broken mast

Last year I purchased a second hand hobie 16 (see pictures here: http://www.thebeachcats.com/modules...mp;file=index&include=view_album.php )

The guy who sold it to me "forgot" to mention that the mast was broken and rebuild from 2 deferent parts (see pictures here http://www.qcm.co.il/mast )
The fixing is 1 feet below the tang.
The mast is not water tight at all (both parts move)

My questions are:

Is it safe?
Can it hold the pressures?
Is it common to rebuild a broken mast like this?
What could happen to the crew or the boat in case the mast broke while sailing?

My biggest concern is for my 3 year old and my 6 year old boy's safety.

Thanks,

Erez

If you are considering doing any sailing with them in the ocean, I would find a new mast.

Caveat empteur, but how did you miss it in the pre-buy inspection? It looks pretty obvious from the pics.

I have never liked repaired masts. I dropped the mast on a P18 I had once and put a good ding in it. I bought a new tube and swapped out the parts, no question. Would not even consider using a compromised mast tube. Also there is the watertight issue, as you mentioned.
We have discussed on the forum how you CAN repair composite to perform and weigh as good as new. You should know that there is no way to do this with an alloy aluminum tube. Strength, stiffness and fatigue life will all be adversely affected, whether the repair is welded, riveted with doublers or bonded.

Jimbo

Agree with all the above

Throw it away and start again. Well actually get a new mast section and transfer everything over.

And welcome to Catsailor.

Where can I buy the tube section for a Nacra 5.2 mast? I have some significant corrosion close to where the diamond wires attach at the upper end of the mast.

-Rob V.
Panama City
Nacra 5.2

From performance Catamarans, the maker. When I bought my P18 bare mast tube it was about $600, IIRC. That's a bundle cheaper than a complete mast assembly, but it will take quite a few hours to swap all the parts. Buy a 100 box of the SS rivets. You will use most of them. Seal everything, including each rivet, with 3M 5200.

Jimbo

Jimbo, dont you mean monel rivets, not stainless? I always heard that SS rivets in aluminium had problems with corrosion..

That is my problem right now. Where the diamond wire connects up on the mast, the two different metals corroded the mast to where I have a few small holes. I know that SS on Al will corrode, but I think that Al o Al will corrode too, right?

When are they going to come out with carbon fiber rivets?

-Rob V.
Panama City
Nacra 5.2

I dont know about alu on alu, but the spike in alu rivets are often plain steel, which corrodes badly. In addition, alu rivets are not that strong. Monel rivets are both strong and pretty neutral against steel and alu. I believe monel rivets are what most riggers use, but perhaps Jimbo know something I dont about stainless rivets.

Carbon fiber rivets? Carbon really corrodes alu, and besides, it would not be very practical as carbon probably is to stiff to deform when you pull the spike.

Thank you all for your replays



Well, it was my first boat and my first time sailing so...

How ever since than i have lernt a lot about masts and I can say that it is forbidden to use Aluminum & Stainless Steel rivets on a mast.
The problem is corrosion, oxidation & Shearing Stresses .
Use only Monel, its a must!!!

Erez

True but everything comes with SS anyway. All the boats I've ever had were built with SS. You could use Cherrymax also, which are stronger than SS and made of aluminum but Monel and Cherry rivets are pushing 50 cent each. Both are a real ballbuster to pull. You really need a air hydraulic puller.

Jimbo

That's backwards. Aluminum corrodes carbon.

Jimbo

Right! It's been a while since I looked on the periodic table. In fact, I have pretty much forgotten how it worked..
But if Alu corrodes carbon, is it correct to say that steel corrodes alu, becouse steel is to the left of alu on the periodic table?

Steel corrodes aluminum, but not because of iron, which is very near aluminum on the galvanic chart; iron is actually only slightly less reactive). Nickel corrodes aluminum a little more aggressively, which is the problem with "stainless" steel. Nickel is the chief alloying agent in SS and many plain steels as well.

