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Thread: Centerboard trunk failure

  1. #1
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    Default Centerboard trunk failure

    Hi all,

    Thanks to some of you who helped identify this boat which is similar to an Atkin's "Sanderling" with an added deck. It is a 16' knockabout that I'm hoping to repair and outfit for week, possibly month long voyages in the PNW, you know, dinghy cruising . Brief history, the previous owner (a Welder) built this with his father who was a Shipwright. Unfortunately all plans are gone and memory of its construction are minimal. Otherwise very happy with this bargain craigslist find.

    The issue is a centerboard trunk leak at it's attachment to the keel (gallon/hr). Having read some information on attachment methods I was not able to find this particular arrangement and thought I'd see if anyone had thoughts on its integrity (evidently not great!) and best methods for repairing or possibly improving it. Last night I took some photos and tried to figure out how it has been assembled and here's what I came up with:

    Screen Shot 2018-05-12 at 12.15.06 PM.jpgIMG-5937.jpgIMG-5938.jpgIMG-5939.jpgIMG-5953.jpg

    Looking forward to any input before I pull out this trunk bracket (term?) shown in blue and possibly the trunk as well. This all houses a very heavy, possibly solid stainless centerboard.

    Cheers!
    Parc
    Last edited by Parc; 05-12-2018 at 02:45 PM.

  2. #2
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    Default Re: Centerboard trunk failure

    That looks like a very nice boat..

    I'd do a process of elimination; put some light silicone (or similar, so you can get it off after a test) around the bottom ally seam, put it in the water. Observe.

    Put some good rubber washers around the CB pin, put it in the water. Observe.

    Etc.

  3. #3
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    Default Re: Centerboard trunk failure

    It looks as though someone used aluminium angles to reinforce a failing joint. It is going to be a fizzing mess of dissimilar metals.
    I suspect that you are going to have to strip out the aluminium angles and see what is going on under the bottom where those screws join the case logs to the keel. I would replace those long vertical screws with through bolts.
    It really is quite difficult to build an ugly wooden boat.

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  4. #4
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    Default Re: Centerboard trunk failure

    +1 on Nick's comment. The aluminum looks corroded, my guess is the side against the wood is totally eaten with thick white oxidization. Ultimately I'd be planning the removal and replacement of the CB trunk.

  5. #5
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    Default Re: Centerboard trunk failure

    Funky aluminum...my first thought.

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    Default Re: Centerboard trunk failure

    Thanks for the quick responses so far! I was going to mention the electrolysis party, glad you picked up on it...From what I can see so far, this aluminum bracket looks to be part of the original build as the machine screws tap into it through the case logs. It's yet to be determined, but the case logs don't appear to carry any screws other than the machine screw. The logs are only about 3/4" wide and laminated to the case walls which both meet at the keelson (keel?). Not jumping to solutions yet, but do you think 3/4" is enough to vertically through bolt with say 1/4" stainless? Or might I be looking at replacing the case logs with wider ones and scrapping the aluminum in favour of what ye say wood or stainless or FG runners to align the centerboard...somewhat novice in this department, but fully capable, just want to do it right. Oh and pardon the diagram above, I modified the cross plans of another diagram and am not sure there are any vertical screws.

    Just need to build a cradle and the gantry crane might come in handy (my dad an I last used it to swap a bmw 325i engine into a '74 bmw 2002, good times!). Will follow up once things are apart. Thanks a lot!
    Last edited by Parc; 05-12-2018 at 05:03 PM.

  7. #7
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    Default Re: Centerboard trunk failure

    I wonder if an athwartship brace at the top of the trunk of some sort could help ,especially with the main sheet going there as well.

  8. #8
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    Default Re: Centerboard trunk failure

    I'm considering that too, although would hate to break up the open floor plan. A bench there might be nice for rowing, currently the push-row seating is on the aft deck facing forward...

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    Default Re: Centerboard trunk failure

    I'd go with wider logs and tie the floors in with the trunk using bigger brackets (knees).

