Strongback construction opinions

[tonycondon]

I have a few limitations on building my strongback. One is that I have limited electrical power in my rented garage and can't run much more than a circular saw. Plus I don't own a table saw or anything like that.

Another is I have a short box pickup.

I was eyeing 2x6's at Lowe's the other night. They've got any length I could want from 8' to 16' in 2' increments. It seems that with some carfeul selection I could even come up with some pretty straight boards.

It seems to me as a canoe newbie that the most important part about a strongback is that the top of it is planar so that when the forms are attached they are square with each other and your canoe gets built straight.

I was thinking about using 8' 2x6's to make a box beam. A nice solid structure but then i thought what would be the best way to get that nice planar top. I could always lay down a couple of 2x6's and plane the top smooth.

Another thought I just had was I could get a sheet or two of nice smooth sanded 3/4" plywood. Using a good straight 2x4 tacked in as a guide I could rip 6" strips with the circular saw and make a plywood box beam with a smooth plywood top. That would probably be easier to get a planar top on it.

Either way the top of the strongback is going to have to be made of two pieces of wood. Would you scarf them together to get a nice smooth transition and then just have to remove the top before breaking down the strongback?

I'm sure i'm seriously over thinking this but want to make sure I have a good plan in place before I start buying stuff to do the job Thanks!

 

[pklonowski]

The real key to the whole strongback/forms assembly is that the forms must be properly aligned. The strongback doesn't necessarily have to be perfectly straight or smooth, though that does make aligning the forms easier.

I couldn't find a straight 2xAnything to save my soul when I needed it, so I took some 3/4" ply, ripped it into 6" widths, then glued them together, concave faces against each other to overcome some of the curvature. I also stressed the assembly while the glue set up, to straighten it a bit more. It was not perfect, but was workable; I just had to shim every form.

Search this forum for stronback, and you'll find Matthew's solution:
http://forums.wcha.org/showthread.php?t=5322&highlight=strongback
Last I heard, it worked well!

 

[Greg Nolan]

If you go the plywood route, Lowe's should have a panel saw that can cut the plywood for you -- probably straighter than you might get with using a 2x4 as a ripping guide. I think there would be no need to scarf the pieces used as the top of the strongback -- just make sure that the butt joint in the top pieces is offset from the joints in the sidewalls of the box beam. The central board of the sidewalls should be a full-length piece (say 6' for a 12' beam), with two 3' pieces at either end. Then the butt joint between the two full length 6' top pieces will be evenly supported by the center of the sidewalls.

 

[Canoez]

Here is a picture of a box beam prototype we've been very happy with in a class that I teach:

Sides are 1/4" Luan and the deck is 8" wide 1/2" ply to give something to screw into. The blocks are sections of 2x6 and are flush with the bottom of the 6" sides. This prototype is 8" long. The final beam is under a Prospector ranger and is 14" long. Joints for the deck and the sides of the box-beam are staggered by at least 2 feet. On the sides, 2' long "patches" were glued over the joint that were 6" wide and overlapped the joint by a foot on either side. These were located at the bottom edge so as not to interfere with squaring up the patterns on the top deck. All joints were glued and where appropriate, screwed together until the glue set. A 180# adult can stand in the center of the beam (supported at the ends) and deflects the beam by a quarter of an inch.

A detailed post on the subject.

 

[tonycondon]

thanks for the input guys. i did check for Matthews strongback thread but it appears he went through and deleted all of his posts. unfortunate.

I will see if Lowe's can rip the plywood into 6" strips. Otherwise I think i might rip them with my circular saw but then laminate them up like Paul suggests. Then perhaps some work with a plane on the top to get a good planar surface.

 

[pklonowski]

That's bizarre...

 

[BFC fan]

how about a self aligning, self plumbing mold with built in strong back?

 

[tonycondon]

That looks pretty nifty but I think I'd like to build my own. I'm going to try to go pick up some plywood tonight after work.

 

[tonycondon]

well i went to Home Despot and got two 23/32" sheets of plywood cut into 6" wide strips. Hopefully sometime yet this week we can start laminiting them up into a big ass heavy 6x6 beam. I might scarf the pieces together just for the experience. I need to get good at scarfing as I see a lot of wood sailplane restoration in my future. plus it is a lot prettier than a butt join.

 

[Greg Nolan]

I would think a hollow box beam made from your 6" strips would be more than stiff and strong enough, and much lighter -- and therefore much easier to handle -- than a solid laminated beam.

 

[pklonowski]

I agree with Greg -- a 6x6 beam would be brutal. Mine was 1&1/2" thick, a bit on the shy side.

 

[tonycondon]

ok ok i will build a box beam. you're right it makes sense. I was still thinking in a mode that i wouldn't have accurately cut 6" wide strips but that is now the case. Now depending on the weather i'll get started this week. Supposed to be about 103 or 104 for the rest of the week and the soaring weather is looking promising so i might go flying...

 

[Dan Lindberg]

Tony,

I also recomend a box beam, for lots of the reason already given.

But, you don't have to cut the wood all that precise.

What you need to end up with are your stations aligned, in all 3 axis's.
The common way of doing that is to stretch a string along the beam, to establish a straight line, and to establish a consistant height. it's easier to get this height if you use shims/spacers under the stations. And the use of shims means that the beam doesn't have to be perfect.

I believe most builders use a cleat between the station and the beam, ie, the cleat is attached to the beam, and the station attached to the cleat.

If you drill the holes in the cleats oversize, that gives you some adjustment to locate the stations.

I'd also use at least 2 layers for the top surface, to give you adaquet thread engagement for the fasteners holding the cleat to the beam.

Dan

 

[tonycondon]

well we started building the beam tonight. perhaps tomorrow ill have a chance to put up some pictures.

 

[tonycondon]

well i finished building the beam last night. it turned out surprisingly straight and flat considering that I built it. I've got a couple of sturdy worktables that I built last fall and just clamped the partially built beam to those to keep it straightish. Now i need to find some varnish to seal it up (any recommendations? Does it matter?). then shim up some 2x4 pieces to make cleats, then forms!

Here is a lousy cell phone picture.

 

[pklonowski]

I didn't put any finish on my strongback, but it was all plywood, so might not have mattered. It was "in place" with forms & boat on it, for several months, over a hot humid summer and into a cool dry winter, so it cycled through that much well enough...

 

[Greg Nolan]

It looks like it should do the job -- one of the advantages of building a plywood box beam is that it generally will come out pretty straight and true if the plywood sheet was ripped with straight, true cuts.

Seems to me that any finish is optional, unless you will be keeping the form out-of-doors for extended periods of time -- then maybe use something with a UV inhibitor. Otherwise, I'd use whatever leftover product I might have on hand, or more likely, nothing at all.

 

[Canoez]

I would NOT apply finish to the box-beam strongback. If you do, you will probably not coat the inside of your strongback. When you have changes in moisture, it will tend to warp as the wood will not absorb moisture evenly.

If anything, I'd put a little bit of wax or tape on the bottom of the cleats used to attach the stations to the strongback. That way if any epoxy drips down as you are 'glassing, you will not bond the station to the beam.

 

[tonycondon]

ok ok ok no varnish on the strongback. was just going off what i read in Hazen's book. We started putting the 2x4 cleats on last night and got a couple taut lines run so we can shim in the rest of them and get everything pretty close to square.

Thanks for all the help folks. Don't worry I'm sure there will be plenty more newbie questions to come

 

[tonycondon]

well we got the forms cut and will probably start setting them up this weekend. On Saturday we'll rip, bead, and cove our cedar. So now that it is starting to get cold outside we'll be ready to start stripping!