Why these screws?

[Greg Nolan]

Last week, after five years of using our 1931 15’ Old Town 50 pound canoe with several cracked ribs, holes in the planks, battered gunwales, and an old canvas that had been repainted several times, I removed the canvas, outer rails, keel, and exterior stems from as a first step in doing a complete renovation.

Before: Now:

In looking closely at the inwales to see what needs to be done with them, I realized that two screws in the center of each inwale actually seemed to serve no purpose whatsoever. I had never looked closely at them before, and always assumed that they went through the inwale into a rib and then on into the outwale, to provide some extra strength where the center thwart was mounted. But the screws do not even go through the inwale -- they are just as long as the inwale is thick, and even if they were longer, they would go into the empty space between ribs.



This is of some concern to me, because one inwale has damage -- cracked in two places -- one partial crack perhaps reparable with glue, but the other a complete break that would have to be fixed by scarfing on new wood. But if these two screws and their holes don't do anything except weaken the inwale, I probably should just replace that inwale rather than trying to repair it -- and maybe should just replace both.

I checked another 15’ 50 pounder, from 1934, and found that it, too, had the same thing -- two screws just the length of the thickness of the inwales, at the center of the inwales, not holding anything.



Any ideas on what these screws might be for?

 

[Grandlaker]

Greg There to keep the inwale from splitting lengthwise with the grain. The stress on the diamond bolt when you set that center thwart on your shoulders to carry the canoe. I have seen it done at seat bolts too

 

[Larry Meyer]

Putting in screws there for that purpose seems a strange, even counterproductive, solution to the problem. The holes weaken the grain, much more than the screws could reinforce the grain. If anything I would take the screws out, and glue in wooden plugs.

The splits in the inwales may be salvageable. Steve Lapey uses a jig to router out a thin line on the underside of the inwale, spanning a crack or stress fracture. He then glues in a piece of wood to span the stressed area. The result is likely stronger than the original sections of the inwale.

 

[Benson Gray]

These are common on Old Town canoes from the 1930s as shown at http://forums.wcha.org/attachment.php?attachmentid=24820&d=1149194189 and in your pictures. It is assumed that they were intended to reduce inside gunwale splits as Dale mentioned. This seems like somebody's good idea that didn't last long.

Benson

 

[Greg Nolan]

Dale and Benson --

Thanks for the explanation. It seems counter-intuitive to me (as it does to Larry), but I suppose the factory must have been having problems in this area, and was trying to fix the problem without increasing weight. I will say that the sheer on the canoe I am restoring continues to be fair, even with inwale breaks, and a crack in this area on one side, and even though the canoe gives evidence of having been roughly treated before I got it (a fair amount of replaced planking under the canvas I removed, and several cracked ribs and a few holes in the planking, presumably after that repair work was done and the canoe was re-canvassed). The other 50 pounder has gunwales that are fair and in good shape, notwithstanding being owned by a kids' camp for many years. So it seems that the screws at least don't do much harm, and maybe do some good.

I think I will replace the inwales on the boat I am restoring with ash, rather than spruce, to avoid messing around with repairing a number of dings and cracks, and for the extra strength of ash, at the cost of a slight increase in weight (a pound or two?). The spruce outwales appear to just have damaged end tips, so I'll probably just scarf on some new spruce tips. But they are painted, and who knows what will show up when the paint comes off?

 

[H.E. Pennypacker]

\It seems counter-intuitive to me

To me it doesn't seem counterintuitive or counterproductive. The screws are perpendicular to the diamond-headed bolt. If the bolt is going to cause a crack, it will be vertical and along the length of the gunwale. The screws are there presumably with the purpose of preventing spread of a crack or preventing the crack altogether. It's possible that Old Town added them to impress customers even if there wasn't an issie. In any case, I've seen quite a few of these and never a problem caused by them.

As for whether they help or not, here's a simple test... Hold two pieces of wood, one in each hand. Press them together side-by-side and then let go of one... prediction - it will fall to the floor. Now press them together, add a couple of screws from one into the other and let one go... prediction - it won't fall to the floor.

