CS17 #226

[Charlie Jones]

ROFLMAO- oh come on Ray- the wire melts in two in about a half second and breaks the connection.

It's a non problem.

[Apteryx]

Well, if the wire holds and burns the boat away you still get to recover the wire.

Sometimes you just gotta think outside the asylum.   

[Tom Lathrop]

Sam Devlin has been using this trick with the battery for a long time to heat wire ties for removal.  It's safer than using a propane torch.  Ask me how I know.

[paul_stewart]

Holding a hot soldering iron against one end of the wire also works.  If using a soldering gun,  you can even remove the 2-pronged heating element and let the wire play the role of heating element  --if you desire a red-hot wire. 

 

[Apteryx]

As of this evening I have the bottoms and topsides laid out on 2x4s in 'wing' formation ready to glue 'em together forward, near the stem.

On the 'hull panel instructions' sheet there is a note 'Drawing above shows both sides of the boat stacked ready for drilling... Wire loosely ONLY between arrows to "open" hull.'

I'm not sure which arrows are meant - what portion of the keel joint should be wired prior to unfolding?

Also, I believe the bottom stringers should go in after the hull is at least taped, and perhaps after the keel batten has been set.

The instructions say the two forward stringers 'are on a diagonal which matches the developed straight line when folded.'

Does that mean the forward stringers line up, sort of like a frame?  Not sure if I'm expressing this well - do the stringers end up being vertical and 90 degrees to the waterline?  I'm not quite sure how to visualize what happens to that part of the bottom, since there is considerable twist there.

Finally, it seems the forward stringers should not extend all the way to the keel seam.  Is that correct, and how much gap should there be?

On a different subject and always overthinking the obvious, I'm planning to augment the extra bracing at the initial taping, the extra pieces temporarily supporting the dovetail joint at the bow.  What I'm thinking is a couple of 1x6s sandwiching the stern topsides and bottom, screwed together in the gap between the pieces (or maybe just lashed).  The idea being that a helper and I will be able to pick the pieces up with a greater degree of safety for that initial piece of tape.

My boat cradles will get straddled by my recently built scaffold.  I hope that will make it easy to flip the hull as needed, and while the wings are flopping around I'm hoping the scaffold will keep them under control.

As always, many thanks for the expertise offered here.  If I ever approach the craftsmanship here, I'll certainly repay in kind! 

[Apteryx]

I just got off the phone with Graham, so I figured I would post the answers I got from him.  Hopefully I got this all right.

The 'only' in that 'wire loosely' statement doesn't mean to just wire part of the centerline, it means to wire the whole centerline - but loosely so it will unfold without damage to the edges of the plywood.  The keel from stern all the way up the stem gets wired in a loose fashion prior to unfolding.

The stringers go in after the hull is taped and glued, and after the keel batten.  The developed straight line the stringers follow is a straight and relatively un-curved line that lies along the angle shown in the plans.  The stringers are there to provide support for the hull shape.  Graham explained it to me using a batten and a nail as an example.

If you bend a batten over a nail you don't get a constant curve because the bending force is concentrated at the nail, and there is no bending effort beyond the nail - the force isn't carried any further out.  The stringers carry bending force from beyond the edge of the plywood to support the designed curves.  Looking at the resulting structure with that perspective gives me a lot of confidence in the strength in that part of the boat.

I cannot describe how cool it is to learn why structural elements are the way they are.  Many thanks to Graham for taking time to walk a neophyte through some history behind engineering decisions.

Yesterday I found myself standing on a boat dock.  Work and obligations have kept me at some distance from water for a few months and listening to the waves lap the shore I started to question whether I should find some way to just buy a gunkhole-capable camp-cruiser.  Then I started thinking fast, shallow draft, and light weight.  Some factory-built options, maybe, but what's as fast as a Core Sound and stiff, too?  Catboats are friendly rigs to sail, the cat-ketch has to be more so.  Then I started thinking about planing at 12 knots and my mouth went dry.  Oh, yeah, that's gotta be good!

3-D soon.  ;D

[Ken_Potts]

  I found the angle of the stringers by taking a straight piece of scrap lumber and holding it against the inside of the hull.  I twisted the piece of wood until the gap underneath the middle disappeared.  That gave me the right angle in relation to the centerline so I could measure for the length of the real stringer.  I found that the farther along I was in the building process the less I used real measuring tools.

[Apteryx]

Ken, that sounds like a perfect way to locate that developed straight line.  When I do that in my shop I'll announce for all to hear it's your innovation.  I'll probably be the only one in earshot, but I'd hate to steal an idea without giving proper credit.  ;D

Graham had another tip that sounds good.  Stresses on the forward bottom pieces are a lot less if the forward 3 feet or so are soaked a little.  A hot wet towel on both the inner and outer surfaces will help soften up the wood prior to unfolding.

I'm not sure how long to apply the hot towels, though.  If I had to guess I'd say maybe ten minutes would get enough water into the wood fiber to make a difference.  Does that sound reasonable?

Think a couple of days would be long enough to re-stabilize the moisture content, after the boat is unfolded?

[Ken_Potts]

I don't think that's my innovation but I can't remember where I heard it...

Now I'm not giving proper credit so neither can you.  ;D

[Apteryx]

I'll host a glass to absent friends, in that case.  ;D

I have a couple more questions - yeah, just two more, that will do me. 

