Just Starting: Attaching Forms to Strongback

Submitted by Michael74 on Fri, 04/19/2019 - 08:35

I just ordered Great Auk kayak plans and read Nick’s book, “The Strip-Built Sea Kayak” as first steps.  I also ordered a 2x4x16 foot LVL construction beam to use as the strong back (I read that this was a better option than a 2x4 because it is more true).  My naive question is how I attach the forms with actual 2” by 4” cutouts to the LVL, which has 1 1/2” by 3 1/2” true dimensions.  Should I try to center a 1.5 x 3.5 inch hole on the form instead of cutout the 2x4 hole, or just try to shim the larger hole to the smaller LVL?  Also, should I screw the forms to the LVL, or is it better to make box spacers to space the forms and hold them in place.  I’m sorry if this is an old question.  I did a search and couldn’t find the answer.  Thank you for your help and any other advice you can offer.  This seems like an impossible project looking forward from the beginning.

Everyone does it different, this is my method.

First, I have never seen a strong back straight no matter what it was made of. I make mine from 3/4" plywood, 3.5"x1.5"x16' with rabbitted corners and stagered joints. Attach the strongback to your table / sawhorses, whatever, in a level position. I mark the center of each end then stretch a piece of thread from one end to the other. This is the line you will follow with your forms this way it does not matter if the strongback is straight or not. I run a piece of 2" masking tape full length on both sides of the strongback and, using a machinests square, I draw a line full length on both sides. The distance from the top of your strongback to this line is determined by whatever kayak you are building. These are lines you will also follow with your forms. 

If the world was perfect you would start with a perfectly straight strongback, cut perfect holes in the forms, slide them in place and you would be good to go - - never going to happen :) I cut the form patterns slightly oversize and spray glue them to 3/4" particle board then sand them to final size. I cut the strongback holes approx. 1/4" oversize both in height and width. There will be indexing lines on your form patterns that you will use to align the form with the lines you just drew on your strongback. 

Slide the forms - in the correct sequence - on to the strongback in their approximate location.

I mark the vertical location of each form on both sides of the strongback. I screw 3.5" long blocks on this line. I use longer blocks (determined by the width of each form) to attach to the top of the strongback using a square. I now have perfectly square blocks to attach each form which I have predrilled for drywall screws. Attach a center form aligning the index lines to those on the strongback using one screw in the vertical and horizontal strips attached to the strongback. 

Using leftover strip pieces from previous kayaks I attach short strips to the indexing lines on each form with spring clamps that stick up 3-4" past the form. After temporarily attaching  the remaining forms stand back and look at these alignment strips to see if they all look good. Put a full length strip on the sheer line clamped into position. If the sheer strip has a nice even flow to the curve and all the strip pieces align you are good to go. If they don't make adjustments until they do. At this point having a good looking sheer strip is more important than wether the form lines are in alignment. When you are happy, add another drywall screw to each form.

Thanks Randy, this makes a lot of sense.  Rather than cut the typical 2x4 inch holes in the form and then use shims, I should cut 1/4 inch wider and taller than my 1.5x3.5 inch LVL.  I’ll have a center line on the top of the beam (thread line) and drawn lines on the sides that coincide with the indexing lines on the plans.  I can use blocks attached to the strongback and forms to line them up side-to-side and top-to-bottom.  Ultimately, I need to pay attention to the flow of the sheer strip to decide if everything was done right.  I think I understand - I’ll have to just jump in when everything arrives.  Thanks again!