Lumber dimensions vs strength

[Longbow]

The plans for a 15' plywood skiff I'm building call for 7/8 in. Douglas fir for the frames. I have a source for DF, but only in 1x4 and 1x6 dimensional sizes. So this will be 3/4 in. thick. I'd like to ask some of the wood experts on their opinions about this difference in size. Would the 1/8 in difference in the width of the frame make it significantly weaker? Should I make the frame slightly thicker to compensate? My plans also call for a 2.25 in. thick stem. I was going to laminate (3) 3/4 in. pieces together to get the correct thickness with the seams running bow to stern. Is this a good idea or should I try to find a solid piece?

[paladin]

Laminate up the pieces from the fir stock. The laminated piece will be stronger for the same dimension that a solid piece of lumber. When you laminate the wood, make sure the grain from one plank is opposite the grain from the adjacent plank.

[David G]

Sounds like you're meaning 7/8" square for the frames. What boat? Is this 7/8" material reinforcing the perimeter of a plywood bulkhead/frame... or standing alone? My initial reaction is that 3/4" will be fine, and yes - it probably wouldn't hurt to beef up the other direction to recover a bit of the loss.

As far as the stem - what paladin said. Do you know what he means by opposite grain?


"I don't know if God exists, but it would be better for His reputation if He didn't" -- Jules Renard

[Longbow]

The boat is Jordan's Footloose. I've been switching back and forth between it and Stambaugh's Bay Skiff, but I put it to a vote and my family preferred Footloose. The frames are specified as 7/8 fir with 3/8 marine ply gussets in the corners. As far as the grain for the stem, if I cut 3 pieces from the same board, I'm assuming that he means to rotate the center piece 180 deg. so that the growth rings don't line up. Are 3 3/4 pieces ok to laminate or should I go with more, thinner pieces?

[ssor]

The fore and aft demension (thickness?) is not as critical as the width ( from skin side in) You could easily compensate by going 3/4x1.

[Bob Smalser]

....Jordan's Footloose....

.... Are 3 3/4 pieces ok to laminate or should I go with more, thinner pieces?

....with 3/8 marine ply gussets in the corners.



http://www.jordanwoodboats.com/images/flb.jpg

It's usually best to laminate using an odd number of lams, as they better equalize seasonal movement stress, so your lam schedule is fine.

Normally I'd suggest you add a frame station if you have to reduce scantlings, but I think you'll be fine without it. Many designs use the same number of 7/8" frames as yours, only with 7/8" planking in traditional boats twice as heavy.

I'd nix those chinzy 3/8" plywood gussets and lam up some real knees using your Doug Fir. They are important to the hull's structure and plywood knees that thin are easily cracked at the butt joint or broken out entirely.

No need for fancy forms. You can steam or boil the lams in place to take a set....then glue and clamp them in place using slip sheets....then remove, finish and reinstall permanently:

http://pic20.picturetrail.com/VOL12/1104763/8408305/113221960.jpg

http://pic20.picturetrail.com/VOL12/1104763/8408305/329355471.jpg

http://pic20.picturetrail.com/VOL12/1104763/8408305/113600135.jpg

[TerryLL]

If you use the 3/4-inch stock for the frames you will reduce the strength by about 15% compared to the 7/8-inch specified by the designer. You can recover some of the strength lost in the thickness by increasing the width of the frames. But you will also be losing gluing surface, and more care will be required when driving fasteners.

The 3/8 planking specified will yield a stout little skiff, so my guess is that there is plenty of residual strength designed into this boat. The design weight is 300 pounds, fairly robust for a simple open plywood boat.

Laminating the stem from three pieces will give you a stronger timber than using a solid piece. Again, 2.25" is robust for a 15' boat, indicating the boat will be quite stout.

[David G]

Now that I know it's a Footloose, I'd firm up my opinion that you can reduce to 3/4" frames with no great worry. One of my fellow Coots has had a Footloose skiff for many years, and they are - indeed - quite stout.

Only other comment is: if you're new to boatbuilding in general, you might be put off by Mr Smalser's notion of making steamed knees. That's understandable. But, it's actually not that difficult. If you're interested, sing out, and you'll be directed to the resources that'll make it doable for you.


"When dealing with the insane, the best method is to pretend to be sane" -- Herman Hesse

[Longbow]

Thanks for all the great info! Regarding the gussets on the frames, maybe I'm using the wrong word. The frames are assembled from Fir and at the chine there is a piece of 3/8" ply on each side of the frame, sandwiching the joint, similar to the way roof trusses are made. Are we talking about the same thing here? Forgive my ignorance, since I have no experience with laminated knees, but it seems like a 3/4" wide knee wouldn't offer much lateral support. Or were we talking about the transom knees?

[TerryLL]

Gusset is the correct word for the reinforcing plates for the frame joint at the chine. This type of construction is very common and results in a strong and rigid connection between the bottom and side parts of the frame. Epoxy and bronze ring shank nails work just fine. Here's a pic of gusseted frame construction on a large dory with fir framing. These frames also had a filler piece between the gussets.

http://i325.photobucket.com/albums/k365/TerryLava/CapeAnnConst2.jpg?t=1238099316

[cat_duh_maran]

Try Anchor (http://www.anchorhardwoods.com/default.asp) in Wilmington. They'll have everything wood you need to build a boat including DF in larger dimentions and marine ply.
Bruce