Extending C21 in shorter sheet plywood

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Cap. Nando
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Extending C21 in shorter sheet plywood

Post by Cap. Nando » Tue Oct 13, 2020 9:00 pm

Greetings from Brazil.
I have downloaded the C21 plans and I read them top to bottom, (or should I say Bow to Stern?)
I have two questions and a concern:
Q1. To extend the C21 10%, all I need to do is to is to add 10% to most of the X axis measurements? The only ones not extendable in the X axis are those that define angles, correct?
Q2. Can I get a motor bracket for the C21? Where can I get the plans for that?

Finally my concern. In Brazil the local market only offers plywood sheets measuring 2200 mm x 1600 mm. I need to solve the nesting problem in which I will extend these plans I got within these ply sheets.
Any recommendations? :help: :help: :help: :help: :help: :help:

Thank you for your ideas.
Best Regards



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Re: Extending C21 in shorter sheet plywood

Post by joe2700 » Tue Oct 13, 2020 10:28 pm

If you need to extend plywood sheets you scarf them together to make one larger sheet. There are many online guides and videos you'll find by searching for "how to scarf plywood". Note you do lose some overall area from the overlap. There are many techniques using different tools so check out different ways for doing it and pick what works best for you. You might have some better options someone else can suggest, but if you do need to make larger sheets out of smaller ones, a scarf joint is the way.

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Re: Extending C21 in shorter sheet plywood

Post by Browndog » Wed Oct 14, 2020 5:44 am

If you want to extend the length by 10% you would be better off scaling the entire boat up by 10% in all dimensions.

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Re: Extending C21 in shorter sheet plywood

Post by jacquesmm » Wed Oct 14, 2020 9:04 am

I would do what Browndog proposes: scale all dimensions by 10%.

For the plywood, you will have to redo the nesting for some parts but that is not difficult.

For the bracket, we do not sell plans, it is a liability problem. You could find a local welder to build one. Dimensions are available online in bracket descriptions.
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Cap. Nando
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Re: Extending C21 in shorter sheet plywood

Post by Cap. Nando » Wed Oct 14, 2020 7:19 pm

Greetings guys

Thank you all for your input. Share knowledge is the secret for progress.
Soon I'll be starting my fishing center console boat, 23 feet long
Rgs

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Re: Extending C21 in shorter sheet plywood

Post by TomW1 » Fri Oct 16, 2020 8:27 pm

Yep do not try to build a bracket have one welded for you from a design or buy one from a company that makes them, Armstrong comes to mind. No need to scarf just use fibergalss seems. Building the C21 up 10% in all directions is a great idea, I love the C boats. You will have a great boat :D
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Cap. Nando
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Re: Extending C21 in shorter sheet plywood

Post by Cap. Nando » Sat Oct 17, 2020 6:48 pm

Greetings Tom

I have a welder as my best friend and he is already searching for Stainless Steel examples on the internet. There are a lot to guide us up.
My seams will be bi-axial 10" tape, 400gr/M2.

Thank you very much for your input

Nando

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Re: Extending C21 in shorter sheet plywood

Post by TomTom » Sun Oct 18, 2020 9:26 am

Have you looked at the Abaco 23 as an alternative? Or the Novi 23 but built without the cabin. These are boats that are already 23 feet in length. You could modify the sheer line on both to look more like a C21... but this may be easier than doing a stretch?

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Re: Extending C21 in shorter sheet plywood

Post by Cap. Nando » Sun Oct 18, 2020 9:11 pm

Hello TomTom:

Greetings.
The boat will be going offshore anywhere from 15 to 60 miles. I need a reliable design when you do not see the landline. The C21 has an high bow line, high free board bow and stern, and its beam is bigger than both of the ones you mention. My dream was the GX28 or the CX25 but they will over stress my budget. In Brazil motors are 75% more expensive than US.
rgs

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Re: Extending C21 in shorter sheet plywood

Post by TomTom » Mon Oct 19, 2020 12:25 am

Cap. Nando wrote:
Sun Oct 18, 2020 9:11 pm
Hello TomTom:

Greetings.
The boat will be going offshore anywhere from 15 to 60 miles. I need a reliable design when you do not see the landline. The C21 has an high bow line, high free board bow and stern, and its beam is bigger than both of the ones you mention. My dream was the GX28 or the CX25 but they will over stress my budget. In Brazil motors are 75% more expensive than US.
rgs
For what it’s worth, I have a C19 which I fished with many miles offshore here in East Africa. We too have the problem of very expensive engines, no coast guard etc. I have taken the C19 up to 27 miles offshore and have never felt unsafe. For all sorts of reasons - I have stretched my C19 - but I have yet to test it. My feeling is a longer, leaner C19 will be better for what I use the boat for. It is now 22 feet long stretched. I stretched it after I had built it - rather than scaling. So I had to add frames from the transom. For me, the C19 has always had plenty of beam as it was. I would not want more.

I don’t know what kind of fishing you do, but we do a lot of trolling for blue marlin with lures and we need to maintain about 7 kts minimum for hours on end. A short, fat hull is an inefficient way to achieve this. Our mooring charges are not related to boat length, and I came to realise a longer thinner hull would have suited me better.

My friend Payan built a GS28 after we had this discussion over a few beers. I watched all the stages of the build and have fished on the GS 28 many times. The GS28 is a superb boat for the fishing we do here.

I could not believe it when we launched the boat for the first time with twin 85 HP Yamaha 2 strokes and it would plane on one engine and do 16kts.
His fuel efficiency for our kind of fishing in a 28 foot boat would probably beat my C19 if we had the same engines.

I also believe that a 150 HP single engine would be plenty for the GS 28 built light. There is almost no hump as she comes on the plane.

So if you are considering the GS28 and are worried about the cost of engines to power it, I can tell you that I don’t believe you will need bigger engines than a stretched C21. I also don’t think you will find the costs of the build as different as you imagine, and if you are a fisherman who trolls lures, your difference in fuel economy will make it up pretty quickly!

Also - you do not need to build a bracket on a GS 28. There is so much space in that boat!

My vote would be a GS28 - hands down!

I’ve often thought Jacques should do a smaller GS28 - like a GS 24!

Here is a link to Payan’s GS28 build.... viewtopic.php?f=12&t=61975&hilit=Gs+28+Payan&start=20

And my C19 stretch .... viewtopic.php?f=12&t=62817&hilit=C19+stretch&start=270

You can see some of the photos of her launched to draw the waterline. She is now a big boat!

I hope some of this helps. Whatever you decide, you are embarking on an amazing journey.

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