When can the temporary bulkhead be removed?

How does the #10 machine screw help?

Can the temporary bulkhead be removed once the seams are closed?  My plan at this point is to fillet the chines and keel seams, then lay short strips of fiberglass tape between the ties.

I'd like to remove the temporary bulkhead at that point - is that a reasonable thing to do?  It will be easier to tape the seams, I think, with the bulkhead gone.

On the 'hull assembly' page in the CS17 plans, there is a comment, 'A easier way to tighten the center seam at the chine line (forward) is to drill through the two sides and insert a #10 x 24 machine screw and tighten until it is just fair...'

Does this refer to a chunk of all-thread run athwart to pull the chines flat against the forward bulkhead, to adjust the topsides profile?  Or is this to close the gap at the centerline seam at the keel?

I have a temporary bulkhead cut, and am getting close at my snail's pace to wiring up the hull.  Oh, yeah, can't wait!  ;D

[PeterP]

If you cut out windows at the chine and over the keel batten then you can leave the temp in the boat until you are ready for the seats. Don't be timid and cut the windows good size especially the one over the center line. The windows will allow you to run tape and even the keel batten without removing the temporary thus avoiding the risk of doing work on a distorted hull. The risk here is to put too much glass over a warped profile and not being able to recover later. Make sure all your bulkheads are not bowed/buckled along both x and y - run 1x2 to straighten them out if need be - they are under a great deal of compression. I don't think you will need short pieces of tape to hold the boat together to pull the wire - I got by with short tacks of fillet. And this is where the #10 bolt comes in. In some areas you need so much wire to hold stuff together it becomes difficult to do any filleting. Also the wire has a tendency to bust out. Hence the bolt and couple of fender washers. Much better holding power this way. I am too tight to buy bolts- it's scraps of ply for washers and 3" dry wall screws for me. Good luck PeterP

[Apteryx]

I think I'll cut those windows a little larger, thanks for the tips.

So the #10 screw goes in place of one or more wire ties?  Just trying to visualize.

Appreciate everyone's help!

[PeterP]

Esthetically, the bow can be bit difficult. And it is easy not to get right. Some builders let the chine crease come all the way into the prow and that does not look  so good. Your crease should peter out about foot short of there. In other words the the bow should be visually flat for about 12" -16". Especially if you plan on topcoating with polyurethane. The high gloss will magnify  the slightest bump and imperfection. On my first butterfly attempt I ended up grinding back the chine tape to iron out the crease. I re-taped later after the prow was done to maintain the designed strength. On my next attempt I screwed a piece of 2x4 across the chine about a foot from the prow to hold it flat through the butterfly. It worked well. The bolts have two advantages: much greater holding power - especially when combined with oversize washers, and they are infinitely tweakable. The bailing wire has an infuriating tendency to break at the least opportune time in the wost possible place. Especially, when you have to nudge them in and out  few times. Good part of boat building is about making compromises that look the best so you do a lot of tweaking. PeterP

[Apteryx]

Thanks for the tips!

[Apteryx]

Regarding the chine crease, I've been thinking about making a form of butt splice at the dovetails - the joint that puts the 'wings' on the bottom pieces prior to stitching.

My thinking is that stiffening that up would help put the chine crease back where it belongs, and it might help with strenght.

If there is an impact on that seam, where the bottom-to-topside junction is flat, is that a vulnerability?  Farther back where the chine develops it seems like the boat must be stronger from the shape.

So, would it be worthwhile to plane down an area along that joint?  In other posts there's been talk of butt-splicing by filling in a hollow at the joint with thickened epoxy.  The downside is that form of joint tends to be stiff, which might be desireable at the bow.

Could be I'm just overthinking things, too.  It happens sometimes.  ;D

[lbrewer]

I'm not sure that I understand what you are visualizing.  The seam at the butterfly is already a butt joint by my way of thinking. Well, in your case it's a dovetailed butt joint.  Are you thinking about backing the seam with a piece of ply rather than the fiberglass tape?

[Apteryx]

One method of butt joint I've seen here is to take down the plywood through the first ply and part-way into the second.  The dished out area, spanning the butt joint, is filled with thickened epoxy.

The butt joint at the bow is generally done with fiberglass tape on each side.

At this point, I should probably reconsider second-guessing Graham. 

[Hirilonde]

One method of butt joint I've seen here is to take down the plywood through the first ply and part-way into the second.  The dished out area, spanning the butt joint, is filled with thickened epoxy.

Hmm, if that means what I visualize it isn't a but joint, but a half lap.  I'm having the same trouble as Larry understanding what you mean.

[lbrewer]

I assume planing down an area that spans where the two pieces join together is to end up fair with the existing surface?  I've done that so that fiberglass tape would be easier to fair  in some instances, but only if I am sheathing over it with FG cloth.  If you just use thickened epoxy to fill the hollow I wouldn't trust the shear strength without tape.   

I just grind off the tape from the outside of the butterfly joint, after all the inside taping is done and the bulkheads installed, then sheath the outside of the hull.  I do prefer to use 45/45 biax tape for the seams.

Larry

[PAR]

I described that particular butt joint in a previous posting. It needs to have tape to work. I use biax with the thickened epoxy. Clamped under a board, it'll produce a fair, smooth joint that is stronger then the surrounding plywood, which is all you really need.