Thread: 'Stringers' in old style boats

Originally Posted by Aberdeen

Would someone like to comment on the original purpose for the use of 'stringers' in the old style construction of sheet ply on frame boat building?

Was it for strength; to help form the ply; keep the frames aligned until the ply sheets have been secured................. ???

I know why the really old boats used 'ribs' for clinker & carvill type construction, but given that modern stitch'n glue boats tend to be devoid of stringers - is it now practical to reduce the number of, or totally leave out, these longitudinal pieces of wood ?

I built a Hartley "Blue Fin" about 15 years ago, and I'm pretty well versed on the subject. The stringers do several things. They provide a flexible rigidity to the frame structure. That is, the frames are held in place at a number of points, rather than just at the chines, keelson and gunwale. Then, the stringers provide surfaces for glue and nails as the ply sheeting is attached.

If you think about the process, imagine what the first sheet of ply on one side of the boat would do to the shape of the boat as it was being put in place if there were no stringers on the opposite side!

The trick in building boats of this type is to work one side against the other so that there is equal torque on both sides, keeping the hull straight. The chines and stringers keep things straight as the ply is attached.

I honestly believe that once that boat had its ply sheeting on it, it would have done very well without any frames in it at all! Because of the huge amount of gluing surface provided by the stringers, the boat will be rigid and it virtually cannot come apart. Finally, if the stringers are carefully put in place, which is to say equally spaced on both sides, there is an assurance of a fine curve, as it will be dictated by the underlying framework.

Mike

From a structural stand point the stringers serve as longitudinal webs within a girder, which is pretty much what traditional boat building is all about. Typically the keel is the lower flange, the athwartship elements transverse and the longitudinal elements webs, within the girder and the deck the upper flange, usually also made up of many pieces. The hull skin offers some longitudinal stiffness, especially when using plywood, but not as much as you'd think when fasteners are the only attachment method. The framing structure is the basic stiffness of the hull, with a plywood hull shell nailed to it.

It took a while for engineers to recognize the physical properties of plywood and develop building methods to take better advantage of the stuff. The older style, traditionally framed designs with plywood planking, generally could receive solid wood planking stock as a direct replacement for the plywood. In fact, the solid wood planking (replacing plywood) will make a lighter version of the same boat, but you'd have to deal with seams.

As designers recognized the need to approach these new, big "engineered wood" panels, they started using slightly different techniques, such as batten seaming, molding and torturing plywood all became possibilities.

When epoxy got a solid foot hold, things changed dramatically. Now you could create a series of flanged pieces, forming a structural grid that could accept the anticipated loads. Once cured the tabbed, taped, filleted, sheathed and bonded elements literally form a giant thermo plastic molecule (yep, just one big sucker, well okay only the chemically bonded areas, mechanical bonds are a separate molecule), with a wooden core. This homogonous assembly of bits approaches the engineering at a very different angle then traditional methods.

As for the question of the use of stringers, well, it depends on the framing system devised by the designer. Generally they fall into two categories, longitudinal or athwart framing systems. Most boats carry athwartship framing, meaning fairly closely spaced frames with equally spaced stringers, a centerline keel, chines and/or longitudinal engine bed stiffeners if necessary, sheer clamp/shelf and deck beams (with their own set of stringers). In this case the stringers hold the frames in position, provide fastener surface for the planking, but most importantly, transmit longitudinal loads.

This load transmission is hard to understand, but essentially it boils down to each separate structural element relying on surrounding and neighboring elements to absorb and share imposed loads. It's this reason why it's very bad to use epoxy on a traditional hull. The epoxy locks down the joints and the separate structural elements become isolated from each other. This gives rapid rise to point loading and stress risers, which can easily overwhelm a skinny steam bent frame if not helped by it's neighboring frames and stringers (for example). If the joints just remained fastened (no epoxy) then they can flex, spreading and sharing loads with the other elements.

This is the watered down version, as it's actually a fairly complicated set of subjects, but hopefully you get the idea.

PAR
Now that was a 'mouthful' to read

Being a Hartley design lover I have tended to keep fairly to the original plans including nails, screws and the like on the last one. It took me a while to come to grips (no pun) as to how good modern epoxies are... used the WEST SYSTEM.

So for the next one, a Hartley VIXEN @ 18', I will probably reduce the use of old style fixings, not entirely though.... be it a traditionalist mindset or just cautious maybe. I then got to thinking how necessary stringers are in the scheme of things given how effective the epoxy was in locking things together etc.....

Although I wouldn't have deleted the stringers, I became curious if they are really needed now...... hence the question.

Lots of designs out there - the Hartley and it seems the Glen-L ones have kept the older concepts. Not a fan of S&G, not sure why though? I think it goes back to my childhood and how my dad used to build his boats. Plus I also like playing with timber, shaping the frames, notching out bits to get that perfect fit and so on. S&G just seems a bland method... it is the fun of creating a boat with a challenge more than just putting one together... ?

