View Full Version : Kayak Building Strongback question

05-13-2018, 11:01 AM
I'm building a Guillemot Double kayak and I have a question about making the strongback. The plans recommend building a 2x4 strongback out of strips of plywood. After going over to Home Depot and Lowes but I'm concerned that I wont be able to make it straight enough since all of the plywood I saw was warped. The strongback needs to be 17'-11" long, so it would have 3 seams. I'd stagger the joints but I really don't think that I'd get it straight enough. So I was thinking of using some man made lumber like an LVL or LSL. These cost between $50 and $35, so not crazy expensive. Has anyone tried using LVLs, LSLs or some other man made beam? If so are they straight enough? How did it work out?

I watched a lot of Nick Schade's videos where he uses an Aluminum strongback. So I searched around and I found someone local selling an 18' aluminum 2x4, but its quite pricey at $215 and I'm not sure if its worth the expense.

Lastly, does anyone have a cheap source for decent spring clamps? I saw some on ebay, but I don't know if they are good enough.

Your thoughts are appreciated!

Todd Bradshaw
05-13-2018, 11:52 AM
2x4? Gee, that's tiny. I must be out of date (just ask my wife). We built all our strippers (including my 18' double kayak) on a 6" x 6" box beam made of 3/4" plywood with internal bulkheads every few feet. It was quite solid. At one point I was building inside a warehouse where we stored sailboats and overflow canoe inventory. I had the box beam mounted on a repurposed grocery store shopping cart, so that I could roll it outside when doing the sanding. I don't think there is any reason to go crazy, expense-wise, for a simple strongback. Use as many legs, cradles or attachments to the floor to lock it in position, straight and level as it takes. For one that small, I get the feeling that the support structure holding the beam in position will probably be more important than the beam itself.

05-13-2018, 04:09 PM
The 2x4 idea is an internal hardback, not external like you are talking about Todd.

If you use a 2x4 of any kind, inside the forms, you still need to support it with something like Todd is talking about.
And you need to have 3-5 supports between the 2x4 and the external beam (6x6?) so that the 2x4 doesn't droop.
The 2x4 plywood beam doesn't have to be perfect, but it needs to be pretty straight.
You can cut the hole in the forms a little oversized. Then you have to align them to each other as good as possible before starting stripping.

A solid 2x4, along with the forms will make this very heavy when you need to turn the boat over to strip the top. Actually, even the plywood won't help all that much.
Did you buy the forms are you going to make your own?

Greg Nolan
05-13-2018, 11:22 PM
The 2" spring clamp that Home Depot sells for 99 cents is one of the best tool bargains around -- more than adequate.

05-14-2018, 03:16 AM
Metal spring clamps sometimes dent cedar strips.
There is limited use for spring clamps in a strip build, but I use the plastic ones from Harbor Freight.

You need something that does not force strips into a straight line around the curve. I use masking tape like Nick Schade shows in his videos. Cheap, disposable, quick. And not too strong.

05-14-2018, 11:41 AM
Todd, sorry... I should have mentioned that I was talking about an "internal" strongback, just like what upchurchmr described. I am planning to build a 16' support table underneath to support the internal strongback so it doesn't sag. I'm more concerned about the internal strongback twisting or warping sideways. To help prevent sideways warping I've read that some people actually screw the supports to the floor. Unfortunately, I'm building over a concrete floor, so that isn't an option for me. That's why I'm considering a LSL or LVL or some other man made material that doesn't warp.

As for spring clamps I saw 32 4" clamps on ebay for $21. but i don't know what quality they are. (https://www.ebay.com/itm/LOT-OF-32-4-INCH-METAL-SPRING-CLAMP/141343969444?_trkparms=aid%3D222007%26algo%3DSIM.M BE%26ao%3D2%26asc%3D44039%26meid%3Dad530e7a3c1e4a1 38dac58833fb458b2%26pid%3D100005%26rk%3D6%26rkt%3D 12%26mehot%3Dag%26sd%3D323190294076%26itm%3D141343 969444&_trksid=p2047675.c100005.m1851)

05-14-2018, 12:05 PM
I built my stripper following Nick's advice from his book. After sliding the forms over the 2x4, I shimmed them carefully to line everything up before screwing them in place. The 2x4 was secured to three sawhorses, which were connected with 2x4s to lock them together. Point is, the 2x4 needs to be rigid, but not perfectly straight if you do it this way.


