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Old 01-11-2017, 11:24   #1
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Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Hi,
There is quite a bit of movement in the front crossbeam. For some reason it has cracked and delaminated the glass (Kevlar) from the underside of the deck. Iím going to remove the crossbeam to get a better look.
Does anyone have any thoughts as to the best way to repair?

This in inside the bow looking aft Click image for larger version

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Old 01-11-2017, 11:25   #2
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Inside the bow looking aft Click image for larger version

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Old 01-11-2017, 11:29   #3
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Some from the exterior
Click image for larger version

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Opposite side of bulkhead in the previous photos. Apparently the inspection hatch was not original.
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Old 01-11-2017, 12:19   #4
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

I can't see much in the way of uni here.

When a load is highly concentrated it is usual to see lots of unidirectional cloth over large coves, spreading out the loads through nice easy to see "fingers". Lots of double bias to help consolidate is good practice.

So I would cut out the affected areas and then work out how the load from the beam goes into the hull sides and deck aft of the beam. You may like to make a new ring frame to help take the loads down the hull sides further. I really like tying fittings onto the boat with many single strands of uni, just like tying something on with string. For a forebeam the tying would have to be substantial but it is a good way of getting a load down to a ring frame or stiffener without relying on peel strength of a foam laminate.

So cut it out, work out where the load needs to go, glue the core back on and then uni the sockets and beam down to stiffeners and ring frames that help spread the load further.

BTW, Kevlar is pretty terrible at compression. Some of the laminate looks like Kevlar. If it went into compression that would not have helped.

cheers

Phil
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Old 01-11-2017, 17:03   #5
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Looks like all the loads are suppose to go from the beam into the bulkhead through the bolted on flange, and from there to the hull sides. All of that seems to be just fine in the pics as no damage is shown there.
The damaged area seems to be only on the deck, as that has been too stiff and therefore carried more load than it was meant to handle. One possible fix would be to make the deck locally single skin around the beam to allow for more flex

If more stiffness is wanted for the beam connection the brackets on the deck should be right at the deck edge, transmitting loads directly to the hull sides (no core in between!) But having more stiffness loads the beam up more, are you sure it can handle that? If not, don't do so, as that might cause the beam itself breaking.

It is certain the front beam is intended to handle loads from the forestay and transmitting those to the hull sides, but that does not mean it is also intended or capable to also handle loads preventing/limiting hulls moving relative to each other due to seaway. That task might be intended to be handled by other beams with more size and thus capacity. Like the ones under the mast and traveler track for the mainsail. You need to check dimensioning of those beams to draw any conclusions on the matter. Information of front beam alone is certainly not enough for that!

Since there seems to be several large size hatches aft of the beam on the deck, it means the deck is not intended to transmit global bending or torsional loading. If the front beam does carry those, the connection need to be capable of distributing those to all the cross section of the hull. That is best done by the bulkhead where the beam flange is bolted on, and it seems to be doing its job just fine, as no damage is shown there by your pics. The problem is the brackets transmitting vertical load to the deck it was not capable or intended to handle, causing the crack. The deck edge could locally handle that if there is no core there, but like already said, it might distribute wracking loads differently than intended, overloading the front beam and all the associated structures, if the global loads was intended to be carried by only the other beams.
Deck should only ever carry vertical loads from people walking on it or from water pressure, not from anything else. Deck can carry horizontal loads from any source just fine, if it's properly dimensioned for that, but that means only a small percent of it's capacity can be compromised by hatches. On your boat they seem to cover a large percentage of deck width, preventing any global loads from being carried.
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Old 01-11-2017, 17:21   #6
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Whenever something breaks up, the broken part has been trying to transmit more loads it was capable of.
There are basically too solutions:

1) reduce the loads it has to carry.
That is almost always the bright choice causing less problems and being the lightest and safest solution. It involves reducing stiffness to route some of the loads elsewhere. Direction of loads must be analysed for that. In this case, loads from forestay must be carried be front beam assembly, and stiffness noes not change those loads. Wrecking loads caused by seaway do not have to be carried by frontbeam at all. On some boats they are meant to carry some percentage of those loads, on others nothing at all. Reducing loads from wrecking hulls to be carried by front beam by making its connections flex more will cause other beams carrying more of those loads. It must be made certain those beams and associated structures will handle those loads.

