Oh, I did it this time....


Unrepentant Canoeist
To make a long story short, my cedar strip took a long tumble in the washout of a Class 1 rapid (rescue demo, for a class I was teaching... honest!), and now it's getting a new layer of 'glass across almost the entire bottom surface, and the inside will get a bunch of patches (or maybe just another big one, like on the outside) after the outer one sets up. :(

Originally, it got 6oz cloth outside, 4oz cloth inside. The outside "patch" will be ~32"wide x 10' long. The boat itself is 15'long, 27" beam, solo. There has been some discussion on this forum about what weight of cloth is appropriate for a given boat, so I'll ask: Should I use 6oz & 4oz cloth again, or something beefier?

Also note that I now have a rubber boat, so the cedar strip will be relegated to lakes and more placid rivers after it's afloat again. I bought the tupperware one so I wouldn't have to fix the cedar strip so often... but it didn't arrive until 4 days after the wreck! Murphy's law, or some corollary thereof...
Pardon the rant, but this is one of my pet peeves and a blatant demonstration of two things: One is the fact that it's very possible to build a nice looking stripper without really knowing what the hell you're doing and the other is that the internet contains a boatload of bad information which should be ignored.

When you build a stripper with 6 oz. glass on the outside and 4 oz. glass on the inside you have basically just guaranteed yourself that you've built a weak boat and drastically increased the chances that it's going to break. A stripper is a sandwich which combines three layers of material (two of glass, placed on either side of one layer of wood). None of these layers are strong or stiff enough to stand on their own and make a canoe, yet when combined properly, the result is a light boat with quite decent strength, durability and rigidity. It will never rival a plastic canoe in terms of taking a pounding, but with reasonable use and care it will be durable and repay the owner in other ways (hull stiffness and efficiency, lighter weight, better shape, better looks, etc.)

You may have noticed.....that unlike a wood/canvas canoe, there aren't any ribs inside your stripper. The wooden core is actually quite uni-directional - meaning that the grain of the wood is providing much more strength and rigidity lengthwise than it does cross-wise. Cross-wise it's strength is limited to the grain strength of the wood (the resistance of the strips to splitting with the grain) and to a lesser extent the strength of the glue bonds between the strips (which is usually, but not always, somewhat greater than the wood's grain strength). This means that if we bend the bare wooden hull until it breaks, the break is most likely going to be lengthwise and in the wood, rather than at the strip-to-strip glue joints.

The wooden part of the boat really isn't that much different in strength and stiffness from the planking on a wood/canvas boat, but rather than bending 40 or 50 wooden ribs in to add our cross-grain and transverse strength, we do it with thousands of cross-wise yarns of fiberglass, sandwiching the core. We also gain surface hardness, abrasion resistance and some lengthwise strength, but the cross-wise reinforcement is the biggest addition that the fiberglass is contributing to the project. By separating the fiberglass layers with a reasonably durable, thicker, non-compressible wooden core, we create the sandwich and in the process, we gain hull stiffness, abrasion resistance, moisture resistance to keep the core wood dry and a whole lotta' cross-grain strength and durability.

Once we're out on the water, there are a variety of forces that our sandwich must resist or stand up to in order for our boat to survive and work properly. Excessive flexing is one and over a period of time it will weaken the hull and could even break it. Running over rocks can obviously do some damage, both in terms of abrasion and impact as well as possibly causing extreme flexing. Unless you routinely put on spikes and jump up and down in your canoe, you'll find that most of these dangers come from the outside of the hull - and most of them are pushing inward from the outside. Thus, the inside lives a fairly quiet, worry-free life. Or does it really?

The tendency of builders to think that it does and use lighter glass inside as a result, in order to save weight, may seem perfectly logical. The rocks are outside the boat....right? We want some beef out there to keep those rocks from gouging the core and abrading the hull...right? This is true, but unfortunately, that's about all the help that the outside glass can provide - and basically it boils down to providing hardness and a suitable amount of skin thickness to shield the soft core. The forces acting on the outer fiberglass layers are putting them in compression. Fiberglass has low compressive strength. It's hardness means that it will be much harder to scratch or gouge than bare cedar would be, but it is not in a position where it can provide much real hull strength (the fibers are basically being pushed-on or pushed together).