05-14-2018, 12:06 PM
"Warped plywood" curved long ways? You can cancel out the curve if you glue the curves face to face but beyond all that, you would be amazed what you can do with a very tight string, a level and patience.

For something long and skinny like a double kayak or a rowing shell preparation for the strong back is everything.

I was looking and haven't found any pictures of the strong back for this build in particular.

I assume you're making 2 by 4 lumber out of layered plywood to get the length and straightness you need, but hardly anything you get or make is going to be straight in that much length.

05-14-2018, 12:17 PM
WoxBox is spot on!

Greg Nolan
05-14-2018, 01:04 PM
As for spring clamps I saw 32 4" clamps on ebay for $21. but i don't know what quality they are. (https://www.ebay.com/itm/LOT-OF-32-4-INCH-METAL-SPRING-CLAMP/141343969444?_trkparms=aid%3D222007%26algo%3DSIM.M BE%26ao%3D2%26asc%3D44039%26meid%3Dad530e7a3c1e4a1 38dac58833fb458b2%26pid%3D100005%26rk%3D6%26rkt%3D 12%26mehot%3Dag%26sd%3D323190294076%26itm%3D141343 969444&_trksid=p2047675.c100005.m1851)

Clamp size usually refers to the clamp capacity -- those 4" long clamps shown in the link you gave would usually be called 1" clamps -- from the photo in the eBay offering, they appear to be of the same quality as the 2" Home Depot clamps (which are about 6" long) -- nothing special, but adequate for clamping many things, and a good value. Damaging soft material is a potential problem with any clamp -- the clamping pressure with a spring clamp is not adjustable (unlike C clamps, F clamps, or some other types), and might be too strong, although the plastic covering of the clamp tips does provide some protection. If 1" spring clamps will serve your purpose, the eBay clamps seem like a reasonable deal, but I'm not at all sure that they are what you need for your project.

05-14-2018, 02:50 PM
Wox is showing exactly what I was trying to say.

The only thing I like is to use a laser for alignment.
After going crazy with a string - the laser worked for me.
But you still have to check position side to side, then height, then side to side, repeat until it doesn't change. With some concern that each form is not twisted around the long axis.

You can make the ply beam straight enough, fairly easily. For me the hard part was finding a place to make the long beam. Use a string or laser to check straightness as you build.
You can actually make a better beam with solid wood on the top and bottom, ply on the sides. Solid wood keeps it from sagging between supports better.

You also should check the beam for sag when you put it on the table. If you don't shim the 3 supports to a straight line you can get more sag than you can take out by shimming the forms.

mick allen
05-14-2018, 06:33 PM
It’s not a big deal and builds like these can be setup in all sorts of manners: some may be slightly more appropriate depending on the anticipated construction or shape issue . . .


The functional and aesthetic utility of an independently rigid internal spine/strongback is that it is a setup that enables a compact, rotational, and movable construction arrangement. A process that requires fixing to the floor with wide sawhorses is neither compact, rotational, or movable while also enabling continuous alignment assurance. A process that utilizes an external rigid box beam does however allow mobility [ie wheels] and although not as compact , great access, but no rotation, and can obviously only enforce alignment in the initial setup orientation. [makes a good and handy long table too]

A rigid internal can do all the above as well as can be hung [for all manner of rotation], rotated on jawed uprights [from sawhorses if desired – or anything], rotated on rotating jawed uprights, or just held in slings or on soft surfaces, as well attaching to/hooking on a temporary set of wheels for inside, outside, diagonal placement etc etc.