2) make the failed part stronger.
This solution always carries risk that it will next fail on some other part on the load path, as no part of that was originally built or designed to carry those loads from the begin with. And making stronger also make it stiffer, causing it to carry even more load in the future than it ever managed to do before it broke. On worst case scenario it might even mean losing boat or lives! It always means thorough analyses on loads and strength of all load carrying members on the whole boat to be carried out. Not just the failed part and those nearby!

In all cases panels best carries loads in the direction aligned with surfaces, not perpendicular to those. No high loads can be transmitted through core material, it can only be used to support the skins perpendicular to the loads. Ie to prevent skin buckling or in column buckling.
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Old 02-11-2017, 15:29   #7
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Thoughts...
After all the technical comments are thrashed out, I think the same fix should be carried out on the port bow.
For obvious reasons I would assume?
Not to start any whizzing match, but I find it difficult to see built in flex solving this problem. Meaning , flex where ? I wouldn't own a boat with a flexing hull at the beam connection.
Shall we assume you have inspected all other beam to hull sections ?
Are they perfectly fine? If so , why just the strbd bow ?
Who is the builder/designer?
What's the manufacturer's take on this failure ?
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Old 03-11-2017, 02:52   #8
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

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Originally Posted by thruska View Post
Who is the builder/designer?
Its a classic.

Possibly one of the best designs of all time.

Design 150, do I need to say any more.

Was it built to the design spec?, that is the question.
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Old 03-11-2017, 05:50   #9
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Quote:
Originally Posted by Seaslug Caravan View Post
Its a classic.

Possibly one of the best designs of all time.

Design 150, do I need to say any more.

Was it built to the design spec?, that is the question.


The builder was Hutchesonís in NZ. Theyíre supposed to be good builders. The sister ship, Deguello, Lock Crowthers personal boat seems to be the same design and construction in talking with the current owner.
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Old 03-11-2017, 06:00   #10
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

I got the crossbeam off. I donít know if it was done intentionally, but epoxy had a very good bond at both the deck cradle and to the crossbeam flange to the deck. I think this maybe part of the problem. If flexible sealants were used instead, the loads would not get transferred to the deck. Click image for larger version

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Old 03-11-2017, 09:38   #11
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

Quote:
Originally Posted by Seaslug Caravan View Post
Its a classic.

Possibly one of the best designs of all time.

Design 150, do I need to say any more.

Was it built to the design spec?, that is the question.
Your operative word there is 'possibly' , ' possibly' not.
You pointed out yourself ' built to design specs' .
Yes you could have said more. Perhaps provide a link?
I own three classics , really doesn't mean sqat when pertaining to the build quality.
Regarding builders ? Well so called reputable yards have been know to make mistakes. One i know could care less. Dont ask me how i know lol.
Thanks
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Old 03-11-2017, 14:36   #12
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Re: Problem with front crossbeam/bulkhead/deck....thoughts on repair?

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Not to start any whizzing match, but I find it difficult to see built in flex solving this problem. Meaning , flex where ? I wouldn't own a boat with a flexing hull at the beam connection.
There are no structures anywhere that do not flex under load at all.
Not in your boat, not in any other boat and not in somewhere having nothing to do with boats.
The issue is about relative stiffness between parallel load paths. The absolute value of flex is not important at all. If path A has flex of 0.48 mm and path B 0.02 mm, you do not notice either flexing. But as path A flex 24 times as much as path B, it will mean path B takes up 24 times the load of path A.
In the end that means path A takes up 4% of total load, and path B 96%.

In this case the bulkhead is intended to carry perhaps 96% of vertical load and transfer and distribute that to both hull sides and along its depth, not just at the top corner like deck brackets would do. That can only happen when the load path from the beam into the brackets to the hull sides at top will have stiffness no more than 4% of the other path. 5 mm thick single skin deck laminate with 50% fiber/resin ratio can be at least as flexible in vertical direction while still carrying out all the required loads in other directions, most importantly fore/aft loads assuming those are transmitted by the front beam instead of some central structure.

Parallel load paths will always distribute loads in proportional to their total stiffness in direction of the loads! There are no exceptions.
And since the route involving bulkhead is the longer route, it will not be the stiffer one unless there is some intentionally built in flex in the other path.
1 mm will most likely be enough, but actual value can not be guessed on the internet forum. It needs to be calculated or just make it on the safe side, ie more than the minimum actually needed.
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