Repairing ribs

[rjp]

I'm refitting a 32ft Buchanan design which has ribs of bent Canadian Rock Elm. A few ribs have cracked on the outside radius at the turn of the bilge - not right through, about a quarter to half the thickness of the rib. I'm proposing to cut out a length to the depth of the crack and laminate the area with thin strips of suitable wood, bonded with epoxy, then install new fasteners in the affected area.

Can anyone comment on the suitability of this approach? What wood should I use as I believe CRE is difficult to obtain? Should I use the same size fastenings (10g copper nails/roves) through existing holes in the planking after drilling pilot holes through the new timber, or would it be better to plug eveything and start again?

Sorry for the number of questions, but I haven't done anything like this before!

John

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[Peterduck]

The repair plan that you propose sounds fine to me. The species depends upon where you are, and what timber is durable and available. In Britain it would be oak, I guess. The fasteners [I presume we are talking rivets here] could be the same size if the old ones were in good condition and the holes were tight on the rivets. If you find that the same size rivets are a loose fit in the holes, then either bung and drill again or use oversize rivets. I would prefer the bung method. Make sure that the grain of the timber does not run out of the side of the laminate plies, or you will have them breaking as soon as you try to bend them. If the curvature is tight, soak the plies for a week or so and then heat them with a heat gun as you bend them around the curve. I would also make the outer plies longer than the inner ones, like the leaf spring on a car, so that there is no sharp change from original frame to repair. Such a sharp change would be a weak point in the future. Good luck.
Peter.

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[rjp]

Thanks very much for that, always nice to have a second opinion. I have a small quantity of American white oak to hand with a nice parallel grain so I'll use that. I'll step the laminates as you suggest.

John

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[roly_voya]

For minor cracks I have also sucsessfully use epoxy by drilling 5mm hole in center of crack and using a plastic medical sarynge injected epoxy until is come out of all parts of the crack. Cirtainly works as temp repair and not had problems of cracks opening again where depth is less than 25% of thickness.

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[mikesharp]

Are the cracks recent or original? They may have been there since original build.
An alternative repair that does not disturb the original frames too much is to sister them by building a new frame alongside each one (you could use laminated white oak for that as well) and then bolt them to the cracked frames for support. The Gougeon Brother's book (West System Epoxy) explains both options.

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[rjp]

Thanks for the replies. I was aware of the 'sistering' solution but felt that repairing the existing ribs might be a tidier job. It could indeed be that the cracks are original, they certainly don't look recent. About four ribs in all are affected, three in a row one side and another opposite one of these. I feel that's too many to ignore, even if they have been there for years (the hull is about 40 years old). They coincide with the maximum depth of the bilge and the maximum curvature as well. Cracks are on the curve and just above the floors, which are galvanised steel. This of course raise another issue - we've been talking about using oak, but I believe there can be corrosion problems with oak and iron. Would this apply to galvanised bolts? Above the level of the floors there are copper nails and roves, so no issues there I guess, but part of the repair would have galvanised bolts through.

I wondered about using epoxy to fill the cracks, but one rib has split longitudinally a little way beyond the crack, albeit only an inch or two, and I wouldn't be surprised if there wasn't some dirt or even oil traces inside the crack to affect adhesion, so feel it's probably better to cut back to clean wood.

John



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[Peterduck]

Sistering is worse than untidy; the stresses are relocated to the ends of the sister rib, and the original rib then breaks there, so that it is now broken in two places. If, as you say, the rest of the hull is fastened with copper rivets, then to use galvanised bolts will invite electolysis on board; always an unwelcome crew! The zinc of the galvanising will be the first anode, and will disappear quickly. The mild steel of the bolts will be the secondary anode once the zinc has gone. I actually use mild steel plates as anodes on my old ketch,and they work [ie, disappear] just fine. The tannic acid in oak also has a deleterious effect on iron, and both the iron and the oak suffer. I'm a gloomy old sod today, aren't I?
Peter.

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[Mirelle]

I agree with Peter about sistering - best avoided. With an iron ballast keel and mild steel floors, however, I reckon one can get away with steel bolts reasonably well, if the heads are sunk into the planking and puttied over - and if the bilge is kept dry and free of stray electrons, of course! The option is bronze bolts through the mild steel floors which is "asking for it!"

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[rjp]

Thanks for the additional contributions Peterduck and Mirelle. Interesting how the picture is evolving. I'm stuck with the mild steel galvanised floors and the galvanised bolts through them. I've drawn a couple of bolts and although they are rusted on the heads with no trace of galv left there though there is still plenty of metal there so I'm not too worried at this stage. No wasting of the shanks at all. Everything above the level of those floors is copper nails/roves. The keel is lead by the way, so no problems there. Oh, and there are no anodes, not deliberate ones anyway!

I had read about the problems with sistering previously, which was why I was seeking to avoid doing it. I wrote to the designer, Alan Buchanan, a couple of weeks ago and received a reply just yesterday. His second suggestion was sistering, 'if you really feel you must do something'. Interestingly his first suggestion is to do nothing, on the basis that the ribs are only 'half-cracked' (In fact only one is even half-cracked, the others are a quarter-cracked or less). So perhaps I'm being over-cautious to contemplate a repair at this stage. On the other hand I'm going to need an insurance survey sooner or later, so how would a surveyor view this?

John

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[chippie]

Re your galv bolts; I have galv bolts through my hull into mild steel chainplates, like you it was the head that rusted first. To overcome this I coated the heads of the new bolts in epoxy prior to painting, my view is that it is one more layer to get through before rust sets in.

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[rjp]

Thanks, that sounds like a useful tip.

John

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