However the issue is that spine rigidity. Rigidity can be obtained by using good materials [alum/steel beam] or reasonably sized boxbeam [the largest reasonable size that will fit within the majority of the forms – say 2’-0” [60cm] or so] in from either end [plus a box/triangle resists twist better than most other shapes] . As most kayaks look like squashed, flattopped circles in section, the best orientation/location for a large box beam will be wider rather than higher and closer to the deck – say 4x6 as an initial starter – but to fit the purpose. Might as well make it bigger rather than smaller – depends on material.

Run a tight string across the beam faces to check for max deviance and place a few horiz sticks across a wide face and sight down to check for amount of twist – and then make the holes in the forms large enough to allow alignment with those deviancies [sp, heh heh] in mind [prob abt 1/8” [or .25 cm] all around – might be 2x that, no big deal]. Align forms by yr preferred method, laser, sight, level [if fixed in place], stringline . . . [whether internal or external, I prefer 2 widely spaced permanently attached internal stringlines thru ½” diam bullseyes: the 2lines for twist check and the permanent bullseyes so indiv form alignment can be checked/realigned in seconds at all stages of the build]

Anyway, there are a whole host of methods that are used or can be used successfully, but to me there is an aesthetic and functional logic to that minimal internal setup. The requirement for that logic is simply a rigid spine.
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05-14-2018, 07:35 PM
Keep in mind, too, that once you get a few strips on the molds, the whole assembly gets very rigid. When it's time to flip it over for the other half, there's no way it will shift out of whatever shape you've built into it -- straight or crooked!

05-14-2018, 07:36 PM

I don't get your comment.

"A process that utilizes an external rigid box beam does however allow mobility [ie wheels] and although not as compact , great access, but no rotation, and can obviously only enforce alignment in the initial setup orientation."

Not as compact as what? My rigid external box beam (6x6) is mounted on wheels, supports a set of forms with an internal box beam (plywood 2x4) held in 3 forks attached to the 6x6.
They are shimmed and screwed to the 2x4 with the hull up to start with. When I finish the hull stripping, the screws are released, the forks removed, and the boat rotated to work on the deck. 3 cradles (supporting the hull) are attached to the 6x6. The cradles have simple straps (like a seat belt) which provide support which conforms to the hull. At this point the location of the 2x4, forms, and hull stripping has long been established and there is no need to force the initial setup orientation. You couldn't move it if you wanted. If the hull shape is not right, you messed up long long before - its too late to care, since you need to keep the internal beam in place with the forms to make a matching deck.

Please note that if you want to "rotate" the internal beam, you are going to have a problem once you get the shear strips on. The forked supports would run right thru the shear strips if you turn the "tool" 90 degrees. Or right thru the hull stripping if you want to turn it 180 degrees. Also you would need new forks (for 90 degrees), since it would have to go around the 4" side of the beam. Now you would need to re-level the new forks so you don't bend the tool and stripping - due to possible sag of the tool. I don't see the point.

4x6 beam is really too large to support many of the forms. The 2x4 runs out of space as it is (at the ends) - some of the specified forms will have very large holes compared to their size, seriously weakening the last few forms. This should not matter if you are careful, but making the internal beam 3x the one specified is just making all kinds of needless problems.
A 2x4 has been proven practical for thousands of builds. 6 for me personally.

There is no loads on the tool/ stripping which cause a need for a wider beam - just a waste of effort.

The real logic is that lots of boats have been made with both methods, successfully.
Take your pick, but don't gold plate it, unless you are going professional.

Edit: Wox beat me too it, less wordy too.

mick allen
05-15-2018, 01:09 AM
As said “don’t gold plate it”, “the . . ass’y is rigid”, “it’s not a big deal”, “there are a whole host of [other] methods” . . .

The “compact” comment just refers to the 3d shape shape/volume of the kayak not needing any particular external framing for the assembly to keep its shape[and is lighter more compact?]. To me, that’s the simple ungilded approach that allows some extra space and flexibility, if desired.

And I agree that rotation is totally unnecessary if there’s no advantage to it, but I see some advantage in some situations.

