Stem length (and structural integrity)

nickb

WCHA member #8947
I've begun planking my very first w/c canoe, and despite the unheated garage have been getting in a couple of hours of work every evening. It's sort of my own design - a modification of an existing design, so to speak, and i realized when I built the form, because of the way my station molds were laid out, my stem channels may have ended shorter than could be desired. I don't know if there's a set length stems need to be - I'll end up with about 4 ribs bent over the stem - but it seems like in some designs the stems go much farther inboard. My particular design is a flat bottom, flat water boat, and will not be fitted with a keel (at least i don't plan to). I am thinking of modifying the form (after I see what boat number 1 looks like) to accept longer stems. Just wondering if this would pose a structural hazard.

Relatedly, I've heard it said that in a boat this long (17') you need either half ribs, or a keel for the structural integrity of the boat. I hadn't planned on either, but was only planning for flat-water day use - purely recreational - probably isn't a boundary waters type boat. If I throw in a floor rack would that be kosher, or do I really need to add a keel to it?
 
Nick,

I don't think it matters. I have not counted how many ribs go over the stems on the canoes I have here so can't say for sure about that.

I built an 18' form and did not put a keel on the one canoe I built from it. My ribs are 5/16" and planks 5/32". It seems plenty strong. I doubt a keel gives much strength, structurally. Especially compared to close spaced ribs or half ribs.

Awhile back I reviewed the serial number forum and found that Old Town put keels on 98% of their canoes. When the factory puts a keel on a canoe you know there is a very good reason for it. More recently it seems the keel vs no keel camps are split 50/50. So, my two cents; leave it off. You can always add it later if you think it is needed for your purposes.
 
4 ribs should be OK. Make sure that you use some small fasteners through the ribs into the stems.

The canoe will be OK for structural integrity with no keel or half ribs. What is affected is the stiffness of the bottom. This is governed by the flatness of the bottom, the width of the canoe, and the thickness of the ribs. Oh, and you personal comfort level for bottom stiffness.
 
The bottom is pretty flat (the ribs rebounding are making it a little less so, but still) and I think it's about 32-33" wide, with significant tumblehome. I used all the specified dimensions in the "blue bible" for structural elements - 5/16 ribs and 5/32 planking, so I'm hoping that's enough.

I don't have any particular aversion to keels incidentally, I just aesthetically like seeing the curve of the bottom of the boat uninterrupted. And paddling on flat (also deep) water with a design that has little to no rocker I probably wouldn't have as much need for one.

At worst maybe I could paint it blue and yellow and sort of turn it into a, you know, floating trampoline?
 
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Still planking... I seem to remember mr "greatlakes" upon completion of his Pal saying that every step was easier than he anticipated (most impressive). I, on the other hand, am finding a new challenge to figure out at every step. That's what I get for being essentially a musician with no woodworking skill trying to build a boat. Of course, I'm already planning how I'm going to fix all my mistakes on my next boat, and then the one after that will be even better, and then...

In the meantime, I'm getting close to goring, and trying to not get too many big gaps in my planks (especially near the end - very fine entry...)
 

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If this is your first boat then you should feel quite proud of your accomplishments. Don't sweat the mistakes. You'll hide them. The only person that will know in the end is you.

As an aside, don't be shy about setting the tacks somewhat aggressively when you are planking. Don't worry if you dimple the wood a bit as long as you don't get too carried away and split the plank. The deeper you set the heads now, the easier it makes clinching. When all is said and done, when you sand out your planking, you really don't want to be sanding the heads of your tacks. To get the tacks to turn better on your form and get a better set you may need to close the gap between the ribs and the form a bit.
 
Yeah, the ribs are a little high... Better readjust my ceiling brace...

I thought you re-seated all the tacks with the clinching iron before you really sanded the hull in earnest? I realize I may be making my life harder down the road, but I also get that second shot to really seat tacks well, yeah?
 
Ya, you re-hammer the tacks again, once it's off the form. And then soak the planks so the hammer blooms disappear, before sanding OR NOT. The outer hull will be covered with filled canvas. Unless you have major gaps or bumps, you don't need to sand much, certainly not completely smoothly. That being said, any bumps or raised/differntial plank edges will show through the canvas.
 
An easy help is to round the hammer head so that it impacts hardest in the center.

BTW, you said that the bottom was flat! That's hardly flat. It will be OK.
 
Well, yeah, the ribs are rebounding a bit. Let's say the bottom of the form is flat :) As to whether I'm doing this exactly right, well...
 
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