Pre-gluing stringers in stitch and glue panels

I'm starting construction of a simple dory-hulled 23 foot cat. The design calls for two stringers, in addition to the sheer clamp. My plan is to epoxy these stringers and clamp to the topside panels before I wire them up, and I wonder if anyone has any tips for doing this successfully.

I have a 24 foot table on which to do this glue-up One thing I've wondered is whether or not it might be useful to put some pre-bend in the panel by blocking it up at both ends slightly. This might be risky, since the bend would not correspond exactly to the required shape. But in any case, the bend required is not extreme, since the max hull beam at the gunwales is 44 inches.

 

I've always inserted stringers after the panel is bent into shape (or dry fitted them in place prior to stitching). Once the stringer is glued into place, it and the panel become an effective T-beam (or I-beam if both sides are attached) and bending the plywood becomes more problematic.

You could tab down the middle of the stringer in a flat section of the panel and leave the (to be later) curved ends free to move while bending the panel into shape. Once the panel is in place, tab the rest of the stringer down, then fillet.

I try to avoid building in problematic stress into the design to simplify fairing and sanding. Every time you do something that can affect natural bending of panels adds fairing time.

--
CutOnce

 

You wouldn't be the first to do this. If it's an easy bend it should be fine.

As for pre-bending it, my guess is that would be fine too. But I would want to be very consistent so each panel behaves the same.

 

That's interesting, and I see your point. Skinny long hulls may be doable this way, because the bends are not extreme. In this case, the maximum bend is 2 feet over a 23 foot length. My stringers are 3/4" X 1" fir-- not very stout. The ply is 1/4". There are some nondevelopable areas in the design along the topside-bottom joint, but since there is no stringer there, I think the ply can be tortured into compliance fairly easily-- I had no trouble planking similar hulls at a shorter length.

One of the reasons I decided to go this way was from watching John Harris at CLC bend his topside panels for his 31 foot proa, which have had the stringers pre-glued to the panels.

http://www.clcboats.com/life-of-boats-blog/madness-in-the-boatshop.html

Of course, his panels are longer and his hulls are skinnier. I may have to think more about this, but it would greatly simplify assembly if I could pre-glue.

 

I would do as CutOnce said. Once you glue that stringer you will have a hard to impossible time forming the panel into a different shape

 

...there would be increased tension on the panels that is not needed, do them when the shape has been dertemined. That way you lock the shape into the panels instead of forcing them into shape.

 

Ray,
Check out this site. He pre glues almost everything

http://www.thecoastalpassage.com/cheapcat.html

 

Basil, thanks-- I've seen that page, and it's impressive. As I said, John Harris is doing the same thing with his proa, so it seems it can be done.

I'm not sure that the stress induced in the panels is a bad thing. It seems to me that this stress would stiffen the fairly floppy 1/4 " ply. just as the cambered futtocks I've used do, or tortured ply hulls do.

Remember that these are not big stringers-- they're 3/4" X 1', laid with the 1 inch side against the panel, so they bend pretty easily. John Harris is glassing his panels too, before bending and wiring. I'm only going to epoxy coat my interior surfaces, to save a little weight..

 

Think Duflex

Gday Ray

Over here there are truckloads of boats with bent composite panels. I did the deck of my 7 metre cat in 15mm foam on the table. Then I bent it to the deck curve - no worries. Schionning, Oram and other builders get a lot of Duflex and curve it to build cat hulls. As your ply will be about the same stiffness as a comparable Duflex or foam laminate then I see no reason no to do this.

Like you I think there may be some pre stressing that will occur. If this is good or bad I can't tell straight off. It may be that getting the stringer already into compression may stiffen things up.

cheers

Phil

 

I had no problem bending the 1/4" side panels of my 20' Tamanu with pre-installed 3/4" x 1" stringers. On one project I had just the sheer and chine stringer and on the second I had a third stringer between them.

 

Gary, thanks. I suspect your hulls are more developable than mine are, if that makes a difference. But it's good to know you haven't had any difficulty with the pre-glued stringers and log.

In Freeship, the developability check shows some stress across the midships area of my panels, but this should not be a problem, since my bulkheads are convex along the topsides. More concerning is that there is stress along the chine forward. With Slider's very similar hulls, this was not a problem, and the planking went on without any difficulty at all. I'm worried that making the stringers pre-glued will remove some of my ability to torture the ply into shape along the chine.

Of course, the big attraction of this approach is that the stringers are more easily glued to the panels while they are lying flat on a table. On the other hand, fitting the bulkheads then becomes more difficult. Without the stringers to contend with, fitting the bulkheads would be dead easy, since these are simple flat-bottomed dory hulls. I've just about decided to notch the bulkheads and install them, then slide the stringers through the notches from the transom. I'm concerned that it will be more difficult to get a good bond between stringers and hull this way, but I plan to fillet and tape them anyway, so maybe not a problem.

What do you think?

 

If you use screws to pull the stringers back against the skin (like in CM which also adds the stringers later) you should get a good glue bond. The Kismet K24 uses a similar method to what you describe with the hull panels made up flat with stringers attached and bent around the two main (notched) bulkheads, the stem joined and transom fitted. Other bulkheads are added later fitted to the inside of the stringers. packer pieces are then glued in to fill the gaps between skin and frame. Kind of a Wharramesque approach but if it works??

Just curious as to why you're taping the stringer fillets as well? I would have thought the fillets alone would be plenty.

 

Put the stringers on the other side

 

Glueing stringers on works fine for the the Crowther Buccaneer 24, frames are pre notched. side panels are wrapped around main b/head, stem and transom fitted then rest of frames push hull out to final shape !
RR

 

quick build

It really speeds up the building process, some of the Bolger boats are built the same way. B