Question about replacing and/or modifying warped keel from kit

[Landlubber Mike]

Hi everyone,

 

One of my next builds is going to be the Corel Unicorn.  I plan to start it in a month or two, but was taking a look at the kit now to see if there are any issues that I might have to address.  One is that the keel is slightly warped.  The keel is made of either walnut or mahogany, and is a full piece that includes the bow and false keel - areas that will not be planked over.  I'm planning on replacing most of the kit wood and going with swiss pear for the hull, which would include the bow and false keel.

 

So, I'm trying to figure out what I should do.  I was planning on cutting off the bow and false keel, and building new ones out of pear to attach to the kit's keel.  But since the keel is warped, I'm wondering if I should just get myself new plywood since I'm going to be building the bow and false keel anyhow (rather than trying to straighten out the keel from the kit).  My gut is telling me that I should just go ahead and reconstruct the keel so I don't have to worry about warpage.  I have a scroll saw, so I think I should be able to cut a new one out without much problem.

 

Does anyone think this is the better course?  If I was to stick with the original keel, to un-warp it, do you soak it and clamp it between two straight boards for a few days?  

 

One more question - when constructing the bow and false keel, should I make them slightly larger so that the rabbet is cut into the pear, or should I construct them so the rabbet line ends at the plywood (i.e., the rabbet would be cut into the plywood)?  My Caldercraft kit had walnut pieces for the bow and false keel and I believe took the latter approach which seems reasonable to me, but I thought I would see if anyone had a better suggestion.  

 

Alternatively, should I just reconstruct the keel out of a single piece of pear, so that I don't have to worry about cutting out separate pieces for the keel out of plywood, and the bow/false keel out of pear?   I wasn't sure if pear is solid enough to serve as the keel when most kits I've seen use plywood.

 

Thanks in advance!  

Edited March 15, 2014 by Landlubber Mike

[gjdale]

Mike,

 

You've pretty much covered all the options! If you stick with the kit supplied plywood, then soaking and clamping between two flat surfaces is the way to go (with the possible addition of some heavy weight on top). Replacing with new plywood is also a viable option. Replacing it with pear should be no problem either, strength wise (just check out some of the scratch builds where pear has been used for all of the framing/keel). Kits use plywood because it's cheaper.

[russ]

Mike:

If the warp is just very subtle, then it might be straightened out. However, I question whether or not you can really get the warp out at all. Plywood is very fussy that way. Good luck.

 

Russ

[CAd My Fingers]

Mike, I've had mixed results with soaking plywood keels and then putting them between heavy sheets of glass (used glass vs boards because the glass is straight--picked this up from Don Dressel's planking book). Also, there are a couple of posts/threads on the forum that discuss scoring the keel to get it straight that looked like a viable solution. Personally, I think cutting your own new keel is the way to go, esp. if you're planning on upgrading/scratching a lot of your kit anyway--then you know the foundation of your build is both true and solid.

 

Cheers,

Jay

[Landlubber Mike]

Thanks very much guys, I really appreciate it.  I think I'll end up cutting out my own keel - now I'll need to see if Jeff at Hobbymill can give me a big enough sheet of pear to work with

Edited March 16, 2014 by Landlubber Mike

[russ]

A lot will depend on if it is a simple bend or if there is any twist. A bend might be corrected, but a twist is VERY difficult to work around.

 

Russ

[Landlubber Mike]

It's a bit hard to tell if it's merely a bend or has some twist to it.  The keel definitely has an arch in the middle so that the middle is up a couple of millimeters off a flat surface.  It looks like there might be a slight twist to it as well on one end, which has me a little nervous.

[CAd My Fingers]

Mike, Russ is right on about twists. I did a build that had a bent and also twisted plywood keel. Did the soak-and-glass bit that I mentioned above; while that got rid of much of the problem, it didn't get rid of all it. Long story short, the slight bend and twist that still were there caused a number of issues as I went further into the build (plus, overall it just was a crappy kit). In the end, the build came out well but I fought with a lot issues due to the warped keel.

 

Since you have a scroll saw, I'm still recommending cutting a new keel--if you have the spare wood, you can give it a few goes 'til you get the keel you want. On the one hand, all may work out OK after you fix the keel you have; on the other hand it also may set you up for a lot of hassles further into your build or when it's done (will the twist continue to warp out?)...why chance it?

 

An alternative to using Swiss Pear for your keel is Baltic Birch plywood--I think it's been mentioned in a few forum posts, and you'll definitely find stuff about it using Google.

 

Cheers,

Jay

Edited March 16, 2014 by JMaitri

[bogeygolpher]

If you are concerned about the strength of a pear keel, try making a replacement from a new, flat sheet of plywood using your scroll saw.  It would certainly be a lot cheaper.

[Landlubber Mike]

Thanks very much guys, I'm going to go with cutting the new keel.  A few hours of work will save me a lot of aggravation in the end.  I'll probably also stick with plywood for the cost.

