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Nacra 5.2 Beam Strap Lug  Bottom

  • Last year when I rebuilt This End Up I reinforced all of the beam strap lugs by wrapping additional glass over the top of all of the lugs. I have had these lugs crack the glass before and just thought I would take the preventative step while I was fixing everything else. Well the fix was not in as it were. Yesterday we were out having some fun and when we were loading the boat back on the trailer we noticed that one of the inside lugs on the main beam had cracked. After all the work I did last year making this boat very pretty I was not happy about what I knew I was going to be doing soon.

    In the past I had just compressed the lug back into position and glassed over it. This time I am taking a little more dramatic approach. I ground the top of the lug down so that I could completely remove the metal bit. I was a little surprised in that mine is made from a cut piece of bronze strap. But what I had always suspected was shown to be the case in that this square bit had saw cut sharp edges and corners. Not a good thing at all in a high load area. Then I took this lovely little devil over to the grinder and rounded over all of the top edges, rounded the corners on the outside of the lug and then ran the whole thing over the wire wheel to clean it up. Epoxy will bond to this thing when clean, but it is the glass that provide the real strength and without those knife sharp edges to shear the glass I expect the results will be far better.

    So then I took my small right angle die grinder and removed all of the edges in the glass on the top side this time around. That even opened up a small hole in front of the lug. So I will be doing glass work from inside and outside this time around too. I could conceivably skip the work from the inside, but I am wondering if flexing is also contributing to this problem , so will fill the hole from the inside and stiffen the underside of the lug.

    After all of that I will bed the lug in thickened epoxy. Then glass this thing over again. Since this is not the visible side of the hull I think it is going to just get painted, but not faired in. So a close look will show this repair. It is tempting to redo all of these lugs in this same way to save future grief, but ............

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128361&g2_serialNumber=4

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128358&g2_serialNumber=4

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128366&g2_serialNumber=4

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128369&g2_serialNumber=4

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128378&g2_serialNumber=4

    Now the question is if there is any of that dang Perfection paint in the shop.

    Anyway, if you ever crack one of these lugs, do yourself a favor and dig it out and grind those sharp edges off the metal part of the lug.



    Edited by dmgbear55 on Jul 15, 2017 - 05:41 PM.

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    dg
    NACRA 5.2 #400
    This End Up
    Original owner since 1975
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  • Hi Dave,

    Having watched all your videos and done just a fraction of some of those jobs myself I know you have a huge amount of effort invested in this, and it sucks that even after adding reinforcing layers one still broke out.

    I have never understood why they put such a large load into such a small surface area, and have thought in the past that if mine ever go Id look at adding a curved metal strap that goes inside from under the front bolt, following curved fiberglass that the beam sits in, and onto the back bolt; i.e. a mirror image of the strap that goes over the top of beam on the outside, but just inside the hull instead, and would just drill thru the molded in lugs so the bolt passes thru them and instead threads in to a nut on the other side of the inside metal strap. It would spread the load over a much wider area and in direct opposition the strap on top of beam rather than pulling on a very small area of fiberglass (maybe even bed that inside strap on thickened epoxy) . As I recall you made additional inspection holes in the deck so maybe you have access to be able to do something like that? Another option is instead of a streap running from front bolt to back bolt, have a plate from side to side under deck inside the hull so the two front bolts go into each end of a single plate and likewise for the aft pair of bolt, again just increasing the surface area to oppose the upward pull of the bolt. A possible issue with that is that there is not flat area inside the hull between the holes and the curved fiberglass so the holes might be ont eh very edge of the plate.

    Not sure how practical either of those ideas are as I have not had to try them, but just a suggestion...

    Please keep us updated with your progress.

    Anthony

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    Nacra 5.2

    'There is nothing, but nothing as much fun as simply messing about in boats'
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  • I have no doubt you can blend this repair, but after all the work you did last year...ouch!

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    Tom
    NACRA 5.7 (1984 Sail 181)
    Pennsylvania
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  • This failure is likely the result of having inspection ports BOTH forward AND aft of the main beam, not saw cuts on the "lug".

    Nacra starting installing larger and thicker aluminum beam bolt plates in later boats, and for a reason. I suggest you fabricate a larger plate. You have reduced the surface area of that bronze lug quite a bit, only making the problem worse.

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    Philip
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  • There is no relationship of the added forward inspection port and this lug failure. This new inspection port is more than 6" away from this lug. The failure is the result of too much load on too small of an area in the glass. A larger lug or added strap would better spread this load out. But I actually think the lug will be fine once the sharp edges that were able to shear the glass are gone. High point loading with a sharp edge is a very bad thing in fiberglass. I would also argue that the reduction of bearing area is very minimal from the grinding I did on the bronze lug.

    As much as I hate to do it I am also going to grind into the cradle some to allow a small amount of glass to wrap into the cradle. The reason I ground down around the lug area was so that when I epoxy back into this area all of the material being bonded together is very sound and allows the best possible bond. I will be using that forward inspection hatch to add new glass in behind this lug both as added support and to reduce any flexing that might have also been responsible for the failure. It might also be possible to add a plate on the inside of the hull and then add a nut to compress the plate and share some of the load. I may look at this for all of the other lugs so I can avoid another round of this misadventure.

    Blending the repair is going to have to wait for another day when I need to do more work on the hull(s), which will happen at some point in time again.

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    dg
    NACRA 5.2 #400
    This End Up
    Original owner since 1975
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  • Here is where this project got to this afternoon. There are two new layers of 10 oz. cloth underneath the lug support to help stiffen it up. One layer of 10 oz. was added to the top side of the lug support. Then the lug was bedded into the support with thickened epoxy and then faired. Finally a single layer of 10 oz was added over the lug itself.

    I'm going to let this all cure overnight and then add 2 more layers of 10 oz. over the lug. In the past I cut the glass to slide it around the bolt, this time I noodled the bolt through the glass to leave the long fibers intact and that was pretty easy to do. Once I have 3 layers over the lug I will have to decide if I want to do more and with what weight of cloth. For sure I will cover this all with a light cloth to make fairing it all in someday easier.

    In the last image you can see how much radius is now in the lug and the glass around it. It will be much more difficult for the lug to break the fibers under load. It won't be able to shear the fibers anymore. As a belts and suspenders approach I think I can also add a nut and fender washer from the underside to take advantage of the glass under the lug. I will be doing this to all of the other lugs that I can get access to, which excludes those on the front side of the rear beam. My arm is just not long enough to get to those.

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128382&g2_serialNumber=4

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128385&g2_serialNumber=4

    https://www.thebeachcats.com/gallery2/main.php?g2_view=core.DownloadItem&g2_itemId=128390&g2_serialNumber=4

    And I found the cans of Perfection pain, So once this is adequately reinforced I can turn it blue again.

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    dg
    NACRA 5.2 #400
    This End Up
    Original owner since 1975
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