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Building on External Strongback

Submitted by JimD777 on Fri, 09/20/2019 - 10:25

I recently purchased plans for a kayak. The plans call for building on an internal beam. I have built 8 kayaks on an external ("ladder") strongback, and exactly one on an internal beam, and I didn't like it. The form templates have a cutout marked for the beam, and no rocker measurements.

I think that I have a way to figure it out:

1. Find the form with the least distance between the beam cutout and the bottom edge, and mark it "0 rocker". Example: Form Nine, 16 mm=0 rocker

2. The next form, Form Ten, measures 18 mm from the  edge of the cutout to the bottom of the form. 2 mm rocker.

3. Form 11 measures 22 mm . 6 mm rocker, etc.

These probably aren't the measurements I'll get in reality, but I hope I'm showing the general concept. Would this work?

Please, I don't want to start a debate about the merits of Internal vs. External strongbacks! Just wondering if anyone knows an easier way to do it. Thanks


I prefer internal beams but if you use the waterline as your constant the rocker should take care of itself with either approach.  Do the plans have waterline indicated?

Well, the waterline is about the only datum you can trust ... so a Scott says the rocker should take care of itself. But mistakes happen and all plans aren't accurate. One thing I always do is run a batten over the rocker to look for obvious unfairness. I also make a plot of the rocker as a check. I'm looking for big deviations ... not small ones. I've had plans where the rocker was specified but when plotted showed unfairness of over 1/2 inch at a couple of locations.



 Thanks for the responses. Yes, the waterline is marked on the forms, so if I use the method I'm describing, the  waterline marks should all be the same distance from the top string for every form (see photo). I always check the alignment with strips before stripping, and frequently have to move forms.

I can't see a picture but the waterline should be a consistent.  I make it a consistent height from the strongback , but I guess if you have a tight string that is level suspended above the forms the same principle would apply.

Etienne Muller

Sun, 09/22/2019 - 03:30

The simple solution is to make all the uprights to which your forms will be attached exactly the same height. Then use the waterline marked on the form as the placement line for the top of the upright. Provided your strong back is level this will give you a foolproof setup.


Waterline as datum point for upright.

Thank you Et, and yes, I have done it that way once.  I found it difficult to get the 18' box beam level. Another thing that occurred to me is that the cutout markings for the internal beam are positioned to make the beam go straight through the forms, and if I set up so that the top side of the marking is always the same distance from the string, the rocker will fall in place. 


Tue, 09/24/2019 - 13:22

Your plan will work just fine.

You said:

if I set up so that the top side of the marking is always the same distance from the string, the rocker will fall in place. 

Another tactic that involves less measuring is to just use a cut length of stick as a measuring tool between the string and the beam mark on the form.

Or, drill a hole through each form at the same place (top of beam cutout?) and run the string - or a laser beam- through the holes. 

BTW I've found that good polyester or cotton-polyester sewing thread works better than string as it sags less. Magnet wire (thin stuff) also works.


mick allen

Tue, 09/24/2019 - 15:39

As has been said, strongback location, straightness, curviness, levelling and measuring can be made irrelevant if one just aligns the forms and forgets about all the other stuff.

In the example below, permanent stringlines in bullseyed holes allow simple visual alignment assurance during whole build, forms are held in place with washered, wingnutted bolts in large holes so that easy individual form manipulation can be done. The double stringlines can be placed in alignment with each other by visually aligning 2 sticks on any waterline on the 2 outermost forms that the stringlines are attached to.  But it's just one way, any careful process will work.

Sofistry station setup


It seems to be working well. I marked the maximum distance  from the top edge of the //// area ( cutout for the beam) with the red tape on the ruler, and make it the distance to the string for every form, and voila! the rocker falls right into place. I still like to set up two plumbed strings, since my box beam isn't perfectly straight or level.setup