Advice for fixing bodgy repair to chainplate mounting! : /

[Matt--H]

So for some location context, this is here:


...well forward of the mast step, and the stringer in question is a short one as it runs across to the aft of the substantial moulding for the houdini hatch which is under the cover - the stringer is about 12" behind the broom head.

 

[Blueboatman]

You don't need a build up anything like the depth of the stringer. Far easier to remove the necessary section of stringer to make the built up section. There will be no loss of stiffness when a built up section is incorporated where the stringer was. The job will be far easier. If you do as Neeves suggests and make up a thick piece of GRP you simpy need to glass it in place with extra layers of cloth. Very strong and rot proof

Yup
That sounds good too as it’s a short, athwartships bit of stringer.

I see the job is getting bigger as the bad core bits are getting revealed

 

[Matt--H]

I see the job is getting bigger as the bad core bits are getting revealed

A little, thankfully I'm fairly relaxed about sectioning out the roof as needed. I've got plenty of resin and CS around, and some nice woven cloth too

 

[Matt--H]

Found out something interesting regarding the strength of fibreglass...

Specific strength is normally measured in kilonewton meters per kilogram (there’s no SI approved single standard unit for this at this point).
While the exact number for the specific strength of fiberglass varies depending on the specific composite, a good average is about 1307 kN m/kg. For comparison, stainless steel’s specific strength is 63.1, and titanium is 260. (from here: Answered: How Strong is Fiberglass? )

..so whilst we do think about steel "strapping", and wood "reinforcement", actually pure fibreglass would appear to be a much stronger option! (Also, noting the helpful post earlier about the stringer wood really being there to act as a former, rather than being the strength of it.)

M.

 

[William_H]

It looks like OP has pulled out the pad etc already. I would have advocated careful measurement and observation to see exactly what has compressed when the shroud is loaded up. I am guessing that the block of wood and the resin filler were pretty solid and perhaps it is the deck it self that is compressing or flexing.
Now I would advocate that the whole load for the shroud be carried down further to the hull.
This might be unsightly but is effective. A saddle is attached to the 2 bolts. A plate of decent size is glassed into the hull side with a tag emerging from the plate. Fit a wire strap with turnscrew in line to enable the deck and shoud attach to be screwed down to avert lifting. Far easier than ripping apart the deck to strengthen the core. ol'will

 

[Matt--H]

It looks like OP has pulled out the pad etc already. I would have advocated careful measurement and observation to see exactly what has compressed when the shroud is loaded up.

As per the photo, any original block of wood was long gone and some green stuff was pushed in there in its place. The 1st photo in the thread is the other side of the boat, the second one with the green was the issue. Starting to load the shroud had lifted the plate to the position that I took the photo - it didn't return/move back on unloading.

 

[Refueler]

The core material adds very little to the strength of the stringer. It's really just a former for the GRP wrapped around it. The stringer only adds longitudinal stiffness in most cases. A bit like the difference between a sheet of thin steel and the same sheet turned into corrugated sheets.

I am aware of what a core does .... and that its basically a stable filler.

The stringer though - is a structural support for the deck .... and IMHO needs care in repair.

 

[geem]

In post #18 the OP shows the excavated core. It's a very strange detail to have a balsa cored deck plus a stringer and so close to a bulkhead. The area of excavation is right at the end of the stringer close to the bulkhead. The issues the OP has is producing a strong repair that doesn't repeat the mistakes of the original builder by using balsa around a chainplate where any movement of the chainplate will allow water in to the core. The area of the chainplate connection would be best in thick solid glass as it's rot proof and strong. Since the stringer finishes pretty well at the chainplate, replacing this section of the stringer with glass and epoxy will be way stronger than what was there originally. The OP simply needs to glass his new section of thick glass pad in to the existing stringer.

I would probably get a piece of window glass on my work bench, wax it and lay up a few layers of heavy E glass, say 300x 300mm . Once cured, cut the glass into the size you need to replace the core under the stringer and glue the squares together with thickened epoxy and clamp them up whilst it cures. You will then have your block of glass/epoxy to the size and thickness you need. Glue this to the deck head with thickened epoxy and glass in place with some 300g cloth incorporating the end of the existing stringer to maintain stiffness. I presume the areas of rotten balsa away from the chainplate will be replaced with balsa and also glassed in place with 300g cloth.

 

[Daydream believer]

In post #18 the OP shows the excavated core. It's a very strange detail to have a balsa cored deck plus a stringer and so close to a bulkhead.

Does your remedy allow for the end of the stringer to be bonded to the bulkhead. It is that bond that has probably weakened & allowed the stringer to crush up under the cored deck. It may be that the design was not relying entirely on the deck for strength but a mixture of stringer-stringer/bulkhead joint-deck. All acting as one. Just sticking a pad in & ignoring transferring load to the bulkhead etc may not work. But I may have not understood your description correctly

 

[Refueler]

It is not only about putting back strength - but also continuity of stress loading.

Some may think its not necessary to consider - but IMHO it is a valid matter. I always try to avoid hard 'break points' in construction whether it be in my RC models or in boats / house etc.

I get good impression that OP is actually well up on what he will do and I wish him greatest success.