 

[Larry Meyer]

The way I look at it, anyplace you put a hole through wood, you weaken it. I would rather trust the natural strength of wood fiber bonds (grain) than what I would substitute for grain (in this case, screws). Carriage bolts work on a different principal than screws. They are a form of clamp, working on both sides of the wood. With screws, all you’ve got in place of grain is a spiral of shallow metal grooves biting into what you hope are corresponding shallow grooves they’ve cut in the wood when you screw them in.
The function of the thwart is to keep the inwales from spreading. I suspect the center thwart is where this spreading inclination is strongest throughout the length of the inwale.
The biggest stress on the center thwart likely occurs when you are carrying the canoe with the center thwart on your shoulders. Then all the weight of the canoe is bouncing on the thwart and the shocks carried out to the carriage bolts holding the thwart.
In birchbark canoes thwarts were mortised into the inwales and tied in with spruce root. I suspect this is a better way to go, because the “hole” in the inwale was filled with wood from the thwart. Under stress, the spruce root would have some play in it, absorbing shock, rather than stressing the inwale.
The Chestnut Ogilvy likely had the biggest stress on the center thwart of any canoe. It has a lot of thick ribs, slab sides, and a flat bottom. Chestnut dealt with this by using an extra wide center thwart, wide enough to be secured with two carriage bolts on each side.
There was probably never (or rarely) a problem with the thwarts of the 50 pounder because it was a case of fixing something that wasn’t broken.

 

[Grandlaker]

Larry I agree with you that the screws are overkill, The inwale should be sound and of the proper grain laid the right way to support the thwart on it's own. But I did a little experiment of my own today and it's unbelievable how much them 2 screws will hold. I took a 12" long 1"x1" spruce. I drilled a 3/16" hole to represent where the diamond bolt would go, Then I drilled and inserted 2 screws just like in the Picture Greg posted. I then drove a wedge shaped dowel into the 3/16" hole. It took quite a few heavy blows of the hammer to start minor splitting of the wood. I took an identical peace of wood without the screws and it split wide open on the first blow of the hammer. I then went back to the first peace with the dowel still wedged in the 3/16" hole and backed the screws out. It split almost in 2 .

 

[Larry Meyer]

I have to accept the results of your experiment and my compliments for going to the length of doing an experiment. Still, as we seem to agree, cases where the inwale would fail due to stress on the carriage bolt are hard to envision.

I recall once trying to salvage a Shaker style rocking chair that someone had cast off. When I gave it a good going over, I discovered that the prior owner had tried to reinforce wood joints that were failing with wood screws. But these had failed. I resolved to try the same—fix joints with screws – with no better results. Closer scrutiny revealed that the core problem was that the rocker structure – legs, stretchers, etc – had been joined and glued slightly out of plum. When you rocked in the chair, your weight was not evenly distributed and one or two joints would get all the pressure and stress. Thus, whenever you rocked in chair, stresses worked and loosened up the wood screws all over again, which in turn stressed and fractured the wood joints. I gave it up.

I put the chair out with trash . . . and a few minutes after I turned my back on it, another fella hopped out of his car and scooped it up! I often wonder if that chair is still working its way through the town, seeking a savior.

 

[Greg Nolan]

It’s great to have a problem kicked around here by thoughtful people with some experience, and who even run experiments.

Considering my 15’ 50 pounder in light of all of the above, it seems that the screws in question have both harmed and helped in the past.

Five years ago, when I bought the canoe, the 2 screws and diamond head bolt on one side seem to have had no effect on the inwale:



On the other side, there is clearly a vertical break on the inside of the inwale, in line with the diamond head bolt; notwithstanding the break, things appear pretty sound, and there was no observable break in the inwale on the other side of the bolt:



The thwart in the pictures above is the original thwart.

Five years later, the pictures show the ends of the portage yolk that I made and installed -- same length as the original thwart and same width at the ends, and I think I installed it without removing the diamond head bolts. On one side, the screws seem to have caused lateral splits or cracks adjacent to each screw:



This must weaken the inwale in some fashion, but apparently not in any way that has actually adversely affected the canoe.

On the other side, the break in the inside of the inwale appears much the same as five years ago:



but with the outwale removed, there is visible a break or split on the other side of the inwale (which may or may not have been there five years ago) and there is a vertical split running (left in the picture) from the diamond head bolt to just past the point where one of the screws is located that was not there five years ago:



It seems as though the two screws may have been helping to hold the broken inwale in place, and may have limited a newly-developed split, but apparently the outwale has been providing the primary structural support and actually holding the canoe in shape at this point.