The job of holding the boat together then falls to the inside layers of fiberglass. When the hull is flexed or when it suffers an impact from the outside, the inner glass layers are put in tension (essentially the fibers are being pulled on, rather than pushed together). Luckily, fiberglass has very good tensile strength. It can handle being put in tension much, much better than it can handle being put in compression and in this case, it is providing the vast majority of the strength that's needed to improve the cross-wise grain strength of the wooden core and hold the boat together. In terms of preventing serious structural damage from typical canoeing situations, the inside glass layers on a stripper are far more important than the outside glass layers. Reducing the weight of the cloth used on the inside of the hull makes the sandwich much weaker in the place and direction where you actually want and need the most strength. It may look like the inside glass leads a charmed life, but every time the hull flexes in waves or you slide over a rock the inside fiberglass is being tested and is what is keeping the hull from splitting wide open.

Hopefully after all this you're starting to see that using a lighter layup on the inside is usually not a smart move. You might save a few pounds, but you pay dearly for it in terms of hull and sandwich strength. If anything, it should probably be heavier fiberglass than that which you apply to the outside. In practice, matching the outside and inside layups tends to work pretty well and produces a pretty good boat. You should be able to hit a rock at speed, fully loaded, slide over it as long as it's not too sharp and leave nothing more than a scratch on the outside of the hull. Seventy-five percent of the new builders who decide (often by second-guessing the designer's specifications) to lighten-up their canoes by skimping on the inside layup are totally unaware of what they are doing structurally to their hulls and all too often their follow-up forum posts are "My boat broke or split on the inside - what do it do now?"
All I can say is, the 6oz/4oz cloth combination is what came with the kit, and I followed the instructions when I put the 6oz outside, and the 4oz inside. I didn't second-guess anyone on hull construction! I'll confess to installing a non-standard seat hanger system, and using much prettier wood for the decks than was supplied with the kit (bookmatched African Mahogany, vs flatsawn ash).

I have read similar comments a while back in this forum, but didn't see any specific guidelines/ideas/preferences for what weight of cloth should be on the two surfaces, other than a comment that it depends on the boat. Thus I gave dimensions for the boat, and some info on expected intended future use (lighter duty than it has seen).

So I plead ignorance, ask to be educated, and return to my original question: what weight of cloth would be appropriate for this boat, inside & out?

Oh, I should mention that I'm thinking of putting a lateen rig on this boat... assuming it will be able to hold up to such use.
With regular, plain-weave E-glass (which is what most common fiberglass is) on a new build, a full layer of six-ounce over an extra, foot-ball-shaped layer of the same cloth on the bottom, both inside and out, is what I use and would suggest for good general durability. It will be strong enough to take a pretty good shot from a rock and also hold it's bottom shape nicely while paddling. Some people find this double six-ounce layup a bit on the heavy side (typical weights of 50-65 lbs. for a well built 17'-18' canoe) but I'd just as soon have some protection from the occasional unseen rocks that somebody hides just below the surface on perfectly good waterways.

For a smaller or solo canoe, you could probably get away with 4 oz. cloth in terms of being strong enough to hold the boat together and stiff enough to hold it's shape while it's in the water. However, you do need to understand that when it comes into contact with something other than water, it will have reduced strength and durability compared to the 6 oz. cloth.

In any case, get the layup and sandwich balanced inside and out, rather than having one side weaker than the other (especially if it's the inside).

Most of the critical strain from a sail rig will be on the gunwales and localized areas where the rig is attached. As long as the gunwale structure is reasonably sturdy, it should be OK. On a boat that narrow, I might consider a lugsail instead of a lateen. The lateen's long boom and a narrow hull that heels over easily can be an annoying combination with the clew corner of the sail actually skipping along the surface of the water at times. This limits your ability to further ease the sail out when you might need to do so. Lugsails will usually pack a similar amount of sail area, with a similarly located Center of Effort into a package with a shorter boom and more room to heel over before the sail touches the water.
Okay, so as a solo canoe, you're thinking I can get away with a double layer of 4oz, inside & out. Yes? Other option might be a 4oz patch (matches existing interior work) to fill whatever the inside repair will be, and a 6oz top laminate, extending 4" or so beyond the patch?

