Stern chainplate rebuild - thoughts?

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Nick
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Stern chainplate rebuild - thoughts?

Postby Nick » Thu May 02, 2019 11:04 am

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I have just discovered that the corrosion in the deck backing plate on Avy-J is far more advanced than I thought, and it looks as though it will be necessary to build a new chainplate. The raked transom (raked at about 45 degrees) means that this is not as easy as it might be on a flat transom.

The existing arrangement (shown below) has the transom plate embedded in the hull, connected to the top (deck) backing plate with the transom plate and the deck plate triangulated by two struts of metal. You can get the general idea from the diagram below (not to scale - the plates are not as big as shown relative to the transom height)

Image

The simplest?! solution seems to be to create an external chainplate in one piece running up the transom and folding over the deck, then instead of using two separate backing plates behind the transom and under the deck to make a continuous bent backing plate to mirror the external chainplate and to triangulate that in the same way as the existing arrangement.

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Obvious initial questions are:

  • How to get the external and internal plates made up with the bend angle correct
  • Whether or not it would be acceptable to bed the transom backing plate on top of the current embedded chainplate piece and bolt through

Proposed construction material is 6.9mm A4 stainless (left over from bow roller).

As always, the panel's thoughts are all very welcome.
- Nick 8)

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DaveS
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Re: Stern chainplate rebuild - thoughts?

Postby DaveS » Thu May 02, 2019 4:56 pm

Looks like a reasonable approach.

Two thoughts occur. It would probably be best to first make a template out of cardboard to get the internal angles and dimensions right.

To avoid letting water or condensation into the laminate I would be tempted to drill the holes through the hull and deck a little (say 1mm) oversize, paint them with epoxy (or even completely fill with epoxy if easier) and then after the epoxy cures drill them out correct size.
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Re: Stern chainplate rebuild - thoughts?

Postby mm5aho » Thu May 02, 2019 7:48 pm

Dave beat me to the same suggestion. Avoid water into the GRP inside the hole. What metal is in inside? Possibly carbon steel if the exposed bit is, so even more important. Oversize, fill then re-drill.
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Re: Stern chainplate rebuild - thoughts?

Postby BlowingOldBoots » Thu May 02, 2019 8:40 pm

I assume you have access to the inside. As it is only GRP I would cut out the old chain plate, make a new, stronger one and glass back in. I think the friable steel would yield under load and the bolts loosen. You are trusting to luck the condition of the unknown bit, Alternatively, get rid of the crap steel and build up the GRP on the inside. Make your external chain plate but have external webs, like a stem head fitting without the roller, then just bolt to stern and put large 5 mm thick washers on the inside.
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Re: Stern chainplate rebuild - thoughts?

Postby claymore » Fri May 03, 2019 12:05 pm

You wouldn't consider glassing/epoxying a substantial hardwood pad to the underside of the deck and transom then surface mount a stainless plate bolted through the pad?
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Nick
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Re: Stern chainplate rebuild - thoughts?

Postby Nick » Fri May 03, 2019 8:02 pm

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Cutting out the old glassed in chainplate on the inside of the transom seems to be popular, but I am not sure how I can do this without damaging the transom. It is a very confined space in at the aft end of the lazarette, and I certainly wouldn't want to operate a grinder in that position.
- Nick 8)

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Re: Stern chainplate rebuild - thoughts?

Postby Silkie » Fri May 03, 2019 8:45 pm

How about drilling a couple of exploratory holes to see what the transom plate is made of and it's condition? I would have guessed that it's one piece with the backstay attachment and is therefore stainless and probably all right. You may be able to get away with just replacing the top plate. Or am I being misled by colours you have chosen and you think the whole under-deck triangle is one piece?

How about a Dremel if you need to cut it out? It might take a bit longer but it's a lot more precise and less dangerous in a confined space.
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Nick
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Re: Stern chainplate rebuild - thoughts?

Postby Nick » Sun May 19, 2019 10:55 am

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Under-deck metal is in one, piece, a triangle.

Here is a pic a fellow Morgan Giles 30 owner has sent me of his aft chainplate.

Image

This would involve removing the embedded metal in the transom or bolting through it, so a friend has suggested the following (excuse the crap drawing)

Image

It will be a wide rectangular plate tapering to couple of inches wide at the top, bent to the appropriate angle where the dotted line is. Material will be 6mm stainless. That way it can be bolted through either side of the embedded metal, leaving it in place. Just need to fabricate two separate backing plates.

The only tricky bit is getting the slight curve of the transom in the plate.
- Nick 8)

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Re: Stern chainplate rebuild - thoughts?

Postby claymore » Sun May 19, 2019 12:50 pm

6mm plate in a hefty vice ought to give you the desired effect -just bend it a bit at a time until it is how you want it
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Nick
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Re: Stern chainplate rebuild - thoughts?

Postby Nick » Sun May 19, 2019 2:19 pm

claymore wrote:6mm plate in a hefty vice ought to give you the desired effect -just bend it a bit at a time until it is how you want it


I've done a drawing and I am giving it to a man wot can.

It's massive overkill, and the old dodgy chainplate will still be there as a backup. Rudder is repaired, one bulkhead replaced and one half replaced, it's just wiring, titivation, paint and launch now.

(He said crossing several sets of fingers)
- Nick 8)

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Re: Stern chainplate rebuild - thoughts?

Postby claymore » Sun May 19, 2019 2:47 pm

A veritable doddle, Webster!
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Re: Stern chainplate rebuild - thoughts?

Postby Aja » Mon May 20, 2019 9:23 pm

Too late to suggest a split backstay? Forget everything that exists and place a couple of chainplates where it suits.

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Nick
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Re: Stern chainplate rebuild - thoughts?

Postby Nick » Mon May 20, 2019 9:48 pm

Aja wrote:Too late to suggest a split backstay? Forget everything that exists and place a couple of chainplates where it suits.

Donald


Others have suggested the same, but it is complicating the geometry even further. Remember the transom is tiny and raked at 45 degrees.

The extra large chainplate with two rows of bolts avoids all the existing stuff and should prove enormously strong.
- Nick 8)

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Nick
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Re: Stern chainplate rebuild - thoughts?

Postby Nick » Tue Jun 11, 2019 3:34 pm

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- Nick 8)

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Re: Stern chainplate rebuild - thoughts?

Postby aquaplane » Tue Jun 11, 2019 4:57 pm

That looks substantial and neat, it's not always easy to do both at once.
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