Best wood for laminated frames?

Ancient Kayaker,


Can you explain the springback formula that you provided? I have found that springback varies by species and lam thickness. I have actually had a few ribs "over" spring. I am wondering why.

Thanks

Allan

 

Wood varies in its characteristics, even within species, some a lot more than others. Its one reason I dislike some species so much, wildly inconsistent traits even within the same tree.

If you stick with the hardwoods most of the inconsistency issues will diminish, but if your using pines and firs, best of luck ;-)

 

I understand it's a standard engineering formula independant of material, and it assumes the material will spring back straight. If a single lamination is being bent beyond its elastic limit that will not be the case.

It applies best to really dry wood that is not over-bent. Wood with a high moisture content, especially if it is green, will take a "set" when held in a bent condition for a period of time; in these cases the spring-back will be less than the formula indicates. I have taken advantage of this by wetting the laminations, letting them sit for a while until dry to the touch then bending and gluing them over a mold; this resulted in virtually zero springback: best done using a water-soluble glue.

Over-spring (when the lamination rebounds more than expected) could be caused by glue creepage, when the glue yields to shear forces. I have never encountered it although shear force is considerable in a typical lamination. Perhaps the glue needed a bit more time to setup before releasing the lamination from the mold?

 

I need to replace some broken frames on a 55' S&S motorsailer. She is planked in Honduran Mahogany and has steam bent White Oak frames, about 2.25" wide. In front of the frames runs a series bilge stringers about 16" wide - glued together and scarfed in long lengths, such as would make it impossible to remove without removing the entire interior for practically the full length of the boat. For this reason I am considering laminating new frames in situ.

My first question relates to bntii's post mentioning the bevel of the hull planking. How is this addressed when laminating in place?

As far as choosing wood that has same properties as the original, does it have much bearing when the laminate layers are thin and there is a lot of epoxy involved? Also what is a reasonable thickness to be laminating in this case? I imagine quite thin to allow the flexibility to maneuver in behind this bilge stringer situation.

If the original wood is white oak, would live oak be preferable to white? And as im understanding this, its better to have green or wetter wood for bending, and as it dries it will hold this shape.

The conversation generally seems to be about using epoxy for such applications. Any thoughts on Resorcinol?

Thanks

 

Resorcinol requires virtually light-tight joints and high clamping pressure to achieve success.
Those two considerations may be almost impossible given your working conditions/situation.

 

For the installation, three major options:
1. Remove coverboard, push the frames down.
2. Remove garboard, push the frames up.
3. Slide in one lamination at the time from the middle (only works for some boats).

Options one and two work for steam bent and laminated frames. If installing laminated frames as a package, put them in plastic tubing beforehand, to contain the epoxy that will squeeze out. After the glue cures you simply rip the plastic off, and dress the frame by sanding (manual or powered) in situ (this is for frames with double bends that can not be removed for dressing on the bench).
With steam bent or laminated frames there is no bevel to adress, the frame twists to follow the hull. You need to have a way to clamp the frame to the hull, either with wedges and sticks, or by using screws (drill trough the old rivet holes into the frame, then troughbolt).

 

My first question relates to bntii's post mentioning the bevel of the hull planking. How is this addressed when laminating in place?
A couple of ways
1 temperary fill strip.
-laminate
-remove laminated frame and temp fill strip
-bevel frame and permanently install.
2 permanent fill strip and laminate over it.

Option 1 allows for easier dressing of the frame's sides. It is unlikely that all of the pieces will align perfectly
As far as choosing wood that has same properties as the original, does it have much bearing when the laminate layers are thin and there is a lot of epoxy involved? absolutely Also what is a reasonable thickness to be laminating in this case?just thin enough to flex into place with out excessive force I imagine quite thin to allow the flexibility to maneuver in behind this bilge stringer situation.

If the original wood is white oak, would live oak be preferable to white? White if steaming and live for laminate as im understanding this, its better to have green or wetter wood for bending, and as it dries it will hold this shape.
Wet/green bends easier but doesn't glue well
The conversation generally seems to be about using epoxy for such applications. Any thoughts on Resorcinol? There's urathane glue (gorilla) which requires water to cure. But it also requires clamping

 

As both Rumors and Blueknar mention for these alternatives to epoxy that clamping is required. I had imagined clamping also necessary with epoxy. In my limited experience thus far, I have clamped epoxied curved laminates for making the stem of a small boat. As I have seen Louis Sauzedde's Tips from a Shiprwright, he shows a method of screwing all the laminates together into place using sheetrock screws, washers and blocks of wood, as Rumars describes - to me this seems comparable to clamping. The only other way I have heard of is laying one laminate strip at a time, stapling it into place and doing them progressively one at a time that way. Perhaps this is the alternative with less pressure being exerted than a clamping/screwing method? Just in that both posts mention the requirement of clamping for the alternatives, makes me wonder how to use the epoxy to laminate frames without using clamps?
And also, if using the "drill trough the old rivet holes into the frame, then troughbolt" method if it would provide adequate pressure to utilize the Resorcinol or Urathane?

