Mast step truss, inevitably:(

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MarkRyan1981
Posts: 158
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 » Mon Mar 30, 2015 12:27 pm

All prepared!

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I'm going to drill a hole either side of the mast truss, put a tube attached to a hoover in there and clear out any debris, then take a poke around with the camera. Once I am satisfied it is structurally sound I will then stuff it full of Dinitrol using the probe spray extender to prevent any future problems before sealing the holes up with thickened epoxy.

Any luck with your welder Bob?

Bob McGovern
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Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern » Mon Mar 30, 2015 4:35 pm

Good luck with the inspection, Mark! That's one of those little USB boroscope cameras? Please walk us thru the procedure & tell us what works & doesn't -- it would be a great resource for all Ballad owners. Nice if everyone could inspect their truss without major GRP surgery.

May phone the welder today. Warm weather has me itching to push forward on boat work. I've found an attractive substitute for a fully-welded stainless steel truss: Online Metals sells 6061-T6 aluminum I-beams of almost exactly the right height (8", 20cm) in a few configurations:

http://www.onlinemetals.com/merchant.cf ... top_cat=60

The so-called 'American Standard' style has thicker webs available and rounder fillets between web and flanges, both good for our purposes. So instead of building up a truss from flat stock, you could start with an I-beam and remove material to suit! I'm told welding thin stainless is tricky (read: expensive); and while aluminum flat stock would be thicker, aluminum changes properties more than steel during welding. An aluminum I-beam offers the advantage that the longest welds (top and foot) are unnecessary -- already built into the extrusion. :) We could just tack-weld the stabilizing fins in place, and off we go. Aluminum is super-easy to shape with common tools.

Dunno if aluminum (or aluminium, as they keep trying to push) I-beams are easily available in Europe? I reckon we could get a 15" (38cm) section of beam with 0.5" (13mm) thick web for around $85 USD delivered. Then a little 8mm flat stock to weld on for stabilizers. I'll call the welder & see if he is interested in working in aluminum. If not, may find another welder.

Yes or no, I'll post measurements of my prototype design today, just in case or anyone else needs them.

Bob McGovern
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Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern » Tue Mar 31, 2015 2:32 am

Measurements of a mast step truss, taken from a physical prototype that has been fitted onto our 1972 Ballad. All dimensions in mm. A few comments and cautions:

* This truss fits our Ballad recess snugly -- main bulkhead to first bilge divider; original truss platform to underside of GRP 'cap'. It is slightly taller and very much longer than the original truss, and it is possible not all Ballads have truss recesses identical to ours.

* The foot and top plate of this design are wider than the original, to maximize bearing surface; a new GRP cap will be required after installation. If you wish to re-use the original GRP cap, you should reproduce the top plate dimensions of the original truss (c. 82mm wide).

* Materials thickness and choice will vary by owner preference. The original truss was 3-5mm mild steel (1/8-3/16"); 304 or 316 stainless of the same dimensions could be substituted with minor loss of strength. Aluminum should be roughly twice as thick, 7-10mm and 6061 where available. While I think this design will be much stronger than the original, all present are warned that I flunked out of mechanical engineering at university. :lol:

* Rather than surrounding the new truss with solid resin, we intend to merely brace it to the keel laminates with epoxied spacers, and to bolt it fore and aft to the bulkheads. Good drainage of the foot area is still required.

* Finally, no measurements are provided for the mast base plate holes, nor for the holes attaching the truss to the main bulkhead. I will assume these are slightly different for each boat; greater accuracy will be accomplished by templating your original truss in place, then transferring the hole locations from the template to your new truss. Every 1cm error at the mast foot translates into around 6cm error at the masthead, so it is important to put the base plate exactly back where it belongs.:)

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MarkRyan1981
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Re: Mast step truss, inevitably:(

Post by MarkRyan1981 » Tue Mar 31, 2015 12:28 pm

Thank Bob, that's cracking, I'm going to do a write up of the whole 'bulkheads bonding' and 'mast truss' Ballad discussions for http://www.albinballad.co.uk to dispel some of the myths out there and to make sure this information is preserved for future Ballad owners and to allay any worrys of future potential Ballad owners.

And yup, thats right, its a small USB endoscope, around 7mm diameter. The quality isn't amazing (for £9 delivered there are no surprises there) however it will do the job wonderfully and hopefully put my mind at rest (or confirm my worries!). We are off on holiday this coming weekend (unfortunately not on the boat :() and the weather is a bit shocking to start drilling holes in my mast foot, so I'll likely do it when we have a spell of decent weather so I will have an opportunity to let a couple of coats of this rust converter stuff dry before sealing the holes up with epoxy. I will report back when I have some pictures of my truss! :lol:

Bob McGovern
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Re: Mast step truss, inevitably:(

Post by Bob McGovern » Tue Mar 31, 2015 9:48 pm

Some more photos of the prototype and where it goes. First, this is how the design drawn above compares to the original truss:

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More foot front, back, and sides; more top plate back and sides; more web front and back; more & better lateral stabilizers. The longer web means the truss can be bolted to the bilge divider.

