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i used to sail a "borrowed" GBE from a mate and one of my regular crew was an ex german airforce aircraft enigineer . he sprayed the inside of the crossbeams with this fog thats designed for protecting the inside of aircraft wings from corrosion . Quick squirt either end of the beam and it drifts through and coats the surface . It was a corrosion X product but cant remember the name of it 

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Randii, the accepted method for making the sleeves is to buy the same section extrusion as the beam. The alloy beams on most tenant cats were typical mast sections of the appropriate size.  You cu

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I measured my crossbeams and wall thicknesses today. There's substantial variance in the wall thickness, some attributable to the forming process, and some likely a result of dings and age. Accordingly, I'm listing estimated average (min, max) and notes:
- FWD: NG-37 egg shape, 170mm x 115mm x 4mm (3.74mm, 5.45mm)    
- MAIN: ellipse shape, 178mm x 110mm x 4mm (3.6mm, 6.1mm)
- MID: flattened oval shape, 127mm x 83mm x 3mm (2.52mm, 4.1mm)
- AFT: ellipse shape, 178mm x 110mm x 3.5mm (3.1mm, 4.65mm)
My main and aft beams appear to be formed from the same die… that will be handy if I need to sleeve or replace the main beam. Due to the female hull sockets, I’m fairly committed to the outer profiles of the main and mid beams. Reworking either of these beams would be non-trivial – reworking the forward or aft beam less so, since the cradles and brackets are well-exposed.  

For reference, NG-37 Isomat specs are here: http://www.rigrite.com/Spars/Isomat_Spars/Isomat_Masts/NG37_mast.php


I have the study plans for both the GBE and the T9, but I'm still sorting out a matching table for these. The GBE text is internally consistent, but the T9 text has internal inconsistencies and some gaps (specifically around section weights and wall thickness). Weight is important, since the instructions say: “If the above extrusions are not available, go to the next biggest size, do not use lighter extrusions, do not use thinner walls, however some reduction in the outside dimensions would be acceptable.” With more analysis yet to do, my first sense is that:

- the upgraded front crossbeam on my T9 seems pretty stout
- the rear crossbeam looks slightly under-spec in dimension, but likely over-spec for thickness

- I'm a little concerned that MT specced sleeves for the main beam on the T9 and that I have no such sleeving
I've seen Internet evidence of some main beam failures... obviously, with the rig standing through that beam, that's the least preferable one to fail, structurally speaking. I've also seen postings of the front beam failing (not surprising if flying a headsail off a prod cantilevered forward). Any crossbeam failure would be a bad result... but are the mid and aft beams anywhere near as stressed?


I'm curious what this means from the study plans: “The main beam and the rear beam are two-piece extrusions, i.e. they are split longitudinally.” How were they split on original GBEs? My T9 beams are definitely not split.




Tennant's aluminium cross beams are under-engineered in my experience. We had problems right away and I have come across a score of other owners with more or less severe problems to report.
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Hi Randii, In the early days some mast extrusions (which is what your beams are) were made in two pieces because the Aluminium company didn't have the capacity for larger dies. So they made some sections in two pieces with a stepped edge so they could be sort of clipped together, then riveted. Later the masts were made in one piece. Some really big ones were still made that way until recently. They are almost all carbon fibre nowadays.



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Photos of T9 interiors make me believe the main beam is for supporting the mast only as it does not penetrate the hulls fully. If this is the case and you have a 'floating' dolphin striker there are no bending loads on the main beam, only compression. The attachemnt points of the dolphin striker strap would be the only high-load spots. As you may have seen on my web we re-engineered this on our Spyders.


The mid beam may be for secondary loads only, like carrying the aft end of the motor pod. It depends on how it is attached to the hulls. If so it will only see low loads.


This leaves with the front and aft beams for keeping the hulls together. There are three main sources of loads on these beams:

  • Wracking as the hulls are moving out of sync crossing waves at an angle.
  • Rigg loads. Shrouds are attached to the outside of the hulls which will induce some rotation of the hulls. The main sheet loads the middle of the aft beam -- added bending.
  • Daggerboards will also try to rotate the hulls.