Now Rumas, you mention no need to cut bevels if laminating or steam bending frames, where as Blueknar describes a method with a fill strip. Again I had seen on tips from a shipwright he shows how he laminates a frame and cuts a progressive bevel on the back strip. So I am wondering, if there is no need to cut bevels when laminating, why does Blueknar describe a method, and also Louis Sauzedde uses a progressive bevel?

If it were unnecessary why would it be done? On the other hand, I would like to have as simple a solution as possible while preserving structural integrity. And if I don't need to cut a bevel, it would certainly simplify the process.

 

Epoxy also needs clamping, but it needs far less then resorcinol or PU glue, it's more of a "fix it in place" instead of "squeeze for the invisible glueline". Epoxy is gap filling, wich means that the glued surfaces do not need to have a perfect match or be machined smooth. Epoxy is also usually cheaper then resorcinol or PU. You could get the required clamping force for the other glues but why bother? They are much more difficult to use then epoxy, it's not only the fit and clamping, it's the open times, etc. and they have no discernable advantage in this application.

Regarding the bevel, it's mostly an appearance thing, if you don't bevel, the frame twists. The frame follows the contour of the planking instead of beeing nicely square to the keel and each other. This can pose problems when you need to attach a bulkhead or ceiling. Most steam bent frames are not beveled, that's the beauty of the system, you take a rectangular frame and force it into place in a minute or so, no further work necessary.
It's pretty simple to see how your frames were done originally, and how the laminated replacement will look. You need to determine the veneer thickness that will take the bends, so you can just put a stack of them in place and see if they look like the rest.

As for laminating without clamps, you can wedge the veneers against something (the stringers, the opposite side of the hull, deck, etc., you can staple or nail them to the planking and either leave the staples or nails in (plastic, copper, bronze, stainless), or you can remove them before laminating the next layer. Or you can use the existing rivet holes (just make sure your screws are thinner then the permanent rivets you plan to install). If you have to remove planking anyway, there are also other methods like zip ties or actual clamps. It all depends on the actual situation and your preferences, there is no "one true and right way".
You might also want to consider replacing only the broken part of the frame, scarfing a new laminated piece in.
As for what wood to use, I would probably use something in the same strenght category as white oak, maybe iroko or black locust, they are simpler to glue then oak, but oak can also be used.

 

Bevels are essential.
The ribs are perpendicular to the keelson.
But the hull skin rarely is.
The hull and ribs might be nearly perpendicular to each other near the center lines. But the angle increases towards the bow and stern.

If the angle difference is not accounted for then the hull planks will not be properly supported.

 

In some boats steam bent frames are beveled, in others (most?) the frames are twisted when installed to lie flat against the planking.

 

Otoh, if you are bonding frames to plank with epoxy, no bevel makes for a nice glue seam

 

The question of beveled or not lies with the initial construction method. Some frames are steamed to individual beveled patterns, some are steamed oversize to unbeveled ones and beveled afterwards, some are steamed to ribbands and only lightly beveled where needed, and finally some are steamed directly to the planking and not beveled at all. The choices were made according to tradition, technology and other rationalisations. Herreshoff buildt the big series boats like the 12 1/2 with a beveled mold for every frame, A&R (and others) bent big boat frames oversize on an adjustable rig with steel backstraps and beveled the inside and outside faces to high accuracy by sawing. By contrast, lapstrake boats normally have the frames simply dropped in and twisted by hand, sometimes even hotnailed. Most carvel boats with steamed frames were done over ribbands and can show some beveling on the outside face, but the inner face of the frame is not usually beveled. One very popular way of building was by using sawn or steel frames with two smaller steamed frames in between.
When it comes to repair and replace one must simply follow the initial practice, if both molded faces have bevels the replacement needs the same, if not, not.

Unbeveled steamed frames twist and cant and might even have compound curves, but it's not a structural problem, the scantlings and spacing make sure of that.

 

There are two different methods using ribbands. One method bends frames on the outside of ribands before planking, and the outside faces of the frames are faired before planking. The other method bends frames on the inside of the ribbands and the frames are twisted as they are bent so that outside faces of the frames are against the ribbands.

 

It's been awhile, but I'm somewhat familiar with those Choy Lee 35s.
OP, with the original frames on 8in centers, they can't be very big pieces of wood, more like large sticks.
One thing you haven't given is the actual size of the frames.
Assuming you have some working room, to sister a double-sawn frame is not so bad, but to sister some stick-sized bent frames seems counter productive without some "beef" to the original to hold fastenings.
Oh, is their a floor on every frame?