  10. #10
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by Parc View Post
    From what I can see so far, this aluminum bracket looks to be part of the original build as the machine screws tap into it through the case logs.
    It would not be difficult to drill through the case logs into the aluminium and then tap threads in situ. It is sometimes done through a keel into cast iron ballast in order to double up on wasted keel bolts.
    It really is quite difficult to build an ugly wooden boat.

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  11. #11
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by wizbang 13 View Post
    I wonder if an athwartship brace at the top of the trunk of some sort could help ,especially with the main sheet going there as well.
    Quote Originally Posted by navydog View Post
    I'd go with wider logs and tie the floors in with the trunk using bigger brackets (knees).
    +1 to both of these. Especially with a heavy CB wagging about under the hull.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Centerboard trunk failure

    How long did you let things swell up when you noticed the gal/hour leak?

  13. #13
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by Peerie Maa View Post
    It would not be difficult to drill through the case logs into the aluminium and then tap threads in situ. It is sometimes done through a keel into cast iron ballast in order to double up on wasted keel bolts.
    You're right, thanks for tuning my logic. So it could have been an attempted fix...yikes.

  14. #14
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by StevenBauer View Post
    How long did you let things swell up when you noticed the gal/hour leak?
    About 3 hours, and that doesn't count the few times it had been out for day trips before. Generally speaking, the boat hasn't seen much water, but perhaps just enough...are you suspecting the keel?

  15. #15
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    Default Re: Centerboard trunk failure

    If the aluminum is corroded it won't matter how tightly it is fastened to the wood. After you remove the aluminum and examine the surfaces you will be able to decide what course of action to take.

  16. #16
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    Default Re: Centerboard trunk failure

    I agree, no more quick fixes here. I'll followup once I get the CB and bracket out. Thanks everyone for the vote of confidence!

  17. #17
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by Parc View Post
    I agree, no more quick fixes here. I'll followup once I get the CB and bracket out. Thanks everyone for the vote of confidence!
    I think this is the wisest choice. Centerboard cases are often a problem, and often a weak link. It's not a component that offers high odds of a successful 'kinda' fix. I think it's time for dismantling, so as to achieve a full and informed diagnosis. What you do after that will depend. But I do concur on the beefed up logs at the foot. And I think I'd also see if I could gracefully manage some sort of angled reinforcement to counteract the athwartships forces. Maybe something like an isoceles triangle-ish shape at the front. Base attached to a floor timber, and apex to the top of the trunk?
    David G
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    http://www.harborwoodworking.com/boat.html

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  18. #18
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by Parc View Post
    I'll followup once I get the CB and bracket out.
    Great, because a while ago you didn't want to do that.

    Based on one photo on the internet showing a slight corrosion to an aluminium item (of course it's corroded a little, it's been in sea water, this does not mean it is the cause of the leak) you are now going to tear apart a very difficult component of your boat without bothering to do a simple test to find out where it's leaking, as I suggested.

  19. #19
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by lupussonic View Post
    Great, because a while ago you didn't want to do that.
    I thought I recognized the probability of this in my initial post.

    Quote Originally Posted by lupussonic View Post
    Based on one photo on the internet showing a slight corrosion to an aluminium item (of course it's corroded a little, it's been in sea water, this does not mean it is the cause of the leak) you are now going to tear apart a very difficult component of your boat without bothering to do a simple test to find out where it's leaking, as I suggested.
    I'm sorry for not acknowledging your suggestion! it certainly is the best practice for diagnosing a leak. I regret not responding or providing more detail! But I believe the entry has already been diagnosed. At purchase, I was told about a leak at the case/keel seam and shown two different attempts to seal it (sealant is visible in the first photo). His explanation was that his father sealed one section, and later on he sealed another section and claimed that he "got most of it". During my first and only sea trial thus far, while I was not of the right mind to observe the leak (stupidity in hindsight), I guess I was already convinced where it was entering. I deserve a stiff kick in the ass if after repairing the CB, it turns out to the be the CB pin. I recognize the risk and the amount of work ahead, but I also recognize the risk of CB failure at sea. Given the givens, I've simply lost trust in this method of attachment/materials and feel the need to investigate further.

