Plastic ribs in wood boats

I own a 36 Albin cruiser ,just took the water tanks out to repair the rotten lazeret and see that 4 ribs require replacement. looking for an alternitive to laminating oak. heard about using thermo plastic heated and slid into place while hot. anybody??

 

naughtydog,

Those 4 laminated oak ribs that need replacing were installed originally because of their strength. Thermo plastic ribs of the same moulded depth and width would not be expected to be of the same strength. It is not just to make it look good that laminated ribs are used.

 

Thanks for the early reply Landlubber. The original ribs on the Albin are steamed oak, I was looking for an alternative to replacing them with laminated oak as I would have to take the boat apart to install steamed ribs under the two strongbacks. its about 30" from the keel to the strongback (3-2*3's) and then 18" to the round chine.

 

You could laminate a repair over the ribs, after propping them up into the position and the shape they need, or you can "sister" in a repair along side of the cracked or broken ribs. Both of these repairs are quite common and easier then removing the old rib and bending in a new one. There are also other alternatives to these two repair types. One I like, is to carve out the cracked/broken section and to laminate in a Dutchman, typically with a well tapered scarf in each direction (10:1 minium). When completed, this repair looks like the original rib and if painted is invisible. Of course planking and stringer fasteners will need to be removed and re-installed, if laminating a scarfed repair, but this is also normal. A sistered repair wouldn't need the fasteners removed as the repair is next to the old rib.

Steamed ribs are incredibly strong and light weight. Laminated ribs are almost as strong and resist "opening" up with age as a steamed rib is prone to do. Oak can be difficult to use epoxy on. This is particularly true of oak that will get wet or lives in the bilge, where it will be submerged sometimes. Use live or white oak, don't substitute red oak as this species is quite inferior to the others. If electing to use a different wood for the ribs, you may need to increase the rib dimensions to accommodate a weaker species.

 

PAR,

Your comment "Laminated ribs are almost as strong".....

Aren't they stronger?

 

They are exactly the same theoretical strength if both the "grown" lumber and the laminated lumber are flawless. Laminating is stronger than a typical piece of grown lumber because unseen flaws in the lumber will weaken it, while the smaller laminates will have smaller or fewer unseen flaws that are canceled out by each layer bonded together. So yes, laminated lumber is more reliably strong than a typical piece of grown lumber. Laminating is a lot less subject to lumber quality if you inspect each piece at you build up your layers.

Laminated white oak is still stronger for the weight, but you might consider building up fiberglass ribs in-place. Solid unidirectional ribbon will be pretty strong, but also heavy, and pretty rot resistant (but not very pretty).

 

Laminated frames can be interchanged with steam bent, but are not as strong by a small margin. Solid frames have the qualities we enjoy in it as a building material, that is it's ability to repeatedly flex and return to it's original state. The moment you introduce anything to alter this you lose some if not all of this ability. The thin glue lines, within a lamination will resist this flexing, not much, but measurably enough. Though a properly selected glue will likely be stronger then the surrounding wood, it's this very wood that will sheer (along the glue line) because it doesn't have neighboring cells to rely on, in it's structure (as it does in a solid configuration), just an unforgiving glue line. It's the cellular structure and the binder (lignin) that provide wood it's mechanical properties. Disturb these and the properties are affected.

This isn't a personal belief, but simply fact. Essentially wood is nature's uni-directional composite, once you understand it's structure. Pound for pound solid wood is one of the strongest and toughest materials known. In tension it's individual fibers rival steel and surpass aluminum, both of which weigh considerably more. It's a self stabilizing material, meaning that it doesn't propagate internal damage from external loading (like metal and laminates do). Pound for pound it can tolerate and absorb more energy then metals and laminates.

 

I think a good way to describe what PAR is saying is that a bent frame is more resilient on impact than is a laminated frame, which even though it is very strong is more brittle and therefore more prone to crack or fail on impact.

 

OK, I see your point (s), and I agree with that use of the solid wood being "better" for the application.

 

I've only seen plastic frames installed on a couple of boats, one was a real mushroom farm & it looked like a sleazy repair to get a few extra years out of it & seemed to work compleat with self tappers shooting out the back of the frame- ouch! Probably much nicer to install some timber ones I think. Most of the timber boats I've worked on have steam bent frames of two laminations, generally in spotted gum but not glued & fastened with square copper nails & roved- maybe your oak bends better, I can understand that a laminated frame glued with many layers will "fix" the shape & be stiffer & have read that in a steam bent frame the neutral axis of the material moves towards the outside of the curve because of the tensile propertys of the wood dont let it stretch but the fibres/lignin/whatever on the inside of the curve kind of bunch up- maybe thats what gives it the resilience that PARs talking about. We often(not lately) would replace frames in the two layers, you'd put like a ski shape on the ends that go in first so they wouldn't get stuck on the planking & always arrised(champhered) the edges & steamed & run to the job & drive em in with a series of holding blows & clamp & wedge them- often a shaped block was made to the inside curve to help get the curve sweet in a hard turn, if the deck was off we sent them down, if not we sent them up which was much more awkward

 

For speed and economy. Put sister ribs along the bad rib.
Remove all bad wood . Dry rib and plank or what ever your skin is.
Wet out with epoxy diluted enough with acitone and apply a 2 nd or 3 rd coat.
Put the solid sister against the original frame and thru bolt it if you can to the best areas of the original frame.
Another way is to cut yellow ceder on the table saw.
3 inch wide and 1/4 thick if you can.
Wet out the stock and let dry.
Bend and insert into the sister position. Staple or nail with copper nails each layer with a heavy coat of West system epoxy mixed with mico balls.
Make a paste and coat each peice building up several layers.
You can screw from out side boat into this also for added strength.

 

http://images.google.com/imgres?img...+sisters+boat+repair&svnum=10&um=1&hl=en&sa=N

Coast Guard Inspection Fact. READ>

 

I own a timber boat that I bought with plastic ribs from amidships back. They seem fine - very dense plastic that is of a bigger cross-sectional area than the originals. The boatbuilder who did the work seemed to do a pretty good job and I'm quite comfortable with them, even though they aren't very traditional.

 

Thanks very much for your opinions and to skipjackbj for the coast guard link.
Iwill continue the pursuit of the facts on plastic and post them when available.

 

I work on mostly wood boat between 30 and 50 feet. I couple months ago I was aboard a 36 foot Columbia River Gillnet boat. Built in the early 50's.
It had a number of the large plastic ribs being used as sisters. They appear to be more like heavy nylon then plastic. The skipper/owner said it was a joy to install. Further he takes the boat over the Columbia River Bar for fish alot.
He said he was very happy and comfortable even in the 12 to 15 foot swells and current on the bar at times.