Mast step truss, inevitably:(

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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs »

Checked for a gap ... mine is also closed (#1085), with evidence of copious amounts of sealer at the joint. I may drill a small hole at the aft end of the raised portion of the step to see if I can peek into the cavity ... if there is one.
MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 »

I can see why previous owners would have closed the gap up, as that 'should' be the only place water could get in, in reality, all the previous owners have achieved is prevent any inspection.

Bob, thinking about inspecting the new, improved, Bob designed truss (hereafter named Bob-truss!), perhaps we could emulate the original design? Allow inspection over the top of the aft bulkhead? Perhaps even lower it to half height or remove it entirely? I doubt it would have had any structural significance beyond being a barrier for the polyester resin?

I had a quote from my local Stainless company who have a very good reputation, and to fabricate the truss to your design in 5mm 316 stainless will be £216 including VAT and about ten days to fabricate. I investigated using Acetel engineering plastic and the recommendation is to not use it for load bearing applications - so back to good old fashioned steel.

Time to drop the rig and cut out my old truss, what fun!
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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs »

I can see why previous owners would have closed the gap up, as that 'should' be the only place water could get in, in reality, all the previous owners have achieved is prevent any inspection.
Mine looks like a factory-done job to me, part of the step installation. Maybe Albin recognized the deficiency and changed the installation procedure in later models. Some more info from other Balladeers would be helpful. I'll post a photo of mine soon.

Good luck with your re-build, Mark. Take lots of pictures!
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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs »

As promised, photos:

This one is a view of the upper aft part of the bilge bulkhead just aft of the mast step (looks like I skimped on the paint a bit!). The filler material that is oozing from the bulkhead/floor joint looks original. My floor is quite solid, must be well bedded.
Image

This one is a full shot of the bilge bulkhead just aft of the mast step. Tapping on it with a screwdriver handle it sounds solid top and bottom and hollow in the middle section. Time to drill a wee hole and scope it. More to follow ...
Image
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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs »

I drilled a 1/4" hole into the aft bulkhead of the mast step, midway up and slightly off centre to miss the truss. I went in about 6" (150mm).
Initially the material was fibreglass, then about 1" in it turned into a softer hard foam-like substance which seemed to fill the entire cavity.
Here is a video of the hole.
I used a small hammer to tap all around the raised portion of the mast step, and on the floor either side of it. It's solid, and only becomes hollow aft of the rear mast step bulkhead. Needless to say I was much relieved not to find a rusty mess, and am confident Albin must have revised their step construction some time before my boat (#1085) was built.
I also measured the rig tension and it had not measurably changed since set up a month after launch date almost 2 years ago.

It's haul out time for new bottom paint next week which will give me a chance to check the forward hull~keel area for cracks, although when I had the boat at home and removed all the old anti fouling paint the only evidence of damage was toward the bottom of the keel ... probably an accidental grounding some time in the past.
Time to go sailing!
MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 »

Great news Peter, Albin Marin must have seen the negatives in having that void and the potential for water ingress and decided to entomb the truss entirely. Looking at Bobs truss, the part of it that threatened failure was above the resin where water could enter, gather, and sit - if yours had been sealed up from construction, there is a good chance water would never have been able to penetrate and thus your step will good forever and a day. Wish I had found that when inspecting mine! :lol:

It does make you consider the design of a replacement truss - whether it should be entombed in some way. We have to assume, in the future when our Ballads may not be owned by us and loved quite so well, they will be left to stand again, and any cavity around that truss will once again have standing water in it. That being the case, we need to engineer the new truss to be able to 'live' indefinitely submerged. Stainless Steel submerged is not a great idea, Stainless Steel entombed is not a great idea either (think crevice corrosion where the threads go through from the mast foot, http://www.turnstonemarinesurvey.com/FAQ11.html). Also, as Bob pointed out, stainless could eat the aluminium mast foot, so perhaps Aluminium is the way forward?

