Unstayed mast build for small canoe based trimaran.

I am planning to build a replacement unstayed mast for my 16ft trimaran. The previous mast was 13 years old, and failed due, I think, to a combination of corrosion and pushing it too hard when I should have reefed. The previous mast was a total of 18ft / 5.6m tall, carrying 44 square feet (4 square metres) of sail. It is silver anodised, grade unknown. The bottom 12.5 ft (3.87m) of the mast was 2" 16 gauge (50.8mm diameter 1.6mm wall) and the top 5.5ft or so (1670mm) was 1.5"' 16 gauge (38mm dia 1.6mm wall). The top section sits down inside the bottom section a further 10" (258mm). The mast is stepped into a tube from hull to deck, 2ft (600mm) and the boom sat at 47" (1190mm) from the bottom of the mast, just shy of 2ft (590mm) above deck. The bottom 5ft (1515mm) of the mast has an internal reinforcing tube 1.5" 10 gauge (38mm dia 3.3mm wall) which extends about a foot (300mm) above the boom. The internal tube is isolated from the outer with thin plastic pipe, but this stops slightly short of the top of the internal tube.

So, questions.



1) It has been suggested to me that 10gauge 3.3mm is overkill for the internal reinforcement tube, and 16gauge 1.6mm would be fine (and lighter, cheaper, more flex) What are your thoughts?

2) the point of failure was right at the top of the internal reinforcement tube. This coincided with two spots of corrosion. Close inspection of the mast revealed light white powdery corrosion inside the mast, which seems too superficial to be a problem , but two spots of dark grey corrosion, probably starting from mechanical damage to the anodising, which I had taken to be superficial, but had actually rotted right through the outer tube, not holed but dark grey crumbly stuff. Any thoughts on the apparent two different types of corrosion?

3) I plan to make sure the plastic isolation extends well beyond the end of the internal tube to ensure this isnt a point of wear, but I suspect it will always be a point of stress. Again, any thoughts?

4) I plan to use 6082 grade mill finish, and finish with wax. I can get this locally, whereas I have to travel 250 miles for anodised, and it is more than twice the money. I'm also wary of the corrosion that seems to be associated with superficial damage to the anodising. A wax finish is easy to maintain. Any (uk available) recommendations for wax? (without going mad - some of the wax kits online cost more than the alu tubing) Is there anything special about 'marine' wax treatments? (Auto wheel waxes are very readily available here in small quantities, and would have to counter very harsh and salty conditions) . I should note that the boat and mast live indoors, and are only at sea for 2 weeks or so max.

Many thanks for any thoughts.

 

For canoes and dinghy unstayed masts I like fir trees. In a new growth thicket they grow tall and skinny with few branches, often choking out leading to a head start on air drying. Firm with the right amount of spring with the strength of naturally grown concentric circles.

 

Id be looking at a used carbon windsurfing mast perhaps ?

 

Thanks - for a different boat I'd be tempted but because of how I use the boat I'm inclined to stick to the original construction as much as possible.

 

Thanks-I did consider using a windsurfer mast, (I have a few!) but other enquiries have suggested it would need reinforcing be cause of the different loading around the deck tube, and the diameter of the stepping tube in the deck is a limiting factor. I'm unsure about the point loads around fittings and how to fix to a composite mast too.

 

Can you provide a picture of the point of failure?

 

Surely.

 

Corrosion of Aluminum and Its Alloys: Forms of Corrosion :: Total Materia Article https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=ktn&NM=187

"""The most common type of localized corrosion is pitting, in which small volumes of metal are removed by corrosion from concentrated areas on the surface to produce craters or pits. Penetration rates from pitting may exceed 10 times the rates associated with uniform corrosion. Pitting corrosion may occur on a metal surface in a stagnant or slow-moving liquid. Pitting also may result from crevice corrosion, poultice corrosion, deposition corrosion, cavitation, impingement, and fretting corrosion. """


Pitting Corrision, crevice or poultice corrsion would be the cause.
Water entering from above the plastic was unable to drain out and the water accumulated at or around the plastic causing the corrosion, loss of strength and failure.

I would think that the plastic should have had vertical siping/grooves to take the water from the plastic to the mast interface and letting it move down the mast, so long as there is a place for it to go into the bilge at the bottom of the mast pocket.

 

I would just copy the design but with better materials, and agree that the stick likely collected water (is it capped?) that was unable to drain.

That corrosion combined with the boom being fixed to the more rigid lower section, and the upper section being allowed to bend, puts a lot of stress at the sleeved union.

I have seen and had to fix this issue on a couple of Windrider 16’s and other boats where they failed near a sleeved/steeped section.

Hard to tell, but from the photos that mast tube looks like the quality of metal I’ve seen used on flag poles.

 

That's really helpful, Barry - thankyou. Very interesting paper sounds bang on the money as to what has happened.- especially the rate of pitting and similar corrosion.

SolGato - yes - from what I've read the 6082 grade is a widely used all rounder for masts. I've also read that the silver anodising can aggrevate the pitting corrosion from very small scratches etc. I hope the mill finish will be easier to maintain, with a simple applied wax finish.

I shall give some thought to detailing the plastic packing tube in such a way as to allow it to drain, and to allow easy disassembly of the two parts of the mast, to allow hosing out and drying indoors after a sea trip. The old mast is not well sealed, and I'm not a fan of the detail of the top section inserted in the bottom - seems it will almost inevitably collect water, but it is a detail commonly used in unstayed dinghy masts. I will try sealing with electrical tape or similar. I'll also try sealing the rivets for the fittings with PUR adhesive sealant. I expect that disassembly, washing out, drying and inspection will be helpful. More faff, but hey.

 

Concur, that is pitting corrosion you have.
Im sure over time water moister/spray has slowly gained access between the two tubes, and the constant action of the mast rubs against each interior and exterior surface of the 2 tubes.
That would scratch/destroy the anodised/oxide layer and leave little score marks - pits.
The stagnant water then just increases in pH locally, making the solution, in the pits, more alkaline - ergo pitting corrosion.

If you replace, as is, then you should pay attention to this, the bare surfaces that are in contact, as well as attempting to eliminate any source of ingress of water/spray into that section.

 

@Ad Hoc thanks. I'm putting some thought into how to detail the plastic isolation between the inner and outer tubes in such a way that it doesn't form a moisture trap.

I did consider just using a heavier gauge 2" tube for the full height of the mast, to simplify the detailing, but I shyed away from this, as its stiffer bending characteristics would impart more force into the deck in a gust. plus heavier.
Another thing - the fittings are currently fixed with stainless steel pop rivets. There is no obvious corrosion to the aluminium around them, but I'm considering using aluminium rivets to avoid risk of galvanic action.

 

Sounds wise.
But, perhaps the rivets are a possible source of water ingress too?

 

Yes, absolutely. I shall bed them and seal over with PUR adhesive sealant. Likewise all the fittings, end bungs etc.

 

I like using high strength aluminum self sealing rivets for stuff like this.

They set better if using marginal tool for stainless, eliminate the need for galvanic isolation, and they seal themselves with the tang breaking off flush with the outside head.