Mast step truss, inevitably:(

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Bob McGovern
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Mast step truss, inevitably:(

Post by Bob McGovern » Wed Mar 05, 2014 11:06 pm

After months of patiently spraying the SS machine screws on the mast step plate and tapping with an impact driver, I got nasty today and drilled the screws right out. I wanted to inspect the mast step truss, which many people have had corrosion issues with. This is what I found:
Image
Okay, that has to come out.:( I've reviewed Temptress' photos on the Knowledge Exchange but still have a few questions. Is the entire cavity around the truss filled with thickened resin? Or is it just one layer near the top & hollow beneath?

What have other Ballad owners used to fabricate a replacement truss? Stainless steel? Aluminum? I'd really like to use composite if possible, just so corrosion or galvanic welding would never, ever be a problem again. Do you think a box section or I-beam design would be better than the odd, stepped gussets of the original?

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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs » Thu Mar 06, 2014 6:27 am

Yikes ... that's quite a hole!
What's your hull number, Bob?

Bob McGovern
Posts: 276
Joined: Fri Oct 26, 2012 3:08 am
Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern » Thu Mar 06, 2014 5:50 pm

No idea. Boat is 1972, pre-HIN, and the bronze plate is missing. Big genoas carry the number 18, and one may well be original. So I'm claiming Hull #18, unless somebody has better information.;)

Anyhoo, on the old Multiply site there are a number of discussions about this issue, but all the pictures have been lost except for Temptress', which someone (Nicholas?) saved & reposted on the Knowledge site. If anyone has more photos of this area or repair procedure, we would be very grateful for them. There were rumors Albin changed design or materials of this part in later boats, but no one could confirm it. I would wager our truss was either mill steel from the get-go, or it was galvanized but years of moisture & contact with SS and aluminum simply ate away the zinc. At any rate, it has a reputation for splitting the front edge of the keel when it rust-swells and/or crumbles. So this one has to be replaced.:(

The forces involved are likely not that huge, or the engineers would not have specified 1/8" metal. (One good reason to use thicker here, where weight is no issue, is to provide more corrosion margin.) A 1" wooden dowel would likely transfer the loads if it could be kept perfectly in column. Pouring the sump solid with resin would probably do it. But a bit of elegance here would not go amiss. I'm confident 1/4" or 5/16" aluminum plate would suffice, and it would resist corrosion pretty well. might even slap a sacrificial anode on it.;) 2x3" aluminum tubing (1/4" wall) would also carry that load, no problem; weld plates onto top and bottom.

http://www.grainger.com/product/Rect-Tubing-6ALU2

But for some reason, composites strike me as a good alternative here. No corrosion issues, no electrolysis, easy to tab to the adjoining keel and bulkheads.... Any thoughts on designing such a truss from FRP or carbon? We could lay it up from scratch, or start with pre-fab pieces of G10 or fiberglass beams or tubing.

http://www.eplastics.com/Plastic/Fiberg ... een-I-Beam

The critical areas would be where the top plate cantilevers over the vertical piece (tension) and where the vertical piece joins the foot (compression). Keep the thing from rotating, and you should have no troubles, right?

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Sanlen
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Re: Mast step truss, inevitably:(

Post by Sanlen » Sat Mar 08, 2014 10:41 am

OMG :shock: Now that makes me really worried about Sanlens mast step truss. From what i remember when reinforcing the hull/keel/cabin floor some 20 years ago there were slight signs of rust colored water coming from the area beneath the mast step. I thought it was only the steel plate backing the mast step so I sprayed "lots" of rustprotection into that area before filling it with mixed polyester/glassfabric and building new bulkheads in the bilge, also below/aft of the mast step. At least it stopped the rust colored water :roll: . Maybe I should drill a larger inspection hole and check this again.
Best regards,
Jan
#547 Sanlen
Owner since 1993

peterohman
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Joined: Sun Sep 30, 2012 8:24 pm

