Epoxy/glass vs aluminum

Hello, I have 31 feet of mast, but I need 34 feet of mast. The shape is approximately 4 1/2 in by 6 in in a semi Aero form.
The bottom of the Mast is clean and so I propose to wrap 4 foot of it in fiberglass cloth, after having put a release agent on the aluminum, to a thickness slightly Superior in strength to the aluminum. So, assuming that I am using woven roving and no carbon fiber or a vacuum bagging or Ouija boards (i hear stories...), how much thicker does the wall section in fiberglass and epoxy have to be than that of the spar? I'm assuming it is 6061. I'm not looking for exacting precision. Just knowing it is stronger.
I guess I am hunting for some engineer type to say something along the lines of "woven roving and epoxy in the same Mast shape will need a wall thickness X times as thick as the aluminum to ensure equal or superior strength."
It will be the bottom of the Mast so a little extra weight is of little concern. Thank you

 

I am a bit confused - you want to extend the mast length by 3' using fibreglass, and you will have a 4' overlap of the aluminium tube with fibreglass, hence giving a total length of fibreglass of 7'?
Or will the total length of fibreglass be 4', with a 1' overlap on to the aluminium?
How are you proposing to attach the aluminium to the fibreglass?
It might be easier to simply make a 4' long timber base and shape it so that 1' can go inside the ally mast?
Or do you need the whole length to be hollow so that lines and electrical cables can be down through the base?

 

I apologize, I had a rare Moment of clarity in my own head and figured everyone else could follow along haha. I am using the Mast as a mold of sorts. I will make a 4 ft long tube on the outside of the mast and then I will slide it off. Then I will sand the inside of the tube for one foot and sand the aluminum for one foot also. I will need to make a sail track of sorts. So there will be one foot of overlap. I will probably put some wood in the bottom section. Something like a two-by- 6 cut down to fit. I wish to retain the shape for air flow , and aesthetic to a much lesser degree. The boom will attach to wood extending out of the fiberglass section. That section of wood will be about six and a half or a seven ft in length.
I guess I am hunting for some engineer type to say something along the lines of "woven roving and epoxy in the same Mast shape will need a wall thickness X times as thick as the aluminum to ensure equal or superior strength."
I'm certain that if I built up three quarters of an inch in wall thickness it would be superior but I don't want to go that Overkill.

 

I am still a bit baffled.
Can you give us a bit more info about the boat this mast will be used on please?

So the GRP tube will overlap the ally mast section by a foot.
Will you rivet them together?
Or just bond them with an adhesive?
Re cutting down a 2" x 6" timber to use as a plug - if the mast is 6" x 4.5" (approx) surely you would have to glue two sections together first in order to make a timber plug?
You said that "The boom will attach to wood extending out of the fiberglass section. That section of wood will be about six and a half or a seven ft in length."
I am well baffled now by this. If you are extending the mast by 3', why is the timber 6' - 7' in length?
Can you perhaps post a sketch of what you have in mind?

Re scantlings for your fibreglass tube, it would be much easier if you do have a timber insert / plug to make up the additional length - you could still have the fibreglass on the outside if desired. Timber has excellent compressive strength.

 

Me too... a simple diagram/sketch would clear all this up...to avoid any ambiguity!

It is not as simple as that. It is the "EI" you need to address. Which leads to:-
The deflection,
Then shear,
And then the fatigue.

Swapping one material for another and "beefing up" the thicknesses to compensate, is only one small part of the differences that need to be analysed and addressed.

 

My understanding has been that aluminum doesn't resist fatigue as well as fiberglass. This is why I did not included mention. This leaves shear and deflection. Which two of these is the most inferior to aluminum for a given wall thickness in a tube , and by how much is it inferior?

 

The possible problem that comes to mind is the transition point where the alloy ends and it is all glass-epoxy, that is the weak point, the glass will be more flexible, and that join will be a bit of a hard point. Depending on what strain you put on it. I think I would be inclined to do something like this, a rounded taper to to the mast top, if possible, maybe it isn't practicable.

 

It would be easier, cheaper and safer to splice a section of aluminum with the same profile to it. The second best option is a section of a similar profile and an adaptor sleeve.

 

I agree that it would create a hard point. There will be a single Diamond spreader arrangement as it will be a rotating mast. The amount of taper I could impart would be minuscule because the mask wall thickness. This math was off a 30 foot boat. I will be putting it on a 28-foot catamaran I am building plywood. this is a low-dollar affair.

 

I have to disagree. It would not be cheaper safer or easier, for me. I already have the Mast and the fiberglass and epoxy. I have no idea where I could find a similar section as the Mast is old.

 

I guess it is impossible to just simply put a photo on a post. We wouldn't want to make things easy...

 

This Thread is getting way more complex than it need be. Does anyone have an approximate knowledge of the shear and tensile strength of aluminum and fiberglass? I have never uploaded a photo to this site and it seems beyond the cumbersome. Seems like I should be able to click on something and search my computer and tap on the photos. Seems that I have to make an album or a URL or some other silliness.

 

I get to the point where it tells me I have not created an album. I know that I have not created an album. Is there a easily visible link to click on that says create album now? Hell no! That would be too freaking easy! Computers are wonderful devices for creating 100 steps to do something which should only take two steps.

 

For the aluminum you need to know the alloy; they vary enormously. For the fiberglass/epoxy laminate there is no way to tell because it depends on the laminate schedule and the workmanship. Another problem is that you are raising the mast without increasing the mast section size. This means the rigging will probably have to be modified accordingly so the mast doesn't buckle.