Here is a better view of the cleaned-out recess:

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You can see the 'shadow' where the old foot sat, as well as the shrinkage cracks in the resin pedestal. It was cracks like that which allowed water to get into the truss & not escape. You can also see the drain tubes which connect the forepeak & anchor locker to the main bilge. The plastic tube was a casualty of the chipping-out process but will pressed into service once more. Prototype in place (it's only wood, painted silver), with approximate drill locations:

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And with the mast base roughly where it will go, barring the GRP 'cap' integral with a repaired sole:

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Sanlen
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Re: Mast step truss, inevitably:(

Post by Sanlen » Wed Apr 01, 2015 6:27 am

Impressive work Bob! I believe your solution can become "standard" and copied by all Ballad owners that have a defective truss.

I will definitely buy one of those USB-cameras and get a closer look at Sanlens truss. Hopefully it isn't that bad but if it is, I will use your solution for sure ;)
Best regards,
Jan
#547 Sanlen
Owner since 1993

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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs » Wed Apr 01, 2015 3:48 pm

Great post, Bob. I've saved the entire thing to file ... just in case ... but I hope I never need it!
Your prototype had me fooled: I thought it was alumin(i)um until I read that it was painted plywood :lol:
Please continue posting throughout the whole process.
Thanks!

MarkRyan1981
Posts: 158
Joined: Fri Oct 12, 2012 7:47 am

Re: Mast step truss, inevitably:(

Post by MarkRyan1981 » Wed Apr 01, 2015 4:15 pm

Cracking work Bob, I'll make that page on my website to make sure we lock in all this hard work. I was actually just talking to a fine gent who is marine gas engineer on the east coast who used to own a Ballad - he lost her when her mast drove through her step (I've emailed him asking for details), so this is very pertinent stuff that could very well save our boats.

Keep us informed as you go ahead. You say you are going to reinforce the truss with epoxied spacers - will these be glassed in to port and starboard of the truss itself? What material will you use for the spacers?

Bob McGovern
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Location: Wyoming, USA

Re: Mast step truss, inevitably:(

Post by Bob McGovern » Wed Apr 01, 2015 4:42 pm

prjacobs wrote:Great post, Bob. I've saved the entire thing to file ... just in case ... but I hope I never need it!
Your prototype had me fooled: I thought it was alumin(i)um until I read that it was painted plywood :lol:
Please continue posting throughout the whole process.
Thanks!
:lol: The poor creature was slapped together out of so many different species of scrap plywood, it was visually confusing. I had to paint it to prevent eye strain. Fortunately, I always have a few cans of metallic spray paint on hand.

So ... as an engineer who didn't flunk out of university ... have you any comments or improvements on the design, or materials suggestions? I have not tried, yet -- and I won't until the new thing is made and installed -- but I believe I could snap our old truss between my two hands. Since that never happened in service, that suggests lateral loads where the worst corrosion occurred are just not all that severe. By the time you get to the truss/poured-in-resin interface, most lateral forces have been neutralized by the rigging, mast partners, cabin sole laminations, and the truss-to-bulkhead bolts. What's left, then, is fairly pure compression. And pure compression is easy. Even the smaller, original truss design re-made in 3/16" (4mm) stainless steel would support over 70,000 pounds (31,000 kg) of compression at the foot. The new design made from a 6061 aluminum I-beam with 0.5" center web should support over 157,000 pounds (71,000kg). (But I flunked out of engineering, so view those numbers with skepticism. :oops: )

Mark: Yes on the spacers/wedges epoxied to the hull. Probably made of phenolic, because I have lots of scraps lying around. And the keel-to-hull transition area of the Ballad is a full 2.5cm (1") thick GRP. As noted above, I believe the lateral forces on the truss are not all that severe. White oak wedges (or similar) would work perfectly well. Especially if the aft end of the new truss is bolted to the bilge divider. The goal is to keep the truss in column under the mast. There is/was no need to resort to a solid block of poured-in resin: that approach, along with the choice of mild steel for the truss, was what doomed the original design. :(

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prjacobs
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Re: Mast step truss, inevitably:(

Post by prjacobs » Thu Apr 02, 2015 3:44 am

So ... as an engineer who didn't flunk out of university ... have you any comments or improvements on the design, or materials suggestions?
I hope that's not me you're asking, as I am neither an engineer (am a technician) and never went to university. But what you say makes sense to me!

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