I'm not sure if there's a direct connection with the periodic chart; the scale of interest here is the Galvanic chart or anodic index.

http://www.rcboataholic.com/faq/corrosion.htm

In this chart some metals are more 'noble' or unreactive, 'cathodic', and some are more reactive or anodic. Gold is zero on the chart and beryllium is on the other end at 1.85. The farther apart two metals are, the more galvanic corrosion when they meet. The more reactive metal always loses.

Notice where some of the higher alloys of aluminum (like 7075) are on the chart; way up ther near magnesium. And 5052 aluminum is right next to some grades of stainless steel meaning thay are fairly compatible. 5052 and 5056 are commonly used for marine extrusions. Ditto for the "stainless" steels with some high on the chart and some far lower. Remember that the terms 'aluminum' and 'stainless steel' are thrown around as if they each refer to one single material with one set of properties. This is just not true. It matters a lot exactly which alloy you are talking about.

Some SS alloys for example are called 'food grade' i.e. is good for contact with food. Other alloys include lead and molybdenum to facilitate machining. But these are both very poisonous heavy metals so totally incompatible with food.

I have not observed a big corrosion problem on my mast where I did some service back in '91. I used SS rivtes. I resealed both end caps and replaced clam cleats.

The galvanic compatibility can be managed with surface finishes like anodizing. This keeps the reactive metals like SS and Al out of actual cotact, which is why you can get away with SS rivets, to an extent. Aluminum oxide is NOT aluminum; it is aluminum oxide.

Jimbo

Thanks Jimbo, that was very informative. I seem to remember that there was a connection between the periodic table and corrosion. How easy the different materials shed electrons or something like that..

Now, I noticed that graphite was at the bottom of the galvanic table. I tought graphite and carbon fibers was closely related? (we are getting seriously off-topic here, but I think this is interesting)

True, but the full answer is not so simple. Graphite is a form of carbon. Carbon is said to be graphitic (grid or graph-like) when the atoms are aligned in a particual crystalline formation. The properties of graphitic carbon (electrical, thermal, pyhsical) are quite different than for 'standard' non-graphitic carbon fiber, even thought these are both made of the same atoms. It's similar to the situation with diamond which is again a completely different material than graphite or carbon even though it is just another crystalline form of those same carbon atoms. And then there's Buckyball

Jimbo

check ebay. I saw a used H16 mast listed there. You might get a deal if the freight is not too much.

Clayton

Speaking of ball buster to pull. Have you ever just grabbed a rivet and tried to pull it and see how far the mandrel or spike will pull before it breaks? Yes I do mean using a rivet gun of some type.

I've found there are at least two types of mandrels. One that seems to be stainless or monel, and one that seems to be mild steel. Except for the shortest no matter what length rivet I've tried only the one with the mild steel mandrel will pull all the way before breaking. Having the mandrel break early might lead to problems.

Anyone else notice this? And yes I am speaking of monel rivets. I've tried this using a large
scissor type riveter. I have access to a pneumatic I just haven't tried it yet.

Having worked for many years in aviation makes me a sort of rivet conniseur The pull shank is one of the most determinant features of a (blind) rivet. Besides the body material, there are many different types of steel shanks offered from mild to very high alloy to stainless alloys, depending on the compressive force needed. The tensile strength of the shank determines how hard the rivet compresses.

There are even special types of 'rivets' like Hi-locks and Jo-Bolts where the shank is very strong; so strong that a normal puller just won't cut it.

http://www.tpub.com/air/2-4.htm

Also a big puller limits where you can use such a rivet since you must allow clearance for the pull tool to install it. So the center shank is threaded and a special collared nut is threaded and tightened onto the shank to the point of breaking the shank in tension thereby extruding the rivet body; the same thing a rivet puller does, and all you need is a wrench. Despite the presence of threads, these are really rivets.

jimbo

I like that Jo-Bolt concept for the high strength, but is did list "alloy steel". Is that stainless? Could the flush mount version be used in salt water, say, on a mast?

It would appear that the way this is applied would ensure a more secure "pull" than that of a handheld rivet gun (except for a pneumatic). Would I be correct in this assumption?

I think the alloy steel is not SS. I have seen them in what appeared to be SS but it could have been Monel.

Jimbo