  20. #20
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by Parc View Post
    I thought I recognized the probability of this in my initial post.



    I'm sorry for not acknowledging your suggestion! it certainly is the best practice for diagnosing a leak. I regret not responding or providing more detail! But I believe the entry has already been diagnosed. At purchase, I was told about a leak at the case/keel seam and shown two different attempts to seal it (sealant is visible in the first photo). His explanation was that his father sealed one section, and later on he sealed another section and claimed that he "got most of it". During my first and only sea trial thus far, while I was not of the right mind to observe the leak (stupidity in hindsight), I guess I was already convinced where it was entering. I deserve a stiff kick in the ass if after repairing the CB, it turns out to the be the CB pin. I recognize the risk and the amount of work ahead, but I also recognize the risk of CB failure at sea. Given the givens, I've simply lost trust in this method of attachment/materials and feel the need to investigate further.
    So, why not dunk her again to be certain? You do not even need to slide her off her trailer. I'd pull than bead of goop anyway as it is not a workmanlike repair. Then see where the leak is with your own eyes.
    It really is quite difficult to build an ugly wooden boat.

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  21. #21
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    Default Re: Centerboard trunk failure

    Well here's the bottom of it. My tow vehicle is currently not running (parts are due in about a week). Coincidentally, next week I am on vacation and I was planning to get started on this project. I suppose I'll rent a truck and go dunk her.

  22. #22
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    Default Re: Centerboard trunk failure

    Ok, busy couple of days...thanks to lupussonic for encouraging me to put her back in the water (I hope you'll forgive my hastiness!). It was entering in a few locations all around the case/keel seam (3 to stbd, 1 to port and a nice gusher across the bow seam). Back to the garage! Today I was able to hoist her off the trailer and pull out the bracket which turns out to be quite a nifty piece of hardware. This section of the hull is almost entirely covered by the decked trailer and bunks hence the single photo at the beginning of the thread. Here are a few more from today:

    IMG_5978.jpg

    first thing I noticed was an unfortunate lack of sealant and a rather poor looking job, there were several sections where none had pressed out...

    IMG_5991.jpg

    second thing, found that the bracket is made up of 6 parts in total, is about 3.5" tall with 1/4" walls and is made to be flush with the case walls (slightly different from my diagram above). This photo is of the end plate (removed) and a spreader being knocked out (by me). This spreader is fastened only at its top end by an athwart bolt all through the main brackets and case logs. The only thing really protecting it from getting knocked afterward is the plate in front of it. This area was of particular interest as most of the water was entering above this (again, thanks guys, it really helped to follow the right diagnostics).

    IMG_6005.jpgIMG_6001.jpg

    here are the components removed, the main walls are kerfed and welded on the water side (edit: keel ally side) to match the curvature of the hull...they came out very easily (too easily!), but appear to be in fine shape other than some lousy sealant and sand...aha. And more discoveries...the centerboard is in fact 3/8" aluminum.

    IMG_6010.jpg

    how about the other side, kind of hard to see what's going on here, but generally spotty sealant all around! mixed up amongst the polyurethane (??) sealant is a very wet jelly like substance, almost like jellyfish consistency, a clue? the aluminum bracket doesn't quite meet the rabbet on the case walls, but I don't think that's too big an issue. Next, the enviable task of removing this and drying it out (I haven't done research yet so tips would be welcomed! I have scrapers, chisels and a dremmel...chemicals?) (edit: dremmel with a fine wire wheel bit was a winner)

    So far, I was able to see a tiny amount of rot in the upper forward keel (end grain)...somewhat expected...but it looks to me like the CB case can stay, so long as I dry it out as best possible. until tomorrow!
    Last edited by Parc; 05-20-2018 at 07:25 PM. Reason: trouble with images, still learning

  23. #23
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    Default Re: Centerboard trunk failure

    Hastyness forgiven... !

    So, ill fitting, poorly sealed, + mitigating rot.

