Gary King wrote:... Basicly the OD of the tube should be at least the same OD of a full hollow wood mast is at the height of where the top mast plugs into the alu. So you use that to check if your 100mm pole is big enough....

 

Yes, I had planned to do exactly the check you describe. My boat is 6.5m x 2.4m, displacing 740kg empty and with no ballast, just a light cb instead of a keel.

Arne

                                                                   Stavanger, Sunday

You are right Paul.

I had excluded the idea of checking flagpoles etc because I failed to find anything strong enough when I was to rig Johanna. I must remind myself that Frøken Sørensen (Frøken = Miss) has a righting moment only about 1/5 of that of Johanna.

I’ll have a look around next week.

Thanks for the reminder.

Arne

I've not had good luck with aluminum--I have an old binnacle, and it had bad corrosion around the base where SS bolts were used to hold it down. I went to great effort to prep it and the paint is bubbling away at the base again within a year of putting it outside.

I don't think my first layer of prep was correct looking back at it, but I'm just going to put up with it like this for another five or ten years before I try again! So I do understand the fear that this sort of glue job won't work very well.

On the other hand, this sort of job you are talking about should provide a very big gluing surface, and the usual forces will just hold the two parts together in addition. So I think it could hold even though I don't trust that kind of glue much.

If your aluminum pipe flexes too much, I wouldn't hold out much hope for a quick fix with epoxy and cloth. The outer layer takes all the stress first normally. But if there are two layers where one is stiffer than the other, the stiffer one does all the work, and the less stiff one is pretty much dead weight. Just like people who put fiberglass around a wood mast to stiffen it. (An outside layer can provide protection and seal out the weather or UV...but isn't structural strength.)

The only way this would really work is if the two layers are the same stiffness...which pretty much means use the same stuff...like adding an extra layer of fiberglass to an existing fiberglass spar.

Rudolf van der Brug wrote:Hi Arne,

Anodised alu is no problem at all when it comes to glueing with epoxy.

Bare alu needs very good sanding before glue/epoxy will hold.

You can use all sorts of glue on aluminium, provided you sand untill you notice the

hard corrosion layer has gone. You can feel the material underneath is softer.

With the corrosion gone gluing should be no problem.

I did some glassing to alu in the past but it was meant to come loose.

It did, sometimes very easily. I didn't bother to sand all that much as I wanted it to come

off.

At the moment I am working on cargo hatches from aluminium. I am gluing isolation and wooden parts to them using a two part polyurethane glue and vacuum.

Rudolf

Hi Rudolf,

I'm sure you are right and in your workshop where you have a the facilities that you need, you get the results you say. However my experience is that in the field and in the average professional workshop the results are unreliable. I have always use the Gougeon brothers technique put the epoxy on and then sanded and indeed it has stuck but I have no faith in it. I have just seen to many epoxy (both paint & glue) failings when it comes to aluminum.

Arne, I can tell you from hard won experience that nothing sticks to aluminum particularly well. You can improve adhesion considerable by sandblasting the tube before hand but the adhesion is still not as good as it is between epoxy and other materials.

However, I would urge you to look at your design again. A wooden top section puts the most weight exactly where you would rather not have it (note, I'm not saying that it does not work. It does but is certainly not optimal). Check out where your center of gravity is, it may be that a heavier wood mast may still have its COG in a lower position and so may still result in a stiffer boat for you.

Annie's mast made sense for her as good timber is expensive and hard to get here in New Zealand. I do believe that is not the case in your country. Also, you are going to have to make the wood top mast, it really is not going to take all that much longer to make a full wooden mast.

You have a whole winter ahead of you and making a small mast is not really a very big project, so I'd seriously consider building a proper wood mast if I were you.