Sounds like a plan!
I would have also pulled a mold off the hull, but flipping it over would be difficult on the side of my house.

Yeah a few ways to do this.
You could also cutoff the original transom and add in the molded section to lengthen the hull.

The simplest way would be to start with a longer boat and then make an integrated bracket out of the last few feet, but I started with an 18' instead of a 20'.
Maybe the next boat I'll cut a 23' down to 21'!

Btw. It's hard to tell from the pics, but the sides of the tub are slightly tapered in, about 3 degrees; my design is more of an integrated bracket than a hull extension.
It looks like the hull has some rocker built in so my tub tapers slightly up too.

I'm new to this community but have been co-owner of a 1980 vintage 18' Seacraft for almost 20 years. I love the boat. But it's major maintenance time and I've just about decided to do a big refit next winter -- new deck, fuel tank, and perhaps some transom modification. Hence my interest in your work Dave, which is very impressive.

I look forward to reading how this boat rides with the Euro transom modification. For me the hull works great. Eliminating the occasional water intrusion through the open transom would be a big plus if the cost in performance was small.

You are right about the weight -- it's a heavy boat. I would not have guessed how much weight comes from the deck, and I'm looking forward to addressing that. One thing I really do like is the raised platform in the front -- handy for the things we do while fishing -- so I'll probably keep that in the plan.

Skip

Finally got the extension/bracket mold finished; with all the rain and mess ups it took me a lot longer than I thought.
It is definitely a ton more work than closing the transom and making a separate mold for the bracket.
You can't see in the pics but I had to fully support the mold and hull, so there would be no movement between them. Every time I got in the hull or leaned on the mold it would separate!

Oh well, I wanted a project and of course picked a most difficult one!
I learned a lot esp. what to do differently and simpler- next time!


Waxed the mold three times and tried to spray Pva with PreVail sprayer- well that didn't work, so I brushed on the Pva. It came up with lots of fish eyes and streaks. It was ugly, which I could have lived with, even with a bit more fairing work, but there were areas that weren't coated. And finally with all this rain , the layer of Pva just lifted off by itself!

So reading on the West Systems site, they used hairspray as a mold release! Which is water based and the aerosol spray can made it easy to get an even coat on the mold. Two cans covered about 8 coats; trick is to spray light coats, esp. the first tack coat.

After using the stuff, I recommend the extra hold, unscented! Too bad it's clear- makes it harder to see the coats.

Decided to do a test batch first before committing to glassing the mold; sprayed some hairspray on the outside of the melamine board and brushed on some plain epoxy, CSM matt, and 1708 samples to see if it will separate cleanly.
Will see tomorrow if it's a go for glassing or I may have to add wax- again!

Lots of pics, in case it helps anyone else.

Recent pics.

Bracket finally glassed!
 

It took a lot of time to measure and cut all the Matt and Biax and way longer to lay it on.
And of course had to cleanup mold and hairspray/PVA it again.

Three layers of Matt and three of Biax, but with the overlaps, some areas, esp. the corners, have 6-9 layers.
7 gallons of epoxy including the thickened putty for the transom.

Finally with no rain, started the layup Sat. afternoon, continued through the night and got all the way up to installing the core and it didn't fit!
All the layers of of cloth had made the space too narrow and trying to wedge it in, I screwed up some of the cloth in the corners.

Since it was 3:00 am, I didn't break out the grinder, but tried to hand file it; after half hour of that, gave up, cleaned up, and got to bed at 4:30.

Next day checked it out; looks like I'll have to grind and repair in the corners, but overall the tub looks good.
The transom has a nice layer of thickened epoxy and fibers that I'll have to grind flat to attach the core, but at least it won't be so messy.

My concerns are I have some white areas; small bubbles in the last layer of Biax, mostly in the hull forward of the tub; it looks solid underneath. I'll fill in the small holes with epoxy and grind the larger ones out and glass them.

There are some bigger areas, between the strakes and going up the sides, that I'll also grind out and lay Biax.
The sides really stiffened up with the extra glass, so I can imagine how the rest of the hull is now.