Although not and inter/external issue, if the build gets rigid after a few strips either “straight or crooked”, it just might be helpful to employ a referencing method that will show that deviancy early while there’s still time to change. This turned potentially disastrous situations into humour several times for me.

Anyway, as I think we’ve all been saying – these comments are just dancing on the margins of many viable approaches.

05-15-2018, 03:07 PM
I really like Woxbox's setup, I think I'll follow his lead. That looks like it would work just fine and as he says, once I start stripping it it will firm up also.

A lot of good info here. I'll post some pics once I get started, but that won't be for a few weeks.


05-15-2018, 07:16 PM
Just to clarify, not "my" setup. I just followed Nick's instructions, which are quite detailed. I'd highly recommend that anyone doing their first stripper kayak spend $20 on the book. It even includes tables of offsets for three boats,


05-17-2018, 10:31 AM
Oh I have a copy of Nick's book and I agree it's a necessity for any newbee building a strip Kayak. I've not read the entire book yet, but I plan to. That said I did read the sections on cutting out the forms and setting up the strongback. In it, I agree he does recommend using saw horses to support the strongback, but you seem to taken it a step further when you locked the saw horses together with 2x4s and sections of plywood which I'm sure gave you some lateral support and stiffness.

I'm going to use a 17'-11" 2x4 LSL (about $30 for 20'er) as my internal strongback. These are typically very strong, straight and they don't warp, but they do need to be supported. So my plan is to use 4 (maybe 5) sawhorses that are tied together with a few 16' 2x4s and some plywood, just as you did. This will give me a nice place to store some strips and provide some stiffness side to side. My fear was that I'd get the whole thing set up straight and then one of the sawhorses gets bumped knocking it all out of whack, because I can't screw them down to the floor as some others have.

05-17-2018, 12:15 PM
Sounds like a good plan.

05-17-2018, 02:57 PM
Much as I like Woxbox's work, building a 6x6 plywood beam and mounting it on 2 sets of wheels makes a more handy base.
Its plenty stiff, both horizontally and vertically to support the internal beam, and it can be wheeled outside or moved without affecting the support/ alignment of the internal beam.

In my case, I have recognized that the better light outside helps with doing the final sanding for both the wood (just stripped) and the finishing (after glass/epoxy).
I just can't see as well with artificial light as with natural.

The 6x6 also is more compact, allowing you to get closer to the work, especially in the ends.
Its easy enough to add shelves under the 6x6, and strip "racks" attached to the uprights where the wheels are attached.

Saw horses take up a huge amount of space. If you are hampered with a 2 car garage like I am (with tools and benches) something narrower is better.
Especially with the length of that boat.

Have fun.

05-18-2018, 11:26 PM
If you really want to attach a strongback to the floor, glue some 2x4 blocks to the floor. After the boat is complete, knock the blocks loose. Depending on what glue you use, the glue on the floor can be removed with heat, ice, solvent, or??

05-19-2018, 07:15 AM
A good method for gluing blocks to the floor is to use Bondo. Holds "pretty good" but will shear off cleanly with a firm blow of a hammer.
It can also be cleaned up easily.

05-19-2018, 10:20 PM
Hadn't thought about gluing blocks to the floor. My floor is finished with a 2 part epoxy, do you think glue or bondo would adhere to it? Maybe hot glue?

05-20-2018, 01:24 AM
At work, we had a company build a $2M model which was built by using Bondo to set locating jigs to the floor.
Worked perfectly.

I'd put a 6x6 plywood support on wheels. Much more convenient.
Show is a poor picture of a boat in work.
You can see the 6x6 ply box (16'), on supports with wheels, and with cradles for support during stripping of the deck.
The wheels are just on one support. You lift the other end just like a wheelbarrow.
Note also the two shelves for holding tools, junk.
There are 2x4's across the wheeled supports, just under the 6x6 that I use for strip storage (on each side).
Sorry for all the clutter. If you would like a better picture, I can wheel it out into the driveway tomorrow when the sun is up. Let me know.