    You must ruthlessly chase out all rot, then rot proof remaining faces (I like Cuprinol 5 star, and Borax, but I'm in the UK), then replace rotten wood you removed. While doing this (after its dry), you can think about if using existing ally item is a good plan, or replace with something fitting better, taking into account the sealing plan.

    Got a stand off shot of the whole boat? She looks like a real stunner.

  24. #24
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    Default Re: Centerboard trunk failure

    Excellent, I'll dig deeper tomorrow. Thanks for the tips, very much enjoying this learning process.

    Here's from yesterday's dunk

    IMG_4083.jpgIMG_4085.jpg

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    Default Re: Centerboard trunk failure

    jebus good lookin boat and smart trailer set up

  26. #26
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    Default Re: Centerboard trunk failure

    It's got a tabernaker-backernackle too...

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    Default Re: Centerboard trunk failure

    Ok, following up, time for the next level...

    Found a bit more inaccessible rot under the sealant so out comes the case. Out comes the tabernacle! Floors were epoxied in so had to saw them out. Mast post was also epoxied to the front of the case. Just before the post came thru deck it was epoxied to a block under deck, one more sacrificial cut, a slice of the seam and the lot came down and out with a swing to the side. Hoorah! hope I'm not breaking any rules so far, I feel I've inhaled the fumes of a sacred place...

    IMG_6057.jpg

    Above: Depending on what happens with the case, anyone have anything against finishing these in place? or tips on how to carefully remove them? (edit: router for the bulk, chisel and sander for the rest)

    IMG_6048.jpg

    Above: a rotten trough, that was just filled with sealant (eek). I went after the rot with a flapper wheel on an angle grinder.

    IMG_6056.jpg

    Above: here's the cleaned surface that now has 3-6mm troughs. Before removing the case, I scraped the bottom to check for screws but found none. Plugs can be seen from the top, these run stem to stern and I assume tie the keel laminates together or maybe the garboards (they don't run thru the bottom). Really starting to appreciate the thought put into building at this point.

    IMG_6060.jpg

    Above: the up side-down partially cleaned - slightly damaged - case, with rabbet to accept poorly fitted aluminum bracket.

    I mostly trust the thought put into the ally* bracket (*thanks for the new term!) but not the execution. All in all, the CB case seemed very solid. I now have an opportunity to greatly improve the fit. But I'm a little skittish about dismantling the case or replacing the case logs. Could I scarf on some more wood? I may be able to run longer bracket screws up into them, possibly narrow bolts all thru the top, but again, only 3/4" to work with. thoughts?

    Next is to true up the keel surface. Replacing its wood with new is a bit out of my scope at the moment (especially on this concave surface). So from other threads I was thinking thinned and thickened epoxy made meticulously smooth, again only 3-6mm troughs to deal with. Does that sound ok? And what about the wood preservative? thinned epoxy over that ok? After that, a template made and transferred to the case, and likely new offset holes drilled and tapped as this will all be about 6mm tighter.

    Come a long way but feeling just a bit ahead of myself! so I'm leaning on you guys, thanks very much! I owe beer/scotch.
    Last edited by Parc; 05-19-2018 at 11:22 AM.

  28. #28
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    Default Re: Centerboard trunk failure

    I gotta think about this for awhile...the combo of preservative and epoxy.
    The preservative is going to affect the epoxy bond/sealing over time.
    I'd get the top of the keel and the bottom of the trunk sealed up with CPES and epoxy BEFORE assembly. Get em all nice as you can.
    Then , careful alignment of the case. The board should be in and down, the boat propped up on horses.This, so you can be sure it is straight fore n aft , also side to side.
    Spot glue the trunk into position with a few dabs of thickened epoxy.
    Then remove the board and epoxy the new logs.Lots of thickened epoxy (403 fillers) , filets almost everywhere.
    Lotta people do not like to epoxy in CB trunks, but I do .
    That's how I'd do it .

  29. #29
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    Default Re: Centerboard trunk failure

    Here is a trunk I built new for an older boat (20'er).
    Just setting on wet thickened epoxy, lead weights and a simple stick across the top for alignment.
    Some feel using epoxy here is asking for trouble... If the board takes a shock it may crack the epoxy, where it may absorb shock better in a bedded or "sika'd" job.