Originally Posted by cellist

The trick in building boats of this type is to work one side against the other so that there is equal torque on both sides, keeping the hull straight. The chines and stringers keep things straight as the ply is attached.

Mike

Mike,
I also like your thoughts - the idea of them maintaining a balance of each side during construction. I can understand that concept and being a generally over cautious person would worry about things getting out of whack - wood does flex with weather and moisture conditions, at least if one side is planked it isn't probably critical to get the other side done immediately..... although I would attempt it anyway to be sure.

Thanks guys

Taped seam construction (stitch and glue, tack and tape, etc.) have a much higher "goo factor" (my term for lots of epoxy work) then more traditional build methods. This alone can scare off some people, particularly those sensitive to the stuff.

For the stringers the be eliminated you'll need more then planking fastened along the chine log and sheer clamp. You'll need interior (inside the hull shell) furnishings filleted, some possibly taped to the planking, as well as the chine log and clamp (inwale, shelf, etc.) being bonded to the planking. You'll still need some sort of longitudinal stiffener(s). In taped seam construction most designers use seat box and cabinet front faces to provide this necessary stiffness.

A current design I'm working on has just two longitudinal stiffeners, but they are full length of the boat, support the transom, sole, deck and serve a seat fronts too (which is how I got away with just two). You could look at these as two big longitudinal frames. In-between these two stiffeners are several particle bulkheads and outboard of these stiffeners are matching extensions to the inner particle partisions that are tabbed to the hull shell. This grid like structure forms everything inside the boat, lockers, seats, side and fore deck support, mast loads. Of course every joint is filleted with many also requiring tape too. There really isn't a need for conventional fasteners, in fact the whole boat is made from 6mm plywood, with a few 1x2's tossed in to make things easier, but only hardware is held down with fasteners.

Look at it this way the boat has two bilge stringers, no chine log, a modest sheer clamp, no keel, no stem or stern posts, but it does have about two dozen internal partitions, which brace everything up nicely. Technically it's a internally braced box beam, which is different then the girder the traditional builds mimic.

If you wanted to remove a few stringers, every other one could be safe, if you bonded the planking to the frame and epoxied the remaining framework. You'd be making the planking do work the original design didn't intend, but they didn't know about epoxy either. You could up size the planking and skip the stringers, but often those stringers are handy when fitting the planking and furnature.

It is important to "balance" the stresses on the structural frame work as it's being assembled (as suggested). you can easily twist or wrack a hull with a stubborn stringers, clamp or log. I always do the same task on each side of the hull. In other words, portside bilge stringer, then starboard side, port inwale, then starboard, etc. This is also true of planking, where I'll plank so much on one side then move to the other to "balance" her stresses out.

So, a definitive answer to your stringer elimination would be maybe. If everything was bonded, filleted and tabbed, you could eliminate many structural elements, but you have to get those panels supported with something or she'll likely "oil can" on you (or worse). If you rescaled the scantlings to make the hull shell more load bearing, then you could kiss those stringers good bye as well as most frames. Of course this means you're making up a new construction drawing, placing bulkheads where necessary with supports, braces, etc. scattered about.

I do a few of these types of conversions each year and it's not especially hard, unless you're dealing with a racer interested in winning a lot.

In short (like I'm really capable of that) each design should be taken apart and accessed separately, as no "rule" could cover the variables. A few books have attempted to do this, but none that I've seen readily take advantage of "goo and go" (another one of my epoxy catch phrases) boat building.

On your Vixen I'd leave one stringer in the bilge and one in the topside planks. These will help fair the hull, provide local stiffness and offer a handy spot to place a temporary screw while setting things up for planking. If you think about this stringer more carefully, you could set it at the height of seats, so it could also serve as a cleat for the seat tops.

PAR
After your disection of things I will stay with the tried and true formula and definitely stick with the alloted stringer setup. After all, unless one is a pro at these things, why meddle! They are hidden beneath the flooring in most cases, add very little to the weight factor overall, and as part of the construction cost they are minimal.

Keep things simple & don't re-invent the wheel comes to mind when it comes to the average back-yard builder.

I'm not after a high speed record breaking vessel, just a good, stable and solid waterproof platform to enjoy the splendor of the sea.....

Thanks for your input PAR - it is appreciated.

Glenn
'Down-Under'

I'm with you Aberdeen- I like a bit of nice joinery, in fact on my present build I've come to love those stringers, together with their epoxy fillets you can just feel them making the hull more seaworthy. If a boat has a bit of beam it is reassuring to have a skin that won't sound like jungle drums in a bit of weather. I'm reassured that my chines will stay exactly in the shape that I left them.