Ribs and their sisters!

stevemeakin

stevemeakin

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I've found a few cracked ribs that I need to attend to in my Stella (what a surprise eh!). One of the ribs that is cracked is just above the stringer in the focsole and there isn't a great deal of shape at that part and so I'm planning on shaping a piece of seasoned oak and sistering it up - is this the right thing to do? the other cracked rib requires a great deal of shape and although I think the Oak I have could be cut as one piece, I wonder whether this will be ok. I haven't fitted ribs before and have limited tools and certainly no 'steaming facilities'

Thanks in advance
 
Sistering is a 'quick-and-dirty' fix, because the forces which broke the rib in the first place end up being transferred to the ends of the sister and break it again there. A better repair is to cut out that section of the rib with tapered cuts [ie, scarves] and glue in a new section. If there is any shape the new section should be laminated if you cannot steam the rib to shape. It is important to have the grain of the wood following the curvature of the planking. If the grain runs out to one side the rib will be weak, and will break along the grain. If you are laminating a new rib in, you can cut steps in the upper and lower ends of the original rib to match the thickness of the new laminae, and make each lamina correspondingly longer than the one below it. This is an alternative to a scarph.
Peter.
 
Cracked ribs are not uncommon in older clinker hulls, but should be attended to properly and promptly. Otherwise the neighbouring ribs have to work a bit harder, and you can easily imagine the postage-stamp-perforation effect.

I put my trust in John Scarlett and his pragmatic volume "Wooden boats: restoration and maintenace manual" (ISBN 071538077 X) which tells it like it is. Sistering is considered a second best option due to the discontinuity in the path of transmitted forces in the rib. Certainly in my clinker hull the doubled ribs have failed in places, and have been doubled again - what a mess. Out they all came to be replaced properly with fresh oak.

In your case it would be appropriate to scarph in a new section to the existing rib, assuming you can wriggle the new section down the back of the stringer.

Shaping ribs from solid isn't very effective if there is much shape, as the grain will run out at the ends, just where you want some strength to create a good scarph joint. Steaming is the correct way to go, and not as difficult as you might think. The wallpaper stripper / drain pipe combination works well. More important is the timber used. Fresh cut oak with the correct grain orientation - you'll have to find a sawmiller who will cut a piece to order, but if you don't get this element right then all the rest of your work will be in vain.

To sum up: read up, do it right, don't let bodgers and short-cutters tell you otherwise, and if you can't tackle it yourself then put the work out to someone who can. You boat willl thank you for it, and some future owner uncovering your repair will breathe a sigh of relief that it was done properly.
 
the scarf idea has been suggested (thanks Andy). What would be the best way of getting a scarf on the ribs in situ as I can't think of the best way of getting a good edge. Also, when I epoxy them into place, should I put a couple of nails through (with roves), or unless there's already a nail there, leave the epoxy as the main strength?

Thanks again

Thanks
 
I am not convinced that epoxy is the best glue to use with damp oak. Please make inquiries of the glue makers technical experts before you start. The scarps need to be long, 6 to 8 inches if you can, a sharp chisel and a steel straight edge should be capable of cutting a flat straight taper, or try a Japanese pull saw to rough out the joint. Do not be frightened to cut well away from the break, as it is easier to work with a longer replacement piece than faff about with a short bit.
 
I agree with Npm that epoxy is probably not the best adhesive for the job, as the moisture content of the wood will be too high, especially in you green oak replacement parts. I have had good results with polyurethane glues for similar jobs. Be aware that not all PU glues are the same - some of those marketed at the building trade go off far too fast to be used in marine work, whilst others seem to be rather brittle. My favoured brand is Balcotan, and particularly the Fibre-gel series which are thixotropic and more tolerant of slight gaps in the joinery. The long open-joint time is a great benefit.
The length and position of your scarphs will be governed by where you can get access most easily, bearing in mind that the replacement section should ideally cover at least 3 planks before the scaph joints. Stopped scarphs are tricky to cut in-situ, but a sharp chisel, patience and frequent checking will get you there in the end. I've found a small shoulder or bull-nose plane helpful for final clean up of the mating faces.
Aim to get three copper rivets through the scarph joint, with a suitable wedge behind it to close the gap with the plank.
 
You can use a piece of steel pipe with a screwed stopend: Half fill with water, prop the open end up and apply burner/fire under the lower end, stuff a rag in the top. It doesn't matter if the wood is in the water. (Well it was OK with a big bit of Iroko, he added hastily.)
Otherwise a big kettle with a hose from the spout to your steam pipe/chest/box, jam up the gaps with rags.
 
Don't sister. Either take the whole timber out and replace - or laminate in a replacement section of the timber. Laminating saves quite a lot of work making the outside surface good after replacement. OF
 
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