GT23

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fallguy1000
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GT23

Post by fallguy1000 »

Planning to build the GT-23. I built the FS-14 from a kit. And then the Phillip Bolger OldShoe, from plans of course, but using materials (plywood, epoxy, paint, etc) from BoatBuilderCentral. I built a small Glen-L sailboat 50 years ago at age 13. Have also built a coupla self-designed experimental boats of dubious quality.

I have some questions about the plans/methods. I see that Naylene built a GT-23 in 2020, using a strongback. And the GT-23 plans do call for using "a jig"....But then the plans
go on to describe a seemingly jigless stitch-and-glue process, where the initial hull construction is done with hull right-side-up, using the panel shapes and bow and aft transoms to provide the curves and shape of the hull. After adding the stringers and cross frames and sole and foredeck, the plans only then call for the boat to be rolled over, bottom up, and the bottom glassed....and still no mention of frames/molds/strongback. Dimensions of the stations are given, but no direct instruction to set the hull upon any molds... Is this supposed to be simply "understood"? That one of course always uses molds to shape the hull? But if this is so, then in this particular case the shapes/curves will already be largely fixed, because the plans call for the inside keel, chine, and transom seams to be built (epoxied) before the boat is rolled over upside down for the bottom work....
I am confused...
bobby
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fallguy1000
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Re: GT23

Post by fallguy1000 »

Jaysen-I took the liberty of creating a new thread for the OP. Maybe you can do it better.

But he wants to know about whether to build the gt23 upside down or rightside up.

I can't personally imagine any other way than on a strongback and jig.
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jacquesmm
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Re: GT23

Post by jacquesmm »

The GT23 can be built either way, right side up or upside down on a jig. The jig is made from strongbacks and molds, molds can be named frames, it is always the same method, same dimensions.
The other method, building right side up, does not guarantee perfect symmetry. It's fine for displacement speeds and looks easier but it is not. Building on jig is easier.

To build on a jig means with molds on strongbacks.
I dont' show dimensions for the strongbacks, that is up to the builder but I would use a minimum of 2x6's, two layers in order to get sufficient length.
The molds should be located at the frames, use the frames dimensions and spacing as on the plans.
Put the whole thing at the proper height using the DWL shown on the plans. Not too low in order to clear the floor with the transoms but not to high so that you can reach the bottom while fiberglassing. It is a big boat, even with the jig as low as possible, the middle of the bottom will be hard to reach. To set up the jig, I would have the BL at minimum 37" from the floor: the bow transom tip is 36" above the BL.
All dimensions are given from that baseline marked BL.
I recommend temporary molds made from cheap MDF.
When you build that way, the sequence is as follow:
- assemble the jig made from temporary molds except for the transom where you use the plywood transoms.
- plank the bottom then the sides using stitches
- build all outside seams, chien and transoms
- fiberglass the hull skin
- flip the hull with molds inside
- remove the molds, now you have an empty shell
- fiberglass the inside
- etc. etc: install inside framing, floor frames, sole, bulkheads etc.

I hope all that is clear.
I am camping on an island right now and have limited internet access, please be patient.
I can sketch a jig on my laptop but prefer not.
Check youtube with GT23 and GT27. There are several channels that show those boats built on a jig in detail.
I have links somewhere at home and will be back in my office around Jan 18.
Jacques Mertens - Designer
http://boatbuildercentral.com

drbobopp@gmail.com
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Re: GT23

Post by drbobopp@gmail.com »

OK thanks... I can build a jig, strongback and molds/frames using the drawings you provided.....No need to draw one up...

Thanks...

drbobopp@gmail.com
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Re: GT23

Post by drbobopp@gmail.com »

Hey Everybody
Went to Ft Pierce on our RV, rented a U-haul trailer, and came back with 40 some odd sheets of Okoume plywood, gallons of epoxy, yards of cloth from Jeff at BoatBuilderCentral .I've got the strongback built. Had to build it on casters as my shop is not tall enough to turn the GT-23 over. I'll have to roll her outside for that. I have the molds and stringers cut.

My question: I have the (aft) transom panels cut. I have ONE 9mm transom panel, and THREE 12 mm transom panels. I plan to mount a 140 HP OB on the back so as to achieve planing speeds on the flat, protected rivers, bays, and ICW of northwest Florida. I believe that I've read somewhere that the designer recommends an additional sheet of 12mm if the GT-23 is to used as a planing boat? But now I cannot find this anywhere on looking back again... ...Was I dreaming? How many 12mm panels should I use? Three? Or Four?

Thanks, yall.

fallguy1000
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Re: GT23

Post by fallguy1000 »

I can't say for sure, but for 140hp, I'd want 4x12mm, radius the top and glass over it.

Perhaps the idea is to swap the 9mm piece for a 12mm as a 2" transom is about the minimum for 140hp planing..
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Fuzz
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Re: GT23

Post by Fuzz »

I would think 1 7/8 plywood plus glass would hold 140hp with no problem.

fallguy1000
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Re: GT23

Post by fallguy1000 »

Fuzz wrote: Mon Mar 20, 2023 4:39 pm I would think 1 7/8 plywood plus glass would hold 140hp with no problem.
As always. All depends on how much glass..
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TomW1
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Re: GT23

Post by TomW1 »

Jacques rates it up to 150HP as designed, unless there is a note in the plans, go with the plans. He always over designs the transom in case some idiot wants to put extra power on their boat over what he specifies. In all reality that transom could hold a 175 or 200HP motor, but do not know if the rest of the boat could handle it, Jacques just over designs the transom for safety. He does this for all his boats. Many years ago he and I talked about this and general design points he used when he was on vacation here in the mountains.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

Dougster
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Re: GT23

Post by Dougster »

Probably you know this but I recall there are two slightly different plans for the GT23. One is the standard planing hull and the other is the houseboat hull with I think a bit more volume forward. As I recall some folks had issues with the standard hull not floating on her lines (was bow down).


Dougster

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