Also, and this is my bad, the gunnel beam is 27"; after the tumblehome kicks in, it's ~29" beam -- not all that narrow of a boat. Mostly flat bottom, well-rounded bilges, a bit of rocker, located mostly in the stems. Some time when I get a chance, I'd like to run this boat up to Madison & have you make some recommendations for locating the mast step, & start talking about a sail. But I want to get the 'glass on, inside & out, before it goes anywhere...
A double layer of 4 oz. is probably adequate for flatwater, but if it's going to be used on any rocky streams in the future, six ounce makes more sense and the weight difference isn't really very much. How much it would be will depend on the particular brand of 4 oz. used and how much texture the weave has which needs filling.

Not having seen the boat or heard the extent of the current damage or how much of the fiberglass will need to be removed and replaced, it's hard to get very specific about ways to repair it.
A bit off topic, but must add something here. Todd I am continually impressed with your patience and detailed writings in reply to numerous issues dealing with what you have learned over time by doing, not just some brief moment reading a few pages in magazines or a post on an online forum. This was another example of your passion for the industry, if I can use the term. :cool: Now back to your regular programming.
Oh, I know pretty well how to fix it -- I built it from the kit, so this isn't anything much different. There being a lot of damage of varying ages, I'm just replacing the bottom 'glass layer. Just needed to know what someone more expert than I would use for the cloth. Now I know, and I thank you...
To add to Todd's reply.

A couple comments,

Thin glass fibers are stronger per size/weight then larger fibers.
Thin/tighter weave fabric is stronger per size/weight then thicker/looser fabric.
Often little specifics are given when buying glass cloth.
There are many different cloth weaves available.

And I agree with Todd about the strength of the "standard" layup, it isn't strong enough (for general use). Though on my only (and 1st) try at building with that layup, I got way too heavy, 72 lbs for a 17.5 ft canoe. (a rookie build, too much resin)

If I were repairing that canoe, without seeing it, this is what I'd consider;

1st a small patch of 3.0 or 3.2 oz tight weave across the failed glass, either torn or separated, both in and out. (After sanding and feathering the edges.)

Then a larger, football sized layer of the same weight cloth both in and out.

This would get a full 6 oz of new glass across the failures and 3 oz additional over the rest of the football.

You didn't describe a failures but this assumes that there are discrete glass failures that can be covered with patches, Todd can correct me if I'm wrong, but I believe you only need something in the 3-4 inch overlap to regain the strength of the original cloth layer.

As for specifing a given "weight" cloth for a certain size craft, I suspect that is a little to general, as that doesn't consider the type of paddling/use the canoe have or the paddlier skill. And ultimately the inner and outer skins need to be of a certain (unknown) strength, and there are a lot of different layups that could give that strength.

Oh, are you replaced the outer layer or adding to it?
If you are sanding it all off, then I'd go with 2 layers of 3.2 plus 1 more in the football.

The outer layup is already off; the "hole" is ~32" x 10feet. I'm planning to do the sawdust/epoxy mixture as filler, then sand, and 'glass the outside with however many layers experts recommend, and then address the inside. The inside has one crack that's 5'long, and several smaller ones, and then the three places where the rock poked all the way through. Depending on how many more I find as I grind the old glass off, it might end up being one big patch, too.

Expected use will be lakes and placid rivers, and possible get rigged to sail as well. Skills of the paddler: I'm no expert, but have some skills.

My last shipment of glass cloth came from Raka; I see they specify "T" for their tight weave fabric. Any other vendor I should know about?

Raka is as good as any for small quantities of glass.

If you will use a lot, try "Thayercraft", he sells full rolls cheep.

Dredging this thread up because, 18 months after cracking this boat up very badly (includes time for shoulder surgery & rehab) this boat is ready to launch again. It has new 'glass inside & out, and new rails & thwarts (includes a kneeling thwart, no seat). During the many delays around the 'glassing phases, it seems to have changed shape a bit, probably due to humidity changes. The bottom is a bit more rounded, and it looks like the keel line has some actual rocker to it. Will get it on the water this weekend & see how it handles, maybe more than once.

Next project is the '46 Otca... at long last, I get to work on a real canoe, with no fiberglass in the plans.



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