  30. #30
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    Default Re: Centerboard trunk failure

    When it comes time for using sealant, maybe for the ally bit, don't make the same mistake as the previous person. Slather both mating surfaces entirely, present up, put in fixings and tighten to start of squidge..... but DON'T fully tighten. If you do, you'll squeeze out all the goop, and end up with a dry joint. Allow to cure fully, then crank up to full torque, thus tightening against a layer of rubbery gasket.

    I like CT1, it's heavy duty, rated for submersion and sticks like sh*t to a blanket, but not sure if available in the US. Plenty of other options available, but choose carefully.

    download.jpg

  31. #31
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    Default Re: Centerboard trunk failure

    Hmmm, great input, thanks wizbang, I hadn't considered an epoxy bed. Having to cut several pieces to get the case out, it may as well be epoxied down below! I like the simplicity of this method, no need for fasteners you think? I especially like how it simplifies attaching of the ally bracket and renders it a separate part with a different purpose...

    Having determined that the main cause of failure was ill fitting joints, and lack of sealant (not necessarily wringing), weighing all the great advice so far, I think I have a plan. Increase support to the aft post (where mainsheet block attaches) with an added floor bulkhead and A-frame to mimic the others (sketch below) as David and others suggested. I'd be doubling the strength of the aft of case or better as the knee's are on the post as well as the walls in front of the post.

    I'm not a builder (although I think I've caught the bug), just thinking aloud, trying to follow the logic of the og designer/builder re. forces acting on this joint. Please forgive my ignorance or blabbery. I'm guessing that they are mostly athwart (less so fore-aft) sheering/twisting, either below by the CB or flexion in the hull relative to the deck or above by mast and mainsheet. It seems the most important structural pieces are the bulkheads and knees, and to a lesser extent, the case logs themselves. The bulkheads triangulate the case, ribs, planks, garboards, and keel to address sheering forces at the bottom seem, the higher up these go into knees or boards at the top, the better they will address twisting at the bottom seem. I hope I'm not belittling the importance of the case logs, I realize there is a consensus on beefing them up, but if the case were epoxied in and I added an a frame to aft post tied to an added bulkhead, might that be sound enough?

    Finally, the ally bracket. Thanks for the tip lupussonic, definitely will follow that lead! I especially like the idea that the CB has a nice wide firm aluminum face to press against that is also cushioned by sealant. I think this should mitigate some of the twisting (or would that be more sheer?) forces on the case logs by the cb. I'm also considering having the aluminum spacers welded to the front plate (same at the back)

    centerboard.jpg

    Thanks again, pardon the bulky message!
    Last edited by Parc; 05-20-2018 at 07:33 PM.

  32. #32
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    Default Re: Centerboard trunk failure

    blabbery... I come here to talk about boats.... over.
    yea, your sketch shows you are understanding the engineering .
    There are a hundred ways to do things right, if your engineering is sound.

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    Default Re: Centerboard trunk failure

    Quote Originally Posted by wizbang 13 View Post
    blabbery... I come here to talk about boats.... over.
    rog... I must be in the right place

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    Default Re: Centerboard trunk failure

    Quote Originally Posted by wizbang 13 View Post
    Here is a trunk I built new for an older boat (20'er).
    Just setting on wet thickened epoxy, lead weights and a simple stick across the top for alignment.
    Some feel using epoxy here is asking for trouble... If the board takes a shock it may crack the epoxy, where it may absorb shock better in a bedded or "sika'd" job.
    I don't see a problem with using epoxy as long as the case is strong enough to not move. The only issue is ensuring that someone else will be the next guy to fix it.

  35. #35
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    Default Re: Centerboard trunk failure

    Quote Originally Posted by navydog View Post
    I don't see a problem with using epoxy as long as the case is strong enough to not move. The only issue is ensuring that someone else will be the next guy to fix it.
    Haha, knock wood that's not me. In any case, stripping out sika from nooks and crannies is way less fun than a sawzall!

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