Building large stitch and glue boats.

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jpoc
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Building large stitch and glue boats.

Post by jpoc » Tue Jun 24, 2014 8:07 pm

I am aware of the reasons which mean that traditional stitch and glue construction does not scale to larger hulls but how about the following idea for the construction of larger boats using the stitch and glue method:


Step one:

Hull panels cut in the normal way for stitch and glue using a thin grade of marine play.

Step two:

Panels joined again in the normal way for these boats to make long panels running the length of the hull.

Step three:

Each panel laid out flat, with the outer surface laid uppermost. Then flat sheets of foam core material are glued onto these surfaces. The foam core sheets are smaller than the ply panels to allow for the next stage in the process.

Step four:

Stitch the hull together in the normal manner so that you have an upside down hull. Then fill the gaps between the foam core panels where the plywood sheets have been joined and layup the boat's outer skin with glass/epoxy.

Step five:

Turn the hull so that it is right side up and layup the boat's inner skin of glass/epoxy on the inner surface of the hull.


All done. The process is little changed from the construction of smaller stitch and glue boats and should scale up to any size that might be considered for foam cored glass/epoxy hulls.

Using large sheets of foam and gluing them to flat ply panels seems to me to be a lot easier than building a male mould and applying the foam core material as a sequence of planks. For this size of boat, you would probably be looking at a seven panel hull and the core could be radiused at the chines to soften them. A fillet on the insides of the chines would ensure that the overall hull thickness would be maintained throughout.

One issue that I can see is that, absent the outer skin, the hull will need some support to maintain its shape during the early construction phase. Perhaps the internal bulkheads could be glassed in to the bottom panel before everything is stitched together. The bulkheads would then serve to support the hull until the outer skin was laid up.

When cutting the foam sheets, they could first be cut to the exact size of the ply panels and then a second cut could be made to allow for the margins that must be left around the edges of the ply panels. The material cut away in this second stage could then be glued onto the ply after the hull was stitched and glued so that the gaps between the foam panels would be easily filled with a little epoxy putty.

In areas of the hull where more strength is needed, you could use higher density foam or perhaps timber inserts in place of the normal core foam.

Would it work? Is there any interest in designing and building with this technique?



Salvatore
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Re: Building large stitch and glue boats.

Post by Salvatore » Wed Jul 16, 2014 6:27 pm

Hi Jpoc,

I don't think it would work, the reason stitch and glue works on smaller boats is that the pressure created by the joining sheets gives you the shape of the boat. If you placed many sheets one after another you will not get the shaping that you need for a nice boat it would look like a tiled roof. You would still need to build an outer frame to force the sheets to curve properly.

Sam

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Re: Building large stitch and glue boats.

Post by jacquesmm » Thu Jul 17, 2014 9:51 am

There is much more to it.
Let's start with what we need in a large hull: thicker panels. The bottom of a 33' trawler will be about 1.5" thick. We can get the thickness from a thin layer of ply easy to bend and an enormous amount of expensive epoxy. No.
We can get the thickness from the plywood but it will not bend. Or we can try to laminate layers and we will have voids: bad.

A solution exist: foam sandwich.
I built my first boat from foam sandwich in the mid 70's. 38'.
Foam sandwich requires a slat mold, see here:
http://www.bateau2.com/howto/foam1.php
(8 or 9 pages long)

We can avoid the slat mold and assemble as our other boats IF we have a good understanding of the stiffness. The idea is to cut the foam panels for the full length of the hull. Then lay a first thin skin on one or two sides of that foam panel, then handle the full length hull panels as if they were plywood.
I started to work on that method several times. In the 80's, a French guy named Ander Bilodeau developped a similar method but for strips.
A few months ago, I finally tested and assembled data about comparative panel stiffness using ASTM methods. I discovered that ASTM does not work well for foam sandwich and retested for full width (48") panels. It's different. Last week, we started our first prototype: a GF16 in foam sandwich made from pre-skinned panels.
It works: no mold, fair hull.
I need a few more weeks to finish the hull and will post a tutorial.

At first, I will make the scantlings available for our existing plans. Later, I will use the method for larger boats.
In some cases we will still have a plywood cored bottom, in others, all will be foam sandwich.
Jacques Mertens - Designer
http://bateau.com

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Steven
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Re: Building large stitch and glue boats.

Post by Steven » Thu Jul 17, 2014 11:13 pm

Isn't foam panels for smaller boats going increase cost significantly?

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Re: Building large stitch and glue boats.

Post by jacquesmm » Fri Jul 18, 2014 9:49 am

Steven wrote:Isn't foam panels for smaller boats going increase cost significantly?

Yes but what is the alternative?
In a large boat, hull materials represent about 20% of the total cost. If you double the cost of the hull, that's is still max. 10% increase. You will have a better hull, lower operating cost because of the weight and a higher resale value.

Plus, let's face it, exotic woods are not going to be available for ever.
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Re: Building large stitch and glue boats.

Post by jacquesmm » Fri Jul 18, 2014 9:49 am

Steven wrote:Isn't foam panels for smaller boats going increase cost significantly?

Yes but what is the alternative?
In a large boat, hull materials represent about 20% of the total cost. If you double the cost of the hull, that's is still max. 10% increase. You will have a better hull, lower operating cost because of the weight and a higher resale value.

Plus, let's face it, exotic woods are not going to be available for ever.
Jacques Mertens - Designer
http://bateau.com

Marshall Moser
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Re: Building large stitch and glue boats.

Post by Marshall Moser » Fri Jul 18, 2014 11:22 am

jacquesmm wrote:At first, I will make the scantlings available for our existing plans. Later, I will use the method for larger boats.
In some cases we will still have a plywood cored bottom, in others, all will be foam sandwich.
I am definitely interested in the scantlings for a foam version of the FS14. My long term goal has always been to build the FS14 as a way to learn the methods and then build a CS23 or CS25. I would love to build in foam core due to the stigma that is associated with reselling plywood cored boats.

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Re: Building large stitch and glue boats.

Post by Noles309 » Fri Jul 18, 2014 11:27 am

Marshall Moser wrote:
jacquesmm wrote:At first, I will make the scantlings available for our existing plans. Later, I will use the method for larger boats.
In some cases we will still have a plywood cored bottom, in others, all will be foam sandwich.
I am definitely interested in the scantlings for a foam version of the FS14. My long term goal has always been to build the FS14 as a way to learn the methods and then build a CS23 or CS25. I would love to build in foam core due to the stigma that is associated with reselling plywood cored boats.
It will still have the home built stigma and the re-sale won't be there regardless.
Gary
GF-16 Completed

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Walkers Run
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Re: Building large stitch and glue boats.

Post by Walkers Run » Fri Jul 18, 2014 4:56 pm

CS30 ?
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Re: Building large stitch and glue boats.

Post by jacquesmm » Fri Jul 18, 2014 6:24 pm

TW34 :D
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