 

[Refueler]

Does your remedy allow for the end of the stringer to be bonded to the bulkhead. It is that bond that has probably weakened & allowed the stringer to crush up under the cored deck. It may be that the design was not relying entirely on the deck for strength but a mixture of stringer-stringer/bulkhead joint-deck. All acting as one. Just sticking a pad in & ignoring transferring load to the bulkhead etc may not work. But I may have not understood your description correctly

Thank you DDB .....

Continuity of stress / spread of load is a consideration IMHO. Something a solid pad without scarf etc. does not allow.

 

[Matt--H]

Thanks for all the inputs people, much appreciated, and a plan is forming, with thanks to your advice.

One element I'm recognising is that I'm probably going to need a more extensive core replacement for an area about 10" square (looking at the darker marks I can see above the glass coat from undeneath). What alternative core product would be recommended, as I'm a little shy just to put balsa back in again. (links appreciated)

Many thanks,
Matt.

 

[Refueler]

You can buy structural small bead foam - similar to what boat builders use instead of balsa. Easy to shape / glue in place - can be made up in layers to required thickness ...

BUT beware .... many foams are 'eaten' by Polyester Resin and Catalyst .. it literally dissolves in front of your eyes !! So need to check that it is suitable for the Resin you will use. Its one reason why I would usually suggest West System or similar wetting out Epoxy resin for the whole job ... fixing in place parts - core - and glassing over ...

Just for thought ... an old trick to get a reasonable finish to a glass over job - especially overhead :

Have a stiff but flexible board .. thin ply or thick ceiling tile covered in plastic wrap ... once glassing in place - raise this plastic side up and have vertical posts / blocks keeping this pressed against the glassing, following the curvature etc.. Once nearly cured - can be removed leaving behind a relatively smooth finish

Just my thoughts ... many other ways

 

[cherod]

I like Geem's suggestion -

You can buy stainless off the shelf as 90 degree long strips of various dimension -
Jonathan

Referred to as “ angle “ ?

 

[Neeves]

Referred to as “ angle “ ?

I sat wondering what they were called - racked my brain

and then gave up!

Thanks!

Jonathan

 

[srm]

Way back a post suggested new chain plates on the cabin sides.

Having seen the deck photo that would be my preferred option. Strong and simple using the cabin side as a beam to spread the load. As this rig attachment has failed the others must be suspected of the same failure waiting to show, so I would replace them all.

The internal work at the damaged area then becomes the much simpler making good of the stringer and rebuilding the excavated deck.

 

[Matt--H]

Way back a post suggested new chain plates on the cabin sides.

Having seen the deck photo that would be my preferred option. Strong and simple using the cabin side as a beam to spread the load. As this rig attachment has failed the others must be suspected of the same failure waiting to show, so I would replace them all.

I've not seen a colvic Watson with the chain plates on the cabin sides before? The roof is cored, but the sides aren't as far as I'm aware so the extra internal strengthening to put in place to achieve that could be significant.

I'm inspecting all the others (indeed,1st photo is the corresponding starboard side), so am not expecting emerging problems later one (hopefully).

 

[srm]

The roof is cored, but the sides aren't as far as I'm aware so the extra internal strengthening to put in place to achieve that could be significant.

The cored roof is the problem, as the core slowly failed under compression allowing water in that speeded up the failure. Hence my saying the others are also suspect, even though they may appear to be sound now. Good practice is for the area with any attachments through deck to have a solid core to prevent the compression problem. Attaching the chainplates to the cabin side will not have that problem. Glassing in a good quality ply pad on the side then bolting through from chainplate, cabin side, backing pad and a plate of similar size to the chainplate to act as washer will give a stronger fixing that is far less likely to fail.

However, the joy of boat owning is that we all have our own ideas and are free to do as we wish with our own boats. My current boat was fitted out by the first owner. Most of the work is excellent but there have been some bits that I did not understand, or did not work, that I have had to remedy to my satisfaction. Fortunately, not chainplate failure though I do have the start of a leak where one passes through the deck before fastening to a semi bulkhead. A previous boat I owned was yard built, and had some serious design and workmanship flaws that caused problems as it aged.

Perhaps considering how you will use the boat and how long you expect to keep it will help guide your decision as to the best solution. My sailing areas have always been out into the open N Atlantic / Norwegian Sea so I tend to look for something that I will be comfortable with in my worst likely conditions.

 

[Matt--H]

Glassing in a good quality ply pad on the side then bolting through from chainplate, cabin side, backing pad and a plate of similar size to the chainplate to act as washer will give a stronger fixing that is far less likely to fail.

What about an excavation of balsa core and replacement with glass or ply pad at each of the existing fitting locations in a suitable sized area instead though? It would potentially yield a similar result without the need to fill 12 holes in the saloon roof (which will always be a visile repair) and go again elsewhere on the sides? (plus I have windows and portholes to contend with)

I would say, this is a motor sailer with a modest sail area, so the stresses involved are a little less arduous than in other situations.

 

[geem]

What about an excavation of balsa core and replacement with glass or ply pad at each of the existing fitting locations in a suitable sized area instead though? It would potentially yield a similar result without the need to fill 12 holes in the saloon roof (which will always be a visile repair) and go again elsewhere on the sides? (plus I have windows and portholes to contend with)

I would say, this is a motor sailer with a modest sail area, so the stresses involved are a little less arduous than in other situations.

I think you are on the right course. Go for it