Forward chainplate above head sawn-off, problems?

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Jan_Ballad_323
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Forward chainplate above head sawn-off, problems?

Post by Jan_Ballad_323 » Tue Apr 03, 2018 7:44 pm

During this winter I have built and installed a black water holding tank above the toilet. In the process I have sawn-off the chainplate. Caught up in the fun of the work I didn't think twice about it before I started but now I start to wonder if the remaining chain plate will be structurally strong enough? (see picture). It's still secured at both ends to the backing plates(?).
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ibbo
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Re: Forward chainplate above head sawn-off, problems?

Post by ibbo » Thu Apr 05, 2018 8:46 am

Looks like no one is keen to reply, so my say would be I don't like it.

The stay it holds down is the Cap shroud which is the most important one of the stays on the side deck, and I'm sure Albin thought long and hard before fitting that big lump of iron in there.

Maybe an eyebolt in the centre and some tensioners down to each bulkhead would work, or you could winch the boat down to a pontoon from the masthead, and see if it moves?

Mike

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prjacobs
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Re: Forward chainplate above head sawn-off, problems?

Post by prjacobs » Thu Apr 05, 2018 4:27 pm

I think you have made a mistake shortening the chain plate. When I built my holding tank I made it of a shape to leave the chain plates exposed so they could be tightened if needed as the bulkhead wood tends to crush a bit with age.

You could fabricate two triangular plates to go over the existing short chain plates and to add three or four more bolts to spread the load.
Make the plates the same thickness as the existing chain plates, and make spacers to go under them, also the same thickness.
This would have to be done on both sides of the bulkhead. Replace all the bolts with longer ones to suit.
That would give you six or seven bolts to spread the load to the bulkhead and, IMHO, would do the trick. (see rough sketch below)
Hope this helps.
Peter.
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Bob McGovern
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Re: Forward chainplate above head sawn-off, problems?

Post by Bob McGovern » Thu Apr 05, 2018 4:48 pm

I'd be a bit worried by the loss of support. Those metal trusses are annoying and somewhat bodged together, a result of Rolf locating the forward lower shroud exactly halfway between two bulkheads. But they have that triangular depth profile for a reason: they need to transfer upwards pull on a 0.6m span out to the vertical chainplates on the bulkheads. That eyebolt might well see loads +/-2000kg.

Peter has a good idea, tho -- you could sister a removable plate to the front of the truss crossmember, which would restore some tho not all is former strength. Of have the offcut bits welded back on & just figure out a way to fit the tank in around it. You could probably take 3-4cm off the tip of the triangle w/out losing too much strength.

Jan_Ballad_323
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Re: Forward chainplate above head sawn-off, problems?

Post by Jan_Ballad_323 » Thu Apr 05, 2018 6:43 pm

Thank you all for the replies! Highly appreciated. Clearly I need to secure this before going out sailing again. I will splash the boat in a week, I will see what I can do before that.

I'm not sure if I used the right terminology though in my first message.

What has been sawn-off is the metal truss with a triangular profile that runs between the bulkheads. The vertical chain plates on the bulkheads themselves (what I called backing plates in the first message) have not been modified in any way, except that I have tightened the screws (they were not really lose) and put some paint on them.

The bulkheads are the same as before. They are in good condition from what I can judge, but the only modification here is that I have secured them to the hull with glass fiber and epoxy (in the same manner as shown here: viewtopic.php?f=8&p=4700#p4700)

If I understand the design correctly, the purpose of the thick triangular profile of the truss is to transfer the vertical force from the area around the eyebolt to the vertical chain plates on the bulkheads, so that the total vertical force is divided between the three attachments (front bulkhead, mid eyebolt and rear bulkhead).

Removing the triangular profile does two things:

1) Weakens the truss structural strength in the vertical direction.
2) Mitigates the ability of the truss to transfer the vertical force from the eyebolt-area to the end attachments at the bulkheads.

Thus, now the vertical chain plates on the bulkheads will experience a smaller vertical force than before, as this force is instead applied to the area around the eyebolt, which will take major part of the total load?

So the truss has been weakened, but at the same time the load to it has increased. Clearly this could lead to structural failure of the truss. In such an event, the truss would bend, and the force on the bulkhead attachment would instead be in the horizontal direction, in which it is really weak with the current setup.

I will call a mechanical workshop tomorrow and see what they will charge for welding back a part of the piece that I removed from the truss. I would have to removed 3-4 cms of the tip of the triangle to have it fit above the tank, but that could be compensated for by welding on some additional piece on it vertically and horizontally.

I think that's the most attracting solution at the moment, since I quite easily can remove the metal truss between the bulkheads without removing the tank and would restore the design to near the original one.

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prjacobs
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Re: Forward chainplate above head sawn-off, problems?

Post by prjacobs » Fri Apr 06, 2018 12:27 am

Ah ... OK ... I thought you'd chopped the straps going down the bulkhead ... my bad!
I looked at your photo again and see that you "trimmed" the vertical sides of the truss.
You could weld the pieces back in, which would probably be best.

Below is a pic of my mock-up tank that shows the truss. I can still get my hand in there to work on the hoses, and have about a 12 Imperial gallon capacity.
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Jan_Ballad_323
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Re: Forward chainplate above head sawn-off, problems?

Post by Jan_Ballad_323 » Fri Apr 06, 2018 6:44 pm

No worries Peter. I wanted to maximize the volume of my tank, I don't know exactly how much can fit in it, but probably 65-70 L. If it works properly I will put some pictures of the finished work. Hopefully by then I will also have the truss welded back together.

Jan_Ballad_323
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Re: Forward chainplate above head sawn-off, problems?

Post by Jan_Ballad_323 » Thu Apr 26, 2018 6:49 pm

Now I have rectified the potential problem. I discussed the situation with an engineering colleague who's working with structural mechanics. He suggested that I would take the sewn-off pieces, add them to the sides and secure them with bolts and glue. Similar solution as you suggested Bob. So that's what I did. In case of heavy loads, the beam will bend inwards beneath the bolt that connects with the shroud.
I used epoxy glue in between the plates and four bolts of stainless steal, with nuts on both sides of the plates, so that the bolts both keeps the two plates together (together with the glue), as well as provide extra stability in the transversal direction. I am pretty happy and confident about this solution.
It was very narrow to tighten the bolts, but it worked out eventually. Hopefully I will never have to remove the beam again.
The toilet works as it should as well. Ready for the season.
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