Also I'll be tying more glass when I add the stringers, sole, and core the sides, so there's going to be even more overlaps.

Oh yeah, had to cut some epoxy off my armpits and head!

It didn't seem like a big job until I got into it!
Probably been a lot easier to enclose the transom and mold a separate tub, but hopefully everything else will be easier from here on.

Most epoxies that I've used won't dissolve the binder that is used to hold straight matt together, the polyester family works best for this. Biax is not an issue with epoxy because it is stitched together. Your white spots most likely are the results of the incompatibility. I have been in the boat repair/restoration business for 30 plus years and learned this the hard way as well.
Rod

Transom core is in; at least that came out good.
Ground out all the white spots since and repairing them.
Removed a lot of the Biax, and the underlying Matt was good, so laying in fillets and Biax tape on the edges and then glassing again.

Removed the white areas: chiseled out the big areas and drilled/ground out the smaller ones, then grinded and sanded it; ended up removing a lot of the Biax. Filled in the holes with 1708. The underlying Matt looked good; probably soaked up a lot more resin than the Biax.
Then more grinding/sanding. I hate wearing the full suit in this heat and humidity; lose 5 lbs every time.

First troweled on thickened epoxy with Cabosil and chopped glass fibers, onto the edges to make my filets, and filled in some small areas that weren't level; this is what looks white under the Biax and not air bubbles like last time!
This time I rolled out the epoxy on the Biax tape on a wetout board, rather than pouring it in and using a large squeegee; I fileted and triple taped all edges. It went a lot faster- only 5 hours and one gallon of epoxy.

Came out a lot nicer than last time. It's now about 3/16" thick at the flat sections, 1/4'"at the edges/overlaps and 3/8" at the transom corners.
Going to fill in the middle sections where there's no overlap and even up the surfaces.

Seeing that the mold separated from the hull a bit in the outside corners, I took a peek and it looks smooth- way less fairing!
The center of the corner panels are only about 1/8" thick, so I'll build that up with 1708 and probably another layer over everything.

The lower hull is definitely stiffer now than before; the original layup was fairly thin.
Since I removed the liner, I can see how stiffening the sides with Biax and some core will help.

Next to do the inside skin.

Theres a 23 for sale in chicago craigslist with a similar setup. Newer model but neat setup.

I could have saved a lot of work, buying this!

Interesting tub design; doesn't go all the way down to the keel.
Reminds me of an Intrepid.
I have a similar Euro style transom on my 18' Sea Hunt; bilge access sucks and water loves to sit there, or leak past the deck plate.
I can't understand why manufacturers make the tub to hold water, rather than raising the transom to hold the motor and have enough room below it for the cables and steering, and make the tub flat.
I'm putting a level swim platform across the whole back.

If I do this again, I'll buy a 20' and cut out the back.

Transom inside done-finally!
 

Added 1708 pieces between the taped edges.

And filet and 1708 tape on the corners of the hull and covered that with another 1708 layer to get to about 1/4" thickness in the middle of the panels.

Then glassed the inside of the transom- layers of 1708/1700/1700/ with a final layer of 1708 over everything- this layer was a pain, besides being unwieldy to layout, the glass gooped up in places and the rollers that I applied the epoxy with started to break off. At least the white spots are bunched up mat and fibers now, rather than air pockets.
It looks crappy, but it's solid!

The outsides of the corners came out mostly smooth; very little fairing. Leaving the transom piece on until the stringers are done.

Mold off!
 

Decided to remove rest of mold, since I was concerned with undersides.
Overall looks good, panels are flat, and strakes are straight. Some white areas to repair but looks like I won't have to sand/fair as much as I thought.
It looks like the glass edges at the transom is about 3/4" thick!

If I had to do this again, I'd use plywood melamine so moisture wouldn't distort it, thicker panels for the bottom to help keep the shape, and make the tub sides out of one larger piece, instead of the two pieces I used, so seam would be cleaner.

