Purpose of Frames in a Lapstrake Hull

[Bongo Boy]

Folks will certainly wonder how anyone could ask such a dumb question, but with my apology up front, I don't see an obvious answer.

What do the frames actually do in a lapstrake design? The reason I don't quite get it is that it seems that 1) they don't add any (or much) stiffness athwartship (forgive me if I misuse terms a bit, I'm new) near the sheer, 2) I wouldn't think any stiffness in that direction is needed anyway due to bow of the strakes, and 3) the strakes are fastened to one another so there isn't any need to prevent motion of strakes vertically (that is, in the direction of each strake's width).

My background is aeronautical engineering, where stringers and ribs are common, but I don't see any connection between the two designs. Maybe another way to ask this, and I've asked myself this, what would happen if you simply riveted the strakes to themselves only, and not through the frames? Again, since I've never even seen a wooden boat and have seen one built, I just don't know the answer. Thanks for your patience & help.

[Brian Palmer]

In a traditional lapstrake hull, they help maintain the boat's shape (especially if sawn to shape) and they help keep the planks from splitting apart at the edges, especially where they are penetrated by fastenings.

In small glued lap plywood boats (like a canoe) they are not needed for either because of the grain running vertically in each plank and the glued laps.

Brian

[Bob Cleek]

It's a function of size. Small hulls can provide a fair amount of transverse strength from the thickness of the planking and the longitudinally curved plank laps, but as size increases, frames provide transverse rigidity without the need to increase plank thickness and hence weight. One advantage offered by lapstrake construction is its light weight, due to thinner planking, supplemented by lighter framing. Frames also provide "shape," counteracting the curved planking's desire to spring back straight. This reduces the sheer stresses on the plank fasteners. Clinker built hulls flex a lot and are designed to do that, but there's such a thing as too much flexing and frames counteract that.

[johngsandusky]

Just like in an aircraft, a grid structure adds stiffness with light weight.
The light and soft woods used for traditional lapstrake boats in this country lack cross grain strength. The riveted laps would take all of the bending force without frames.

[Bongo Boy]

I think I understand, certainly the bit about reducing the shear load on the fasteners. It helps me if I imagine the strakes made of a highly elastic material instead of imagining they're totally inelastic. Also, until I paid more attention, I didn't appreciate just how close the frames are together in many designs--and how they shorten the 'free span' for any given strake.

Also working against me is my not having ever seen or handled a frame after it has 'set'. So, I imagine a frame that has little stiffness--when in fact I have to guess they're quite rigid, at least when taken in mass as is the case when installed, working together.

Even so, I have to assume a well-positioned thwart is far more structurally-essential than its ancillary role as a place to park your kiester would suggest.

[Peerie Maa]

Most definitely, properly kneed thwarts, breasthooks and quarter knees prevent wracking or twisting. They also stop the boat panting by flexing the keel up with a decrease in the beam and sagging the keel which will pull the stems in and increase the beam.

[donald branscom]

I like what one marine designer said when a client was faced with the choice of thick frames or thin light weight frames. "After you have been out in a big storm I do not think you would argue with me about selecting the thicker frames."

[Tom Robb]

As implied by the lack of frames in glued plywood lapstrake, the frames try to keep the plank timber (little cross grain strength) from tearing on the dotted line.

Theyre also part of the aesthetic, the eye-candy, of those hulls which is lacking in laminated or glass hulls.

[James McMullen]

Once they have cooled off, the steam-bent frames are really pretty darn rigid.
Plus, they look awesome.

[Bongo Boy]

Yes, the aesthetics I totally get.

[py]

Timber is strong in the direction of the grain, but weak across the grain. The frames (ribs) add strength across the grain. In a glued ply boat ribs are not required as the ply has grain in both directions.

[peter radclyffe]

there to stop the wooden planks cracking
& to stop the hull caving in

[Timo_N62.9_E27.7]

I would say that the bent frames add little if any to the rigidity of the boat. They are there just to take the loads that would otherwise split the planking.

