That is definately a job for a hand plane.  Clamp one side to your work table and plane down the other, then switch sides.  Do not plane all the way to the line before doing a fit check.   

I guess I'm using the plane wrong (even after watching youtube videos). The plane just gets caught on the wood, instead of peeling off shavings.

The breasthook is make of plywood, so I'm planing against the grain. Should that be possible?

   Once you get the plane sharp and dialed in, yes you can cleanly plane those edges, but it takes some fiddling.  I'm no expert, but I did read up on sharpening and got my iron a lot sharper. New ones from the store are likely crap edges unless you bought a $$$ premium one. Then you need it square and the right depth in the plane.  I have to keep it really shallow to plane plywood edges.  I'm still not that great, but it is sweet when you get it working right.  I shaped my spars for my skerry with the plane and it was nice getting those long runs along the timber to bring the corners down.  The curlies come up and fall away and it's great.

Really, for this, a rasp can work really well.  Again, do the last bit with sandpaper, and I'd do it w/ a hand block and 80 grit paper.  You need the bevel to fit well agains the sheer plank.

   if you can't get a block plane working, 60 grit sandpaper on a block could do the trick with a little effort.  you could also add a beltsander to your tools arsenal....a beltsander running 60 grit paper would make easy work of this...a beltsander is a pretty useful tool and can often be found on craigslist for less than $100 bucks.

h

Agree with everything already posted but want to add that a bandsaw or a power jigsaw could handle this, too, especially if you get the angle with a bevel gauge and tilt the table to match. If it's a rolling bevel you'll need to touch up the angle with one of the other methods mentioned above.

That said, my MO in the past has been to make a cardboard template and use a simple handsaw to cut the piece so that the edge is plumb vertically mostly making contact all around the bottom with a gap along the top edge. Then I tacked it into place with epoxy/woodflour putty and waited for that to cure. Finally, I filled the gap with epoxy/woodflour and smoothed it into a nice fillet. I always filleted and taped the underside and depending on the boat design the top may have gotten some tape, too.

The advantage of doing it this way is that since it's not fine joinery it's faster and takes less skilled effort and specialized tools. A simple handsaw is all you need and the edges don't even have to be cut to fit particularly well. In spite of that it's very strong structurally. The gap-filling properties of the putty give you a strong structural joint that transfers the load evenly between the pieces, the glass tape gives excellent strength and any irregularity in the shape is hidden by the fillets.

Until you have time to practice and the money to acquire the more advanced tools, this is an easy solution.

Lszlo

I went to a nearby woodworking store (Rockler) and spoke to them. They talked about a number of possible saws that could work. There is amazingly specialized equipment! And it is expensive!

The guys I spoke to at Rockler agreed that a belt sander is probably my best bet, it is not that expensive, so that's what I'm going to try.

   I use one of these for shaping and sanding odd bits. I find it incredibly useful.

https://www.homedepot.com/p/RIDGID-Oscillating-Edge-Belt-Spindle-Sander-EB4424/202459151?source=shoppingads&locale=en-US&&mtc=Shopping-CM-F_D25T-G-D25T-025_009_PORT_POWER-RIDGID-NA-NA-SMART-3047250-WF-SMARTSHOPPING_PL2&cm_mmc=Shopping-CM-F_D25T-G-D25T-025_009_PORT_POWER-RIDGID-NA-NA-SMART-3047250-WF-SMARTSHOPPING_PL2-71700000083187724-58700007049582709-92700063451361524&gclid=CjwKCAjw_tWRBhAwEiwALxFPoVxP-7k0sp02TlgPxhyTF3o8-Vcc5loIuhLbjBSK6TmUPd-iGJ27SBoCpaoQAvD_BwE&gclsrc=aw.ds

Dan

   Oops. Sorry for big (non) link.

End of the story:

I was all set to buy a belt sander, when I had dinner with a friend who had exactly the right tool, a bandsaw (?) with a surface that could be set at an angle:

I did a not quite perfect job of trimming the breasthooks. In some places, I trimmed too much, and I'll have to fill those with epoxy and wood flour (I think). In other places, I didn't trim enough, but I was able to sand those by hand. They fit reasonably well:

   I don't have room for band saws and such but I find a shinto rasp to be pretty darn good and shaving an edge down quick

   Shinto rasp /saw file is a great tool for this type of work.