I have done no T9 number crunching. I don't have enough information for that and it would have been nice with data on the beams as specified in the drawings. I have enclosed a summary of some modeling I did for a Belgian Spyder owner many moons ago after he had broken his front beam.




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I'm providing the info from the study plans for reference. I'm working my way across the web trying to identify the extrusions I have so that I can compare them to what Tennant specified.


The GBE study plans list the crossbeams twice, once in the Fitting List (FWD 100mm, MAIN D15, MID 100mm, AFT D15)
…and another with greater specificity under Aluminum Extrusions. They don’t exactly match, but still have value in context.
- FWD: round, 88.9mm x 88.9mm (weight/m: 1.85kg)
- MAIN: ellipse, 152.4mm x 114mm (weight/m: 3.43kg), D15
- MID: ellipse, 133.4mm x 100mm (weight/m: 2.77kg)
- AFT: ellipse, 152.4mm x 114mm (weight/m: 3.43kg), D15


The T9 study plans specify:
- FWD: round, 127mm x 127mm x 3mm (weight/m: 3.42kg)
- MAIN: ellipse, 161mm x 112mm (D16 Yachtspar: weight/m: 4.24kg), each end sleeved with 400mm D15 (a short sleeve for a short socket)
- MID: round, 101mm x 101mm x 3mm (weight/m: 2.71kg)
- AFT: ellipse, 190mm x 120mm  (BS 21, weight/m: 4.35kg)
I don’t have the full T9 plans (sure wish they’d have come with the boat!) but may prioritize boat bucks for that.


I appreciate this forum!



Edited by randii
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Martin, my T9 does indeed have the shorter ‘bury’ on the main crossbeam hull socket (improved access forward vs. less engagement with the ring frame)... and Tennant's floating mastbase/dolphin-striker. The forward and aft beams hold this boat together, since the main and mid beams aren't affixed to the hulls (at least before the rig is tensioned). I have looked at your slick composite beams, but hope to keep in much simpler (as my composite skills are MUCH more basic!). Thanks for breaking down the loads... I was not visualizing them all. It really underlines how involved multihull crossbeams can be!


My beams seem generally dimensionally similar or larger to what Tennant specced, but I will be Googling to verify the intended wall thickness and section weight. Getting the boat off the trailer and into the water is more a priority than is identifying upgrades.



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randii.  The drawings I do  have for the T9 show the int and main beam having short lenght burys.  While they look similar the GBE and T9 are quite different.  the GBE hulls are much narrower for their length than the T9.  And so the fore beam clamps are much further apart and thus generate more fixity to the hull.

 sorry - keep forgeting to measure my beam - will try and remember tonight.

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i used to sail a "borrowed" GBE from a mate and one of my regular crew was an ex german airforce aircraft enigineer . he sprayed the inside of the crossbeams with this fog thats designed for protecting the inside of aircraft wings from corrosion . Quick squirt either end of the beam and it drifts through and coats the surface . It was a corrosion X product but cant remember the name of it 

CX is good stuff. There are various formulations, but the HD mix is probably best for open-exposure ocean use. http://www.corrosionx.com/corrosionx-products.html I'm not sure how well any of these films hold up to open weathering. Some kind of end cap might help?


Our main beam is 160 by 115, same as the mast.

Any idea on the thickness of your extrusion. Freedom?


I'm going to play with Beam Boy a bit: http://www.geocities.ws/richgetze/. That's a good bit further towards the deep end of the engineering pool than I usually wander, but nothing ventured, nothing gained...

There's at least some partial consensus that the crossbeams were under-specified in early GBEs. The Wildfire and T9s I looked at have much larger-dimensioned beams. Perhaps Malcolm (or builders) up-specced on subsequent boats?


After running down a couple of water leaks, I have a dry boat (hurrah!). Next stop is to get the hulls off the trailer onto dollies for some bottom work and trailer repair.



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