This is so cool to watch.

Been awhile.
Glassed the transom top, filled in the bolt holes, and most of the grinding. Tub came out good; a bit more sanding underneath.

Made the forms to raise the stringers.

Poured the foam.
Will cut out for the rigging and drain tubes.
Then lots of glassing!

Stringers done.
Filleted, tabbed, couple layers 1708, and lots of epoxy.

Keelson removed.
Going to replace with about 6" wide composite board; will act as drain for fish boxes and subfloor for fuel tank.

Next lots of my favorite thing- grinding and sanding.

Do yourself a HUGE favor. Sandblast instead of grind. Way faster and easier. Harbor freight stuff is fine. You will use a lot of sand but be super happy with the result. Getting in corners is easy that way.

Thanks, but I just spent last four hours grinding and sanding; went quickly with 36 grit grinding wheel and 60 grit sandpaper. Did the stringers and mostly between them to prepare for glassing.
Lots of dust but had the full suit, pullover hood, respirator, goggles, and gloves on. What I hate most about grinding is putting on all that gear and the dust and smell. And usually the heat, but at least it's February now and not August! My usual attire is flip-flops, shorts, and T shirt.

Removed the bow eye (had to use a torch) and I'm glad I did; the wood backing plate was rotted.
With the raised sole, I have to raise the bow eye too.
The bow eye is the very last original part of the old boat, other than the hull.
Also ground out a few large globs of putty/glass fibers, with voids underneath; will fill them in and glass over.

Keelson is now gone!

Pic of bow area with bow eye removed and some voids ground out.
Some sloppy glasswork up there; and I thought my work was messy!

Amazing how they built boats in the 70's isn't it!?!

watever happened with this build cool stuff makes me wanna tackle my 75 18sf myself.

I got side tracked with other projects, a new grandson, taking care of an elderly relative, and moving up to Port St Lucie.
Waiting on getting a side driveway done and a large canopy so I can work on the boat again. Hope it works out otherwise I’ll have to have someone paint the hull.
Got to meet Chris/squid lips and his nice boat building setup.

Got most of the hull to finish primer and have to build the pilot house still.

Some older pics.

A few more.

Nice work!

Forgot to update this thread!
Finished her up in early 2019, glassed in the hatches, added a Pilot house with Wynne glass Windows, and painted it.
Then the big money for the motor-200AP Suzuki, hydraulic steering, Bennett Bolt electric tabs, BocaTech switches, wiring, lights, vhf, dual Garmin 1042xsv, GemLux thru hulls, steering wheel, rodholders, and lots more.
She rides well esp. loaded down for offshore. Needs some tab most of the time
Top speed 45, cruises about 30 at 4000 rpm, 4.0-4.5 mpg, depending on conditions.
When it gets sloppy can still run 14-18 mph.
Been Swording 25 miles out a few times and deepdropping quite a bit.
She handles well offshore, but with the wind and rough seas lately, a bigger hull would be better.
Also I’ve been exploring the inshore fishery here in the St. Lucie Inlet area.
I’m obsessed with Snook.
The boats high sides, and Pilot house creates a lot of windage, so it drifts quickly off spots. I'll need to add a trolling motor next.
Also it drafts a bit more than a dedicated bay boat, so tough to get in really shallow.
Who knew when I built this boat I’d be fishing inshore so much? Shoulda made it into a bay boat!

More pics.
 

Will add more as I can.
Anybody want to see anything in particular, let me know; I got a lot more pics.

And some more.
 

More pics.

And some fish pics.
 

Hard to find fish pics w boat.

Dave - Great to see the updates. I found a lot of inspiration for my 75 from the early days of your build and then found it on THT. That is a hearty rig. Wish I was close enough for a sea trial. Nice to see you and your family enjoying it. Nice looking snook.

I had a 1975 18SF as my first Seacraft and I have to say, you were able to cram way more bells and whistles into that hull then I ever could have imagined!
Nicely done!
Also, I love that the color on the bottom of the hull matches the liner. Really nice touch.