse]

I may try a larget diameter solid fiber rod, 1/2" thick and 10ft long, since it's only a $11 experiment.
Biting the bullet is indeed in order. I'm fussing about weight, but I'm forgettting that I'll be picking up some power with this as well. With less deflection, the sail will dump less wind and use more of it to push the boat forward.

[gonzo]

How critical is the extra weight? Are you racing the boat?

[capt_jack]

Perhaps if you also used a spray foam insulator to help bind the rods to the outer structure?

There are fiberglass / composite rods made for the electrical trade known as "glow rods" which are used to fish wires.

A cheaper source of composite rods would be to convert modern "fishing" rods. These are made of graphite and carbon composites but like the glow rods mentioned are built to "flex". The addition of a spray adhesive of some sort would help bind everything and take out some of that flex, perhaps?

In a few years or so you would be able to feed two sets of chemicals into a space and watch them form a honeycomb lattice, ideally constructed of carbon nano tubes: http://pubs.acs.org/doi/abs/10.1021/ar050013n

[science abuse]

Originally Posted by gonzo

Are you racing the boat?

Only in my own mind. The entire boat, assembled and ready to sail, weighs less about 100lb. Minding the weight isn't as much a concern about speed as it is trying to keep it close to its superlight status.

Originally Posted by capt_jack

In a few years or so you would be able to feed two sets of chemicals into a space and watch them form a honeycomb lattice, ideally constructed of carbon nano tubes: http://pubs.acs.org/doi/abs/10.1021/ar050013n

Now that is damned cool!