Over twenty of them... wasn’t bad after I got going. I cut the rib stock to length- about an inch or so more than I needed and cut it to the width of where the bottom of the scarf would be on the existing rib with a block plane. Then I cut the angle on the new piece with a belt sander clamped in a vise upside down. These ribs taper and are beveled (trapezoid shape) and I roughed that in with a block plane. Then I used the belt sander to cut the angle on the old rib matching it to the new piece as I went. Brushed tight bond III on both pieces and clamped. I will have to sand them smooth and touch up the tapers/bevels with a spoke shave. And stain them to match.
I have done this with a chisel before, but find I get a better fit with the sander. I have also used epoxy and PU glue. All work well, but the PVA glue cleans up easiest for me.
This is my method, but as with most things always more than one way.
Bob
Removed the inwales and top planks to expose the rib ends. One side was really rotten under the inwales. Looking forward to the rebuilding bit rather than cleaning and taking apart. 
Mocking up the stem ends today from some ash scraps. I only need the top few inches of each. They are thicker than they need to be. The backside of the scarf will run down the existing stem to give more surface area to glue. 
Well I was supposed to be on a trip this week, but weather took a turn for the worse- “hail and damaging winds” according to a couple forecasts. So some local fishing and wood shop time finally. The inwales are in, rib ends trimmed, and getting the stems shaped. Next- decks, planking and replacement ribs. Not to mention cleaning the shop!