 

One more follow up question.  I plan for the stem, stern post and false keel to be in pear, and I also plan to cut a rabbet for the hull planking.  When cutting out the modified keel in plywood, should I cut it such that the rabbet would be cut into the plywood (i.e., the hull planking would flow right into the start of the pear pieces), or should I make the stem/stern post/false keel a little wider so that the rabbet would be cut into the pear itself?  I would think going with the latter might provide a little more strength to the glued area.  On the other hand, cutting it into the plywood would be easier because I can more easily cut it since one side would be open.

 

Thanks!

[CAd My Fingers]

Mike, both approaches sound reasonable. I'm guessing the kit is dbl. planked, so if I understand correctly you'll be increasing the width on each side by just over 1mm for the rabbet? In addition to being slightly out of scale (~ 2mm @ 1:75 is just over 1/2" real world, so not huge), if you increase the width of the stem and stern posts, you may want to see if those changes will impact the kit's steps for the head construction, bulwarks and rudder fits, and so forth.

 

Cheers,

Jay

[Landlubber Mike]

Hi Jay, sorry I think I wasn't very clear.  I was wondering if I should cut the false keel, stem and stern post right to where the hull planking should end (so that the rabbet would be cut into the plywood), or cut them bigger (and the false keel smaller) so that the hull planking would end up in the middle of the pear pieces, rather than butt up against the start of the pear (so the rabbet would be cut into the pear itself).  If it helps, the keel is 5mm.

 

I think the first approach would make cutting the rabbet much easier, but the plywood edge to which the pear pieces would be glued would be narrower -- and therefore, maybe less area for the glue bond -- for example, 3mm of the plywood matching up against the 5mm pear.  The second approach would make it more difficult to cut the rabbet, but the plywood and the pear joint would be the full 5mm of both the plywood and the pear.  

 

Just curious as to what people generally do in these cases.  Thanks!

[CAd My Fingers]

I think I'm tracking now, Mike on what you're looking at doing.  Where it'll matter are the bulkheads (BH).  Check them on your warped BH former to see if the BHs are flush with the bottom of the former.  If they're flush, then your garboard should be able to flow sweetly into a keel-cut rabbet (am guessing you'll use a slightly broader plank for your garboard).  My initial thoughts are that if your BHs aren't sitting flush with the former, then I'd really consider not cutting a rabbet into your keel.  However, I would test the magnitude of how this will effect the planking if a rabbet were cut.    

 

You can test how things will work/look by tack gluing a couple of midship and bow BHs (before the stem sweep) with PVA (alcohol will dissolve the bond) to the former along with scrap pieces of 5mm breadth keel.  Cut one simulated keel side w/a rabbet and then dry fit a 1st and 2nd layer of planks on both sides to see how they lay if you use a rabbet and how they lay if you don't.  

 

In the attached drawing, I quadrupled the scale of your parts (am assuming 1mm for 1st planking and .25mm for 2nd planking thicknesses).  Hopefully it'll give you a visual of what I was mentioning above about how non-flush BHs may be an issue.  The left side shows how the kit likely was designed to accommodate for planking; the right side illustrates potential issues in this case if a rabbet is cut into your keel.  The forward BH is has much sharper profile than probably your kit forward BHs, but it should give you an idea of potential issues as your near your stem sweep.  

   

 

Cheers,

Jay

Edited March 17, 2014 by JMaitri

[Landlubber Mike]

Thanks Jay, this is very helpful.  I forgot about looking to where the bottom of the buikheads lie - I'll have to take a look and figure things out.

 

Thanks!

[Landlubber Mike]

Sorry for all the questions.  It's a bit harder to find plywood that I would have thought.  Any suggestions on where to buy good quality plywood?  National Balsa has the size I need, and I think I've heard good things about them, but I get nervous when I read the following disclaimer on their website:

 

"Birch Plywood sheets are cut from a premium quality grade product. The 1/64-1/8 6 ply is aircraft grade plywood. The thickness of the sheets are in metric measurements which have been converted to English equivalents, as is all aircraft grade plywood. We always strive to provide the best quality possible, but the nature of these plywood sheets make it impossible for us to guarantee that every sheet will be perfectly flat and straight."

 

Thanks!

[bogeygolpher]

I would suggest there are probably a number of hardwood lumber suppliers in the DC, VA, MD area (Google search) that in all probability also carry different sizes and types of quality plywood that you could pick up and take home.  Also, look for marine supplies.  Those types of services were available when I lived there 20 years ago.

[mtaylor]

Mike,

 

Micheal's has birch plywood and so do many of the R/C aircraft hobby shops.  There's also Woodcraft and Rockler's which have online stores also.

[Landlubber Mike]

Thanks Prowe and Mark.  I checked out Michael's this weekend and they didn't have the plywood in my size.  I'll check out the other two shops, thanks for the suggestion.