What does seem clear is that the stresses of carrying the canoe with the portage yolk have caused some damage. In addition to the lateral cracks on one inwale, the diamond head of both bolts have sunk into the wood quite noticeably. The vertical split on the other inwale may not be new, but it certainly is more visible now. I have used the portage yolk a lot -- and every time I load and unload the canoe from the car’s roof rack, the weight of the canoe is supported on those two bolts alone. While I have not carried the canoe over any serious portages, getting the canoe from the car to the water has often involved a carry of scores or even hundreds of feet.

So . . .

On one side of the canoe, the screws apparently caused or contributed to lateral splits in the inwale -- but those splits seem largely harmless.

On the other side of the canoe, the screws seem to have limited vertical splits, and may have stabilized a broken inwale -- enough to help keep the shape of the canoe stable and keep the diamond head bolt in place sufficiently so the bolt worked effectively while the canoe was being carried frequently with a portage yolk.

I don’t know how the inwale was broken -- it seems hard to imagine such a break without other damage occurring, especially to the outwale. But there it is, and it seems that the screws have done more good than harm.

The diamond head bold has sunk well into the inwales because of the use of a portage yolk -- and the screws may have limited a split on one side that was caused or aggravated by using the portage yolk.

Because I intend to keep the portage yolk in the canoe after restoration, I am now strongly inclined to replace the inwales with a hardwood such as ash, even at the cost of a slight increase in weight, because the diamond head bolts should not sink into a hardwood the way they have into spruce.

 

[Larry Meyer]

Hmm. It is a puzzle because it’s only a 50 pounder and like thwarts and spruce inwales have born heavier loads and like stresses without a problem. I suspect a poor quality of spruce, since OT “fixed” other 50 pounders with such screw treatment, according to Benson. That one side bore up well and the other didn’t, suggests one side’s spruce was not up to the task. Top grade spruce is great stuff, but not all trees are created equal.

Diamond head bolts may be a problem because, unlike round heads, they tend to chisel into the grain more aggressively. I tend to agree that, if you lack confidence in the quality of the spruce, even one spruce inwale, then ripping it out of there is the only way to go.

A possible experiment to reinforce that area of the inwale would be to install a normally dimensioned piece of ash as an inwale, and then widen it by laminating (gluing on) another short piece of inwale where the center thwart meets it. That could provide as much extra width at the center as you chose, up to inch or so for a total of a two inch wide inwale at the center. You might even be able to salvage the existing inwales that way. Thicken them up to two inches wide, for a length of say a foot, and hang your carriage bolts through the new wood. Fill the holes in the old inwales with glued in dowels

 

[Benson Gray]

I agree with Larry that you probably have a lower quality section of spruce on the inside gunwale that split. The Otca models from 1927 and 1936 in my garage both have these screws on all three thwarts with no signs of any splits around them or at the diamond headed bolts. Both are AA grade with mahogany so the quality of wood may be better.

Great experiment Dale, thanks.

Benson

 

[Canoeal]

If you look carefully at some of the 30's boats and the "war built boats" (40s) you will see that wood quality was an issue. The better lumber was going to the war effort and suppling our allies. The grain in some if these canoe gunnels is anything but straight. run out was present as was squirrely grain. Not always but enough to cause Old Town and others (Penn Yan) to add the screws to all of the canoes. Whether you guys believe it worked or not, The builders of good quality canoes thought it worked. I'm working on a HW, that even as late as '48, the gunnel grain is so wild that they can't mantain thier shape. I am adding new, straight grained outwales in hopes the will fair out a bit...

 

[Larry Meyer]

I don’t know the history of the logging industry well enough to be sure, but I suspect spruce supply was an issue in the Great Depression. Unemployment was sky high and manufacturers of all types had tons of back inventory to sell. So it’s very likely OT had to settle for whatever low grade spruce was left in their and other lumberyards because the mills were shut down.

Once you got into WWII high quality spruce was back in high demand and the sale of it strictly regulated. Domestic production was at the bottom of the list when it came to allocation of raw materials. You could not buy tires, etc, for example.