Re: Mast step truss, inevitably:(

Post by peterohman » Sat Mar 08, 2014 1:00 pm

Hi
there is a good article about mast step design in the UK Ballad association newsletter summer 2013.
http://www.ukballadassociation.org/page25.html#2013
/Peter

Bob McGovern
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Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern » Sat Mar 08, 2014 5:18 pm

Wow, that's perfect information, Peter.:) It didn't show up on Google search because of the PDF format, but it is exactly what I needed. Quite a mix of brutality & elegance, there. Removing the aft bulkhead will certainly speed chopping out the resin. And we were planning to cut out the cabin sole for repair & reinforcing anyhow -- the new bulkhead design should solve our keel sag issues, as well.

I have a bunch of 3/4" and 1"t phenolic countertop material I may use for the bulkheads. May or may not use it for the vertical truss; its compressive strength edge-on is not ideal.

Thanks for the referral & to Alan Harris for supplying the photos and text!

Jan: I half suspect that, even if the entire steel truss dissolved, the solid resin plug would hold up the mast & direct its force down to the keel. Assuming that top two inches didn't crumple. We could likely just pour the area solid with resin, or cut away the top of the truss & stick a brick in there, and all would be well. Some of the mast step arrangements on much larger boats are frightful: the Westsail 32 is one example. The only danger with the Ballad seems to be the rusty truss expanding and splitting the front edge of the keel layup.

Mild steel in the bilge -- whose idea was that? :x

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Sanlen
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Re: Mast step truss, inevitably:(

Post by Sanlen » Sat Mar 08, 2014 10:04 pm

Bob McGovern wrote:We could likely just pour the area solid with resin
Decision made - drilling holes and pouring resin if I didn't manage to fill the area completely earlier - thanks for the tip :)
Best regards,
Jan
#547 Sanlen
Owner since 1993

Bob McGovern
Posts: 276
Joined: Fri Oct 26, 2012 3:08 am
Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern » Sun Mar 09, 2014 12:40 am

Some previous owner filled the cavity part way with squirt foam. That was sooo effective. :roll: There's two permanent wet spots on the keel of our Ballad, front edge and stbd side, right about where the foot of the truss sits. Wet and slightly rust-colored. :o Guess I will borrow my friend's industrial hammer drill and go at it. Or: use a Sawzall from the outside & remove the entire offending section. Or: clamp a car battery across opposite corners of the truss plate; wait for the whole truss to glow red hot, then pull it out of the melting resin like a bad tooth. :lol:

Good thing we aren't in any hurry to launch Fionn.... On the plus side, mountain snowpack is around 140% normal this year, so our lakes should get enough water to take the little boat out. Gutting one sailboat is much less stressful when you have another one you can play with.

Image

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Sanlen
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Re: Mast step truss, inevitably:(

Post by Sanlen » Sun Mar 09, 2014 9:18 am

Bob McGovern wrote: Gutting one sailboat is much less stressful when you have another one you can play with.
I totally agree. Only stress comes from my beloved wife now that I have occupied 3 :lol: rooms in our house with boat parts.

Actually, we have two more boats 8-)
IMG_3450.jpg
RJ-85
IMG_3450.jpg (59.83 KiB) Viewed 10103 times
P7141079.jpg
Flipper 787
P7141079.jpg (41.01 KiB) Viewed 10103 times
(Excuse the offtopic)
Best regards,
Jan
#547 Sanlen
Owner since 1993

Bob McGovern
Posts: 276
Joined: Fri Oct 26, 2012 3:08 am
Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern » Sun Mar 09, 2014 4:45 pm

You shall have to change the name of the RJ-85 to Polymer. :lol: We also own a Buccaneer18 racing dinghy, sadly neglected. Together they are Grannia, Diarmuid, and Fionn. It's nice to have the entire love triangle, but three sailboats is one too many.

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