Fixed and kneed thwarts are there to give stiffness, in the traditional finnish boats.

[James McMullen]

I disagree that the frames don't add rigidity to the boat. If you've ever built a trad. lapstrake boat, you'll see a huge difference even between the time the boat first comes off the molds and when you finish steaming in the rest of the ribs where the stations were--and all this is before you get in any risers or inwales or thwarts or floors in. The frames and the planks are the warp and the weft of a basket. Every piece you add gives the boat that much more structure. Once a steam-bent oak rib has cooled, it's really pretty stiff and rigid compared to a cedar plank.

[bruce w]

on fishing boats the sawn frames are used to fit engine beds ,bulkheads and shelves to lodge the deck beams ,olso to fix chain plates and cleats ,large clinker boats are very supple untill the frames are in place and some of the deck beams are fitted.

[donald branscom]

Originally Posted by James McMullen

Once they have cooled off, the steam-bent frames are really pretty darn rigid.
Plus, they look awesome.

Wow. Looks better than the top of a piano. Where do I lay the fish and cut its head off on that bench seat. What about my muddy boots on that bench grinding pebbles into it? Looks like a slip and fall accident waiting to happen.

[bruce w]

Originally Posted by donald branscom

Wow. Looks better than the top of a piano. Where do I lay the fish and cut its head off on that bench seat. What about my muddy boots on that bench grinding pebbles into it? Looks like a slip and fall accident waiting to happen.

you forgot the kedge and 10 fathom of 1/2 ins chain. Still a lovely job you would have to be a moron to take her out in corse weather landing on a shingle beach with a lot of swell beam on.

[James McMullen]

What the client wants, the client gets. My own boat has more paint and less varnish.

[floatingkiwi]

Originally Posted by James McMullen

I disagree that the frames don't add rigidity to the boat. If you've ever built a trad. lapstrake boat, you'll see a huge difference even between the time the boat first comes off the molds and when you finish steaming in the rest of the ribs where the stations were--and all this is before you get in any risers or inwales or thwarts or floors in. The frames and the planks are the warp and the weft of a basket. Every piece you add gives the boat that much more structure. Once a steam-bent oak rib has cooled, it's really pretty stiff and rigid compared to a cedar plank.

Thats right. I noticed a big difference in the folkboat rebuilding the floors and screwing the planks to them and then an even more staggeringly noticeable increase in rigidity by , not adding, but replacing with glued laminated frames in place of ther cracked ones. The sides stood up much more vertically and it didn't wobble any more and like justmentioned,that wuz before any other bracing elements were in place.

[Bongo Boy]

One of the most enjoyable aspects of this new area of interest for me (boats) is to read English words that yet appear to be a foreign language.

[BBSebens]

I think a big factor is whether you go traditional or use plywood strakes. In a trad. boat the frames are essential to help hold the shape. Howeer, in a glued plywood lapstrake boat, one is basically making wood-based fiberglass. there is a ton of glue holding the wood fibers together. I have a 16' peapod constructed in this manner. In my case, the frames are mostly just a place to hang the seats and mount the floorboards. I doubt the frames play an integral part in the structure of the boat. That said, I wouldn't even think about building a boat like mine without frames, because there are other roles than just structure that the frames play in a boat. Of any size.

for the ultimate in lightweight, look at skin on frame.

[James McMullen]

Leave it alone or put a cedar shim under it and refasten with a new rivet or nail. Do not under any circumstances glue the rib to solid cedar planking!!!!! Traditional lapstrake construction with solid wood members must have some flexibility to come and go as the wood shrinks and swells. Trying to glue a rib in at right angles to the grain of the planking stock is absolutely guaranteed to cause you misery later on.

[sismcq]

I was only going to glue the whiteoak to the existing rib and rivet through all.
Sis