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View Full Version : Scarfing a gunwale, then rabbeting the entire length?



YotaTruck
08-27-2014, 09:57 AM
I am refurbishing an old fiberglass canoe that had crappy aluminum coping for gunwales. I scarfed inwales from 1/2" x 1-1/4" stock and glued/screwed them to the inside of the sheer. My original plan was to use the same stock for outwales and then cap the top with some slightly wider stock. Then I thought about rabbeting the outwale to cover the sheer and the inwale, but since I need a 17'-2" outwale I will need to scarf two shorter lengths, or find clear pine in 18' lengths. It's definitely out there, but a lot pricier than simply buying two 10' lengths and ripping them.

Basically what I'm thinking of doing is ripping 1-1/2" x 1-1/8" pieces, and scarfing them using 12:1 joints and a polyurethane glue. Once the glue is cured, the pieces will be rabbeted using a router along their entire length, so that the end profile will look like this:

http://i25.photobucket.com/albums/c68/Badfish740/Outwale_zps2a44b9a4.jpg

Would it make a difference if I did the scarf vertically vs. horizontally, or is going to be too weak either way?

Here is what the two different scarfs would look like:

http://i25.photobucket.com/albums/c68/Badfish740/Scarfs_zps30e12c3e.jpg

Scarfing horizontally looks like it will yield more surface area, and theoretically more holding power?

Peerie Maa
08-27-2014, 02:22 PM
Which way has the most bend at the position of the scarf? Upwards (edge set) or horizontally? I would arrange the scarf so that it is in bending not in shear.

TerryLL
08-27-2014, 03:16 PM
The inwale may have bent on fairly easily, but the outwale is more than twice the thickness at 1.125, so you may have some difficulty getting it bent on. There will be a lot of stress on the scarf, so a longer scarf would be worth considering. And ditch the PU glue and go with epoxy.

I'd go with a table saw to cut that big rabbet.

Fitz
08-29-2014, 10:17 AM
I do this all the time. You will have no trouble gluing the scarf and then cutting the rabbet. Cut the rabbet on a table saw with feather boards to hold the stock vertically and horizontally as you run the stock through. The rabbet may need some bevel to account for any tumblehome on the canoe so that the gunwales will lie flat. I would use thickened epoxy. You will have no trouble bending the gunwale except for maybe the sheer in the ends of the canoe. Either scarf direction would likely work. I like the one on the left in your sketch. Fasteners holding the gunwales on the canoe may help reinforce the one on the right.


Fitz

YotaTruck
08-29-2014, 03:23 PM
I do this all the time. You will have no trouble gluing the scarf and then cutting the rabbet. Cut the rabbet on a table saw with feather boards to hold the stock vertically and horizontally as you run the stock through. The rabbet may need some bevel to account for any tumblehome on the canoe so that the gunwales will lie flat. I would use thickened epoxy. You will have no trouble bending the gunwale except for maybe the sheer in the ends of the canoe. Either scarf direction would likely work. I like the one on the left in your sketch. Fasteners holding the gunwales on the canoe may help reinforce the one on the right.

Thanks! I trust advice from someone who self identifies as a "wood canoe nut" and quotes Thoreau in their signature line :d

StevenBauer
08-29-2014, 04:37 PM
Pine? Most canoe builders use Ash for gunwales. It's a spot that tends to get abused. You might be doing it again soon if you use Pine.


Steven

Fitz
08-29-2014, 08:09 PM
Spruce is a traditional gunwale material. Ash, mahogany, cherry are also good candidates.

Todd Bradshaw
08-29-2014, 10:28 PM
Yes, pine outwales tend to get beat up pretty quickly. I've generally found that the middle of the boat is not a very good place to have a scarf. It's always under pressure from the hull curvature and also gets stressed by the center thwart, portage yoke or other structure. That's the spot where I would like my gunwales to be at their strongest - no matter what their cross-section or wood species. Given the option, I'd rather put my scarf(s) closer to the ends where they tend to be straighter and under less strain.

Did you measure the hull thickness? Most fiberglass canoe hulls are nowhere near 1/8" thick up along the sheer. I doubt your rabbet needs to be that deep.

YotaTruck
08-30-2014, 07:51 AM
Yes, pine outwales tend to get beat up pretty quickly. I've generally found that the middle of the boat is not a very good place to have a scarf. It's always under pressure from the hull curvature and also gets stressed by the center thwart, portage yoke or other structure. That's the spot where I would like my gunwales to be at their strongest - no matter what their cross-section or wood species. Given the option, I'd rather put my scarf(s) closer to the ends where they tend to be straighter and under less strain.

Good point about the scarf location. Since I'll be using two 10' lengths I can simply offset them from the center pretty easily. As for the durability of pine (I'm thinking Southern Yellow Pine, which is a little harder) are you talking in terms of resistance to cosmetic dents and dings or actual structural failure? These wales will be painted, so when they get banged up they'll just get another coat of paint.


Did you measure the hull thickness? Most fiberglass canoe hulls are nowhere near 1/8" thick up along the sheer. I doubt your rabbet needs to be that deep.

I need to measure it as I was not near the canoe and just guessing when I threw the sketch together-the inwale stock size is correct though.

CundysHarbor
08-30-2014, 08:08 AM
The sections look very heavy to me; but I'm not a canoe guy. I'm thinking more like 1/4 inch for thickness for all the pieces in spruce. If such an assembly was epoxied together, you would have a very strong beam. May be I'm missing something.
Dave

Todd Bradshaw
08-30-2014, 10:28 AM
If the grain is decently straight the pine should work and be strong enough, but outwales get banged on docks, racks and other stuff and harder wood holds up better to the everyday knocks. If you look at a typical wood-gunwale fiberglass canoe, like a Mad River, the inwale and outwale sandwich the hull and are screwed together through the hull, typically with something like countersunk #8-#10 flat head screws. The top is just sanded level and varnished, and the upper hull is visible as a thin black line between the two wooden pieces. In many cases, the fiberglass that shows probably isn't even 1/16" thick. One of the good things about composite construction is that it allows you to thicken the construction down low where you need toughness and thin it out up top where all it needs to do is keep splash out. It isn't unusual for a decent fiberglass canoe's sheer line area to be as little as gel coat, a couple layers of 7.5 oz. fiberglass cloth and maybe a layer of woven roving - which doesn't amount to much in terms of layup thickness.


The sections look very heavy to me; but I'm not a canoe guy. I'm thinking more like 1/4 inch for thickness for all the pieces in spruce.

Gunwale cross sections vary like crazy from builder to builder, but that would be awfully light for a recreational canoe. Those shown in the drawing above are a bit on the unusually hefty side though. It would be a lot more typical to ditch the cap on top and essentially cut the whole works level with the top of the hull. As is, they will look drastically better if they taper in height as you approach the ends of the boat to maybe about 2/3 of their mid-boat height. It doesn't take an awful lot of taper, but it makes them look a lot less clunky (usually easier to bend down there, too).

http://webpages.charter.net/tbradshaw/Sails%20and%20Plans/guide2-a01.jpg

I'm assuming that your seats are hung from the hull's sides, because you haven't allowed enough wood to hang them from the gunwale. Which also brings up the question of how you plan to hang the thwarts for the same reason. Drilling vertical bolt holes through half inch pine is pretty risky. At least you wouldn't have to worry about the scarfs being the weakest spots on the gunwales......