Eastport Nesting Pram: untwisting and tacking questions

Builder's Forum
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So, we have an Eastport Nesting Pram all trussed up with copper wire.   We've gone around and have tightened them all up -- overall any gaps have been eliminated.

We believe we have eliminated twist and made the pram "square" by strapping the bow and stern to the sawhorses and then tightening a strap run diagonally with a tourniquet. We've left it sit for about a week now but are afraid removing the tethers will allow it to spring back to where we started and be back out of wack.

Is that likely to happen? Should we over correct, over tighten,  hoping when it is loosened it will spring back some and then be where we want it?

Or should we leave it under tension until we've tacked the strakes with epoxy? Or even further until we've done the next step of removing the wire ties and fillet the outside laps.

I fear if we remove straps it will slowly revert to where it was while the epoxy is curing and result in weaker joints.   Below is some more detail on how we set things up, made measurements, and reasoned them out.

After leveling and squaring up our sawhorses, and then setting the ENP so that it was perpendicular to them, we found the measures taken at the bow from side to side, and then the stern from side to side to be uneven.

(It took me some time to realize that, once properly set up, the horses are in effect winding sticks)

Not surprisingly the longer measurements were on opposing sides, say bow port and stern starboard, meaning that diagonal was longer and we had in effect a parallelogram, but not a rectangle. Meaning that to make it square we needed to make the diagonals equal.

Boy, I hope the above makes sense. If not I could try to post some pictures.

Thanks for any advice and help. 

Signed,

"afraid to loosen the ties that bind" AKA digger

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"...by strapping the bow and stern to the sawhorses and then tightening a strap run diagonally with a tourniquet. We've left it sit for about a week now but are afraid removing the tethers will allow it to spring back to where we started and be back out of wack."

My experience has been that when you have it Straight, Plumb & Square to your satisfaction with the diagonal strap in place, that's the time you ought to commence tacking the panels together. Removing the thethers before the epoxy tacks have cured solid gives things an opportunity to spring back, which ought to be avoided.

If the tethers / strapping make tacking in some places difficult, go ahead and get as close as you can to them. Maybe place tacks on the outside of the parts / panels at those locations where, if they'd not been there, you'd be able to place tacks inside.

Tacking is intended to stabilize position of all the various components; filleting done later makes the parts into a monocoque structure to better deal with the stresses it's designed for. Tacks ought to be sufficient to hold things in proper alignment once SPS has been achieved (however you go about accomplishing that) but those tacks need to have time to cure solid to do what's expected of them.

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Oh and a last bit about uneven. Your measurements don't have to be equal in all respects to provide you with a worthy craft once you're done. Don't go to extraordinary means to bring everything into dead-nuts equality; performance won't suffer if your hull's winded a bit or your stem & stern aren't dead vertical over 100% of their lengths.   

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 Thanks for your input spclark! Your answer is the one I suspected, but I'm glad to have someone more experienced say so. I have a fair amount of woodworking behind me, but the ENP is a first for me with tortured wood and epoxy. I also appreciate being told to not go crazy on getting it exact!

digger

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   Hey Digger,

Congrats on being healthily paranoid at this point.  I believe that your sawhorses only act as winding sticks for your floor.  I did use winding sticks across both transoms to ensure the boat didn't have a twist.  I think the tourniquet you're referring to is a Spanish windlass, which is a classic boat building tool.  Because you have two transoms, you have the additional ability to measure diagonals, which ideally should be the same.  Once I got my boat true, I was able to use epoxy "tack welds" of a slightly smaller radius than the final fillets (e.g. 3/4") to fix the hull shape.  I hope that makes sense.  BTW, I used the same procedure on both my EP and PM videos.  

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   I used a Spanish windlass…huh. I guess I must know what I'm doing elen though I don't know what I'm doing.

Here's my reasoning on the sawhorses as winding sticks, CaptainSkulley. if the horses are lined up properly (in my case I leveled them) and the boat is perpedncular to them, then any twist will be shown by the boat not sitting on them at four points. Likewise, when it does rest on the horses properly there's no twist. So as far as I can see the principle is about same as true winding sticks. Does that make any sense?

digger

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Yes it does but at the same time with a flat-bottomed hull resting on levelled 'horses if winding sticks are placed on the sheer edges you might still see some twist owing to how the panels between bottom & sheerstrake are fitting together.

Why it's a 'balancing act' getting what are given as 'proper' dimensions ( or whatever you've accepted as reliable, lacking anything defined) to come out to your satisfaction before tacking or filleting.

On that Spanish Windlass thing: I did the same on finding my hull sans deck was ~ 3/4" too wide at center prior to fitting deck. Windlass fitted to C-clamps mounted high on frames but still coear of where deck'd soon be placed gave me adequate purchase to pull sheers closer during deck bonding. All's well that ends well!

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Clear... but for that missing <edit> button...

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 Ahh, okay -- that makes sense, Spclark. I wasn't seeing the bigger picture of the entire hull and inconsistancies there may be with the strake placerent from top to bottom and side to side.

Thanks for the explaination.

digger