Aluminum mast repair

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KrabKrusher
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Aluminum mast repair

Post by KrabKrusher »

I removed my radar and the bracket and now have four 3/8" holes on each side of the mast about a inch apart (in a vertical line) and a single 1 inch hole at the front of the mast, slightly above the top 3/8" hole. I'm a bit worried about local strength. It's a pretty sturdy section - Albin Vega. Should I be concerned? Is there anything I might reasonably do to strengthen it? What if I just want to patch the holes, to keep out the rain?
Drew
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Re: Aluminum mast repair

Post by Drew »

Considering that the mast was strong enough before the radar was removed it should be strong enough now. Assuming that the rig is intact the primary loading on the mast is compression. The difference between filled hole compression and open hole compression in a sturdy aluminum mast section like the Vega is academic. Fill the holes and relax.

Drew
Hirilondë
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Re: Aluminum mast repair

Post by Hirilondë »

Drew wrote:Considering that the mast was strong enough before the radar was removed it should be strong enough now.
This is probably true.
Drew wrote: Assuming that the rig is intact the primary loading on the mast is compression.
This is not true at all. Compression is the primary loading on the mast when at the mooring, certainly not the only significant load when sailing.

If there is no corrosion around the holes nor other apparent damage caused by the holes or not then filling them is probably all you need to do. If you are not comfortable determining this on your own then consult a rigger. But you are right to question the situation.
Dave Finnegan
builder of Spindrift 9N #521 'Wingë'
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Drew
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Re: Aluminum mast repair

Post by Drew »

This is not true at all
I respectfully disagree that compression is not the primary load on a mast while sailing. I do not intend to suggest that there are not other significant loads however the limitation is resistance to buckling due to compression loading.

From Brion Toss's The Complete Riggers Apprentice page 140-141:

"All mast design formulas are variations and refinements on Euler's formula, an engineering cornerstone which predicts the behavior of columns under compression, with allowances made for all the significant variables."

I heartily agree that that KrabKrusher is right to question the situation and that if there is any doubt a professional rigger should be consulted. I apologize if the tone of my prior post did not reflect this.

Drew
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mishnish
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Re: Aluminum mast repair

Post by mishnish »

Small craft naval architect and engineer chiming in!

A mast is not just under compression at all! While sailing there is:

1. A pressure distribution along the aft face of the mast from the mainsail causing distributed bending moment
2. Compressive loadings from the boom resulting in a large compressive point load on the mast
3. Bending moment occurring at the deck interface in the case of a keel stepped mast
4. Compressive AND pull out forces from shroud and stay tension
5. Sag owing to self weight when heeled!

In other words, the mast is subject to a complex and dynamic range of forces. When a mast is designed, the designer will traditionally work out the peak loadings the mast is likely to see based upon the static maximum righting moment, then multiply it usually by a factor of 3 to determine the dynamic loadings. then he will work out his mast section based on this.

It is likely that "globally" you have only slightly reduced the mast capacity to withstand these loadings if you consider the cross section where material loss is greatest.

The risk is that "locally" you have created higher chances of cracking, corrosion and fatigue.

In my opinion, the 3/8" holes are not anything to worry about. I would be slightly more concerned about the 1" hole. Where is the hole? on a single spreader rig, peak bending moment (Area of most bending stress) occurs approx 2/3rds the way up the mast to the spreaders. You are fortunate that this hole is on the front of the mast. This places the hole on the "neutral axis" of the mast beam in this bending area. If it were on the sides of the mast, this would be much worse as the sides take all the tensile or compressive load in lateral (port to starboard) bending!

A possible repair option may be to rivet a patch over it, or have it welded up to restore the cross section... saying "it will be fine" is fair enough, but few people ever use their boat/mast up to their boats peak righting moment, which is when youre likely to find out if it really will be fine! What is your approach to risk?! Remember how much you rely on that stick for getting about, and the mess it can make when it comes down, and the potential consequences. This alone should drive your actions. I agree that if there is doubt, a professional rigger should be consulted.

Hope I have not confused too many people!

Best Regards
chris
http://www.contradanza.co.uk - The restoration of a Vivacity 20, Aberdeen, Scotland.
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Rachel
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Re: Aluminum mast repair

Post by Rachel »

mishnish wrote: Hope I have not confused too many people!
I found your information really interesting. Thanks! That's one thing I like about this board: People who understand things aren't afraid to get into the technicalities/realities of them.

(Although now I'm curious how designers account for the point load at the boom, because I can really see how that exists now that you mention it. Is it just handled by the usual mounting-plate type thing where the gooseneck attaches? Is the whole mast thickness sized to accommodate that "weak link"? There's not usually secret structural buildup inside the mast there .... is there?)
mitiempo
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Re: Aluminum mast repair

Post by mitiempo »

On larger boats there is usually (or should be) a doubler at the gooseneck, also on vang attachment points on both the mast and boom. On smaller boats my thought is that the extrusion is a bit over the strength and wall thickness to start with.
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mishnish
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Re: Aluminum mast repair

Post by mishnish »

Interestingly, "traditional" mast design has never really addressed it properly (in my opinion) and seems to have mitigated the affect of the boom loading by saying its point of contact is close to the deck where the mast beam is well constrained, or spreading the load over the length of the goose neck fitting, but also the mast is typically stiffest in its fore/aft orientation, which helps alot. If anything, a simple point load calc would have been done.

It is only fairly recently in design terms that a well engineered approach to the problem has been implemented with new computer based analysis techniques. I had an interesting one lately. A client wished to take his beautiful 1970s camper and nicholson style steel motor yacht and add a sailing rig! Unusual, but his reasons were sound. I attended for a week and developed a design for the rig that would work. All the client wanted to do was be able to turn his big noisy engines off and sail along at 5 kts in a 15 kt breeze, which we did with 250m2 of sail on two masts with two genoas. I themed the rig to look a little like the Lateen rigs that are traditional to that part of the med.

http://www.spars.co.uk/columbus.htm

The major issue for me was the forestay for the aft mast, which owing to space constraints had to come down onto the aft face of the foremast. We solved this problem by slotting the foremast and letting a massive aluminium plate through all the way to the front of the mast, where it achieved almost 1 metre of total contact length with the front mast wall! We then welded it all up. We had to do this as the only other option was to bring the forestay down on top of the wheelhouse, which would have required almost 400kg of structural steel and massive modifications within the wheelhouse, all of which the client was none too keen on!!!! You can see the fitting in the last picture on the site...

Ill admit that, owing to the time constraints on this project, I had to be fairly brief with my calcs and analysis, and I did rely on the experience of a professional rigger to fill in the gaps. When the two camps work together, great things are possible in a short time!
http://www.contradanza.co.uk - The restoration of a Vivacity 20, Aberdeen, Scotland.
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Rachel
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Re: Aluminum mast repair

Post by Rachel »

That's fantastic! I wonder if that concept will become more popular if fuel continues to get more expensive. Of course I would want to shut the engine off when possible even if fuel were free, just to be rid of the noise.

Really interesting reading about your design constraints and calculations.

Rachel
JoeC
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Re: Aluminum mast repair

Post by JoeC »

I would not advise welding aluminum, unless it can not be avoided. Welding can reduce the strength of aluminum by upto or more than 50% locally in the weld affected area.
As others have pointed out, I personally would not worry about leaving the holes as is or at most just plug them to keep the moisture out.
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mishnish
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Re: Aluminum mast repair

Post by mishnish »

Good point about the welding actually, I think aluminium needs some kind of heat treatment after welding to restore properties.
http://www.contradanza.co.uk - The restoration of a Vivacity 20, Aberdeen, Scotland.
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