cedar strips may be too thin

[fisherguytoo]

My bear mtn 15' prospector is ready to precoat and apply FG.
I just finished raising the grain and sanding with #100 paper (by hand) and I
noticed a number of small areas that are very thin indeed :rolling eyes I started
out with 3/16" planks, trying to conserve weight.
I have read the discussions on the planking being only the form for the FG to be applied on and that the strength is really in the FG. Yeah, sure, but, I am beginning to get worried
about the plank thickness. In order to get a really good hull I have sanded too much.
But I need opionions / advce;
1...Am I correct in wanting to correct the thickness of the hull ????
2...what would be the best method of doing this ????
I have the FG on hand from RAKA. It is one coat / sheet of 4 oz cloth for both the
outside and the inside and I am ready to precoat. But, the weather is going to hold me back at least another week, so ordering more FG will not delay me much.

My idea is to order enough 6oz cloth for the outside and use one layer of 4oz on the inside. I have planned to add 3/16"x1 1/2"x approx. 14' stiffeners on centerline of the bottom inside of the hull to act as both stiffeners and a walking surface on approx 10'
centerlines. These stiffeners would be like mini-ribs that are used in wood/canvas
canoes and prvide traction also. Suggestions are hereby solicitated THANKS
:confused :frightened :embarassed

[garypete]

The main structural function of the wood in the hull is to distribute forces to the fiberglass/epoxy layers that give the real strength. If a submerged rock is hit with the canoe underway on one of the thin areas (1/8"?), the thin wood layer may cleanly break before it can transfer the force inward to the inner glass/epoxy layer. And if the force is transfered to the inner layer, 4 oz cloth might not be strong enough to withstand the tension forces trying to push the hull inward at the point of the rock.

At the least, I'd opt to put 6 oz. cloth both inside and outside.

I've epoxied inner half ribs into the floor of a stripper with sucess. I used white cedar, and steamed the half ribs and installed them temporarily into the canoe bottom to set. I placed the 2" x 1/4" ribs 2" apart. Supporting them while they cool and dry is quite a trick and you'll create all manner of push bars to hold the ribs down. After the ribs have dried in position, I marked each position carefully. Mixing some thickened epoxy and using the same push bars, I reinstalled each rib where it had been drying in the hull. It was a messy operation, but it did work.

In your case, the half ribs would help provide puncture insurance for the thin hull bottom.

Good luck.

Gary

"When people figure out what's really important in Life, there's gonna be a big shortage of canoes."

[Doug]

6 oz. inside and out.
Anything else would depend on your comfort & experience with fiberglass.

Doug

[Rick]

FG2.

How thin are the thin spots - you can find out by pushing a pin through until it just appears and then marking the pin with masking tape or using pliers. And where are the thin spots at - compound curves are structurally stronger than flat areas, and might not need reinforcement. Also the area and extent - a small area of thinness might not be much to worry about.

You might want to hold off on any emergency measures untill you try out the canoe in the water. If it oilcans significantly (the bottom flexes up and down when you move around), then it's too thin. Otherwise, it might be fine and you'll have a lightweight boat, since much of the weight is determined by the amount and type of wood that's used.

[fisherguytoo]

:confused After futher review of the problem, in the wee hours of the night,
I could also apply two layers of the 4oz cloth on the outside of the hull and order
6oz cloth for the inside. Benifits;
1...Using the two layers of 4oz would allow me to prceed with the glassing.
2...Use of material on hand and ordering material (6oz) for use in when glassing the
inside of the hull.
3...The extra coat of 4oz and the difference between 40z and 6oz on the inside
would probably equal the weight of the strips that were sanded to thin. Hence,
not a large increase in weight.
IDEAS ?????? :rolling eyes :confused

[Juneaudave]

This post may help http://www.bearmountainboats.com/phpbb2 ... s+strength

Also...Kayakcraft has a comparison of the strength of various glass schedules, laminates and panel thicknesses. In general, it concludes that three good ways to increase strength and stiffness are: increase plank thickness, add more layers of reinforcing fibers; or use stiffer (higher modulus) reinforcing fabrics. I think that your plan to use two layers of 4 oz is a good one. On a 3/16 inch plank thickness, adding a second layer of 4 oz cloth adds only 2.3 ozs per square foot (from 6.6 to 8.8 oz/ft2), but doubles the lbs to failure from 162 to 309.... Juneaudave

[mtpocket]

Juneaudave............
You wrote the following:
On a 3/16 inch plank thickness, adding a second layer of 4 oz cloth adds only 2.3 ozs per square foot (from 6.6 to 8.8 oz/ft2), but doubles the lbs to failure from 162 to 309....

Can you explain how you calculated these numbers (162 to 309)? Thanks, mtpocket

[Juneaudave]

Well...I didn't calculate them...they are on a table in Kayakcraft that was excerpted from an article by J.R. Watson that appeared in Epoxyworks, Number 10, Winter 1998. They tested four thicknesses of cedar (3/16, 1/4, 5/16, and 3/8) with two weights of woven glass fabric (4 oz. and 6 oz.). Most, but not all, of the failures were in tension. In my mind, that implies that to protect a hull from busting up on a rock, preferably the extra cloth weight would be on the inside i.e. rock hits hull, hull deflects, and the inside is in tension. In the case of Fisherguytoo, while the extra layer is on the outside, it will still provide strength and additional stiffness. Interestingly enough, the 6 oz fabric on the 3/16 hull has a weight of 8.0 oz per sq.ft., and yields at 214 lbs (as opposed to 8.8 and 309 for the double layer of 4 oz.). ...Juneaudave

[Doug]

Sorry,
Any tips out there for applying two layers of cloth?

Floating epoxied fiberglass?
Excess epoxy?
Time between applying second layer of cloth?
Shifting fiberglass cloth?

Doug

[hoz]

Iwould apply both layers at the same time. Allow the epoxy to seep through the double layers so you don't have starved spots. But two layers at once is the way to go. No worries about de-lamination later.

[Juneaudave]

I did two layers of overlapping 6 oz, both inside and outside of my Rice Lake. Before I did that, I asked the forum for advice and it was split...do both layers at once...or... lay-up one side, wait, and add the second.

My advice....if I were doing a double layer of 4 oz on the ouside of the hull, I would personnally layup both layers at one time...assuming MAS slow epoxy which has a pretty long open time and the temps were good. If I were using West, I would want a helper to mix. On the Rice Lake, I did one layer of 6 oz with MAS that covered one side and came up about 3/4 the other side, then overlapped on a second layup that came up the other side....giving me 6 oz on the sides and 12 oz on the bottom.

I can't see the overlap seams on the Rice Lake unless I'm looking really, really close!

As I understand it, a lot of the Europeans do both at once, as they cannot get 60 inch wide fabric. Their results are great!

So...On the outside, if you have a slow epoxy or a helper, the correct temperatures, and some experience, I would do both layers at one time. If I hadn't layed up a hull before, I would put one layer on, let it set up and do the second layer.

Just my thoughts, advice is free...Juneaudave

[mtpocket]

A couple of questions in my pea brain


1. How is it possible to get good penetration in the wood after the epoxy has soaked through two layers of cloth?

2. Wouldn't the epoxy kick before the desired penetration is achieved?

3. Also, how would you know if you did not have good penetration?

4. Is this method only possible while using a open or loose weave cloth?

5. I guess you guys have had no problem with removing the bubbles, right?

[Jeff in Pembroke]

Hi MT

I wet out a double layer of 4 oz fiberglass over the interior bottom of my canoe. The 4 oz fiberglass has a tighter weave than the 6 oz (or so it seemed) and I had no difficulty getting both layers to wet out simultaneously with a good bond to the cedar. I used WEST 105/207 and was able to wet out about 1/3 of the interior on one side before having to squegee.
I judged the penetration of the resin to be adequate when the glass turned clear and the shiny wet wood was easily visible - identical to wetting out a single layer of 6 oz on the exterior of the hull. The resin seemed to soak through both layers without any problem.
I gather from other posts on the forum that WEST is a bit thicker than some other epoxies, so I would expect a less viscous brand would wet out even more easily.

Cheers,

Jeff

[mtpocket]

Thanks Jeff,

I don't have any plans of wetting out two layers of cloth at the same time, just curious how you could accomplish such a task. I guess this fall in the category "How did they do that". One final question. Did you have any trouble with wrinckles?

[Juneaudave]

A couple of questions in my pea brain


1. How is it possible to get good penetration in the wood after the epoxy has soaked through two layers of cloth?

2. Wouldn't the epoxy kick before the desired penetration is achieved?

3. Also, how would you know if you did not have good penetration?

4. Is this method only possible while using a open or loose weave cloth?

5. I guess you guys have had no problem with removing the bubbles, right?

All good questions, mtpocket! As with all glassing questions...everyone has a theory. For instance, Hazen in his book Building a Cedar Strip Canoe- puts a football shape of 6 oz down, covered by a full layer of 6 oz and wets both out at the same time. In Rip, Strip and Row; I believe that the author puts each layer on and glasses them one at a time