replacing ribs and planking in clinker dinghy - which first?

Hello there.
I have a friend's 10' clinker rowing dinghy languishing in my workshop. V light - 1/4" mahogany planking on seemingly 3/8" x 11/16" steamed oak ribs, at least 50 years old, a lovely thing, made by Wright & Sons of Ipswich. I have either to repair it or convince him to put a match to it. Several planks are rotten, many ribs cracked at the turn of the bilge, stringers and rubbing bands broken/ cracked, knees coming apart, foredeck needs replacing, some evidence of grp patching internally on planking at transom below the waterline etc etc. Still mostly in good shape and repairable I reckon, a good excercise in patience and perseverance.
Crucially the shape seems pretty good, not twisted or saggy, my question is, how to keep the shape, bearing in mind it's as dry as a bone, been sat on a trailer for years in a barn? Should I do a few ribs then proceed with the planking, just replacing the rotten sections rather than unstitching the whole plank, with the faff of shaping the plank ends, or reinforce the broken ribs temporarily and concentrate on the planks first, that being the correct order of play?
Thanks in anticipation..

 

I have a 70 +/- year old 16 ft lapstrake inboard skiff (Jersey speed skiff) suffering the same problem. ....every rib broken in multiple places and almost every plank below the waterline split somewhere. ....
I bent in all new ribs (next to the originals , using small wooden blocks screwed to the originals to keep the new ones in place), then changed out every other rib (riveting only to laps that are staying, then changed out / replaced the planks, and finally swapped out the other half of the ribs.
I removed the planks to repair / replace sections.

 

Thanks for that Ned - good idea with the blocks though old ribs are cambered on top so would have to screw a strap across two old ribs. I take it you using the hull shape as a former for clamping steamed ribs to before pulling out old and shuffling across new, in which case nail holes? Do you fill and re-drill or put new exactly in way of old? Either way awkward job.

 

I place the new ribs exactly where the old ones came out of and use the same holes in the laps. .. Yes, a bit tedious.

 

and on my larger boat...

 

From a practical point of view we could ask ourselves whether we really need that many steam bent ribs. The shape of the sections are reasonably suited as a stress skinned (monocoque) design. It all depends on how well the strakes are married to one another does it not???? Is it conceivable that more attention should be given to the strake overlaps than having to replace all those ribs? Replace some of the ribs of course, but all of them???

Those were elegant designs for boats that were done in the distant past by superb craftsman that may not have had a formal education in structural design methods. All those ribs are dirt, moisture, fish guts, and bacteria catchers and rot promoters. Many a glued lapstraker has no bent ribs at all and they are mostly more than adequate and a damned sight cleaner in the interior.

 

Natural wood planks riveted together can split along the laps if there are not sufficient ribs holding the planks together.

Many traditional designs are based on decades or centuries of experience with boats which lasted and boats which failed. Experience taught how many ribs were needed.

 

Her ribs are on 7” centers. Yes that is a lot of ribs, but 6-7 inch centers was the norm, so hers are not unusually close. She is a 61 year old inboard power boat of Jersey white cedar planking. When she was built the world didn’t use travel lifts, marine railways were still the tool in use, and they are much easier on a hull. So knowing she would be moved by travel lift from now on I actually doubled up on the ribs at the turn of the bilge where the rear strap will lift her (you can see those ribs in the last picture).
Riveted cedar is not glued plywood, I think it would be an unwise choice to reduce the number of ribs, but that is an interesting thought.

Here is the boat by the way.