Okay, so in that case, how do we prevent crevice corrosion of the new truss? Sheath the whole thing in Epoxy and then make sure the bolts have plenty of silicon on them to prevent water ingress? :P

Edit: Or, as SS is stronger, and the the old Mild Steel truss didn't eat the aluminium, use Duralac on all bolts and rely on the fibreglass foot to prevent galvanic corrosion... perhaps I'm leaning towards that. Glass the truss in along with the securing wedges and aft bulkhead, protecting the truss with both glass fibre and resin from the water that may enter in future years, leave enough space over the aft bulkhead to get a good look at it, and make sure the forward bilges do not drain into the mast well, but into the main bilges.
Bob McGovern
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Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern »

Good presentation of the tradeoffs, Mark. Those questions are what had me initially leaning toward composites for the truss; of all available materials, composites have the fewest issues with water, salt, corrosion, or galvanic action. Not to say 'no issues,' but the fewest issues. The only reasons I've veered back towards metals are detailed in the posts above: provability of a one-off composite truss for the expected loads. (Start there: what ARE the expected loads? What are the outliers?) How will a plastic perform under high shock loads and/or long term compression? Basic GRP is good in tension, but compression isn't really its thing.

Certainly someone can build a polymer/composite truss that will withstand any forces a thirty foot boat can deal out. But the longer I looked at the question, the more I realized I wasn't qualified to say what such a truss would look like, nor how it should be assembled. :oops:

I would say full encapsulation of most metals in anything -- foam, polyester, epoxy resin -- is a recipe for trouble. It's where Albin went wrong. "Mild steel should be okay if we just surround it with enough resin." First rule of boats: Water.Gets.Everywhere. :| While our truss is most eroded at the top of the resin, it is severely rusted from front to back, top to foot. Had it been stainless steel installed the same way in the 1970s, it likely would have looked fine above the resin level but severely rusted below it. Which adds the danger of false security: shiny above, decaying out of sight. Stainless steel chainplates are the famous example. It's the 1" inside the decks that will kill you.

Aluminum also corrodes in salt water, especially when mixed with nobler metals. But most of its non-galvanic corrosion is at the surface, like bronze: formation of fluffy oxides & sulfates, a little pitting, but nothing unmanageable.

So where I am right now on the truss:

-- Metal, because metals are known and knowable. Either stainless or aluminum will be fine. Bronze would be fine. Titanium would be awesome. Magnesium would suck.
-- A well-designed truss in wood, composite, dried elephant dung, or concrete would work perfectly; but I don't know how to design or construct a suitable truss from composite or dung. Concrete+salt water+metal fasteners offers its own set of nightmares. And while I work with wood professionally, I also have some curious superstitions about where it belongs. Our house is entirely steel, for example. :lol:
-- Observing the boat axiom Water.Gets.Everywhere (with the corollary "Water wrecks everything eventually"), our plan is a corrosion resistant material with good air circulation, careful water-ingress prevention measures, enthusiastic drainage scheme for when prevention fails, and easy access for inspection & maintenance.

We're probably overthinking this whole issue, and I believe there are any number of equally correct solutions. But the known consequences of getting it wrong cause a worrier (like myself) to turn over the matter endlessly.
MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 »

The more I consider it, the more I am swaying back the other way to a series of epoxied hardwood bulkheads (Teak? Oak?) with thick layup on top (err, how thick?). As a rule of thumb, I've heard it said the compressive loads of the rig can be assumed to be around that of the weight of the boat, so 3 tonnes... I was considering an alternative to stainless as something like the below (clearly the shape is more complex than this towards the front end):

Image

Image

All bulkheads primed with a few coats of epoxy resin first to keep out water, with end grain pointing up, bedded on West System epoxy putty thickened with Colloidal Silica, installed one at a time. To finally fasten the foot in place, drill oversized pilot holes, inject epoxy into these, and then use beefy coach bolts coated in release agent so they can be removed when the time comes (additional fun reading here http://www.westsystem.com/ss/assets/How ... enance.pdf :geek: ). Humm.

I have one complicating factor to my truss extraction - the marina will only drop the rig when I am in the water!! I really need to avoid the cost of another lift and launch, thus I will be getting the truss out whilst in the drink, which makes me somewhat nervous. What I propose to do is drill four inspection holes at the corners of the section of cabin sole I want to remove with the mast up, and then when the rig is down, I will cut between these hole with a jigsaw set to a specific depth (to limit the changes of me sinking my pride and joy). Then the great smashing will begin to get the polyester resin out... whilst in the water... best wear my swimming trunks :lol: .

Bob, when removing the truss, did you have to do anything to the bulkhead immediately forward of the truss or was it simply removing those three bolts? What do we think of an alternative to SS?

edit: So, if we consider west system epoxy as the matrix and teak as our reinforcement for a composite option, West System has a compressive strength (which is the one we are interested in, as other forces on the foot will be trivial in comparison) of 11,500 psi (http://www.westsystem.com/ss/typical-ph ... roperties/), and teak around 8,400 psi - so considering that, even if the whole weight of the mast were concentrated on one square inch of the structure the compressive strength would be well over 4 tonnes... or am I over simplifying this. IT Manager here - not structural engineer :).
Bob McGovern
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Re: Mast step truss, inevitably:(

Post by Bob McGovern »

Solid slabs of endgrain teak are certainly strong enough... but.... Anyone who has dug out and replaced end grain balsa core knows just what a nasty mess results when water reaches what is, in essence, a bundle of drink (soda) straws and cannot escape. Water Gets Everywhere; Water Destroys Everything. And when wood becomes wet -- or dries out -- it moves. Teak will move about 1cm (per foot) in width (tangentially) and a bit less in thickness (radially) from 6 to 12% internal moisture content. So if any moisture does get at the wood -- even small amounts moving thru the keel laminates, for no plastic is truly 100% waterproof -- the solid wood plug will swell and blow the keel sump open. Hydraulic forces in water-filled wood can split granite boulders.

Teak costs more pound for pound than stainless steel or aluminum; good teak is increasingly hard to find; that much teak would certainly cost more than a skilled welder. And speaking as cabinetmaker with fully-equipped wood shop directly beside the boat, that looks like waaaay too much effort templating, fitting, shaping, re-fitting, etc. :|

Alan Harris documented in the UK Ballad newsletter a truss replacement in wood/wood products which is elegant, quick, and inexpensive:
http://www.ukballadassociation.org/page25.html#2013

Also approved by a friendly Naval Architect. :) If considering wood, I'd opt for a design that looks a lot like that. Strength down the spine, lateral stabilizers at each end, and room for water to escape if/when it bypasses all your careful preventative measures. :( Because Water Gets Everywhere. It's a tribute to water's persistence rather than a moral failing on our parts. Mr. Harris has laid out a really, really fine wood-composite solution.

BTW, white oak was commonly used for floors and framing on wooden boats. Its rot resistance is almost as good as teak's; it's strength is better; and it tends to be vastly cheaper. Roughly 1/5 the price here in the US.
MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 »

Thanks for the reality check Bob - you are quite right, making that area 'solid' would cause more problems that it would solve :oops: . And besides, how can I argue with someone with a workshop that looks like this??

Image

Oh for that space! I have to share my garage with large quantities of storage boxes and about a dozen children's 'ride ons' as we have no loft - not easy when you are handling full sheets of plywood (note to self, order half sheets next time...).

I digress! I have seen that design before, and indeed, it would be much easier to implement. I spoke to a boaty lumber yard locally here, and they suggested the same as you re wood selection, so Oak it is if we go via that route. Lots of epoxy and a tick layup over the whole lot and it would be solid for a good many years to come (forever?).

Image

Back to the stainless option, the bit that leaves me scratching my head about it, and what made me retreat from that solution was as follows:
  • Galvanic corrosion - I presume our current fixings through the aluminium mast foot are stainless steel? What is to stop these eating the mast foot?
  • Crevis corrosion - If we presume water will get everywhere, we can assume it will get UNDER the stainless truss if it were epoxied down. It would also get at/under any wedges we used to support the truss.
  • Lateral support - The lateral forces are going to be negligible, however I am still unsure of what sort of wedges I would employ to bond the SS truss laterally in to the hull? Something like the below?
Image

My rig is due to be dropped on the 11th of June, rig back up on the 25th of June! So decision time is rapidly approaching. :lol:

I'm a bit miffed I will have to do the job in the water - wailing on my hull with a club hammer whilst afloat is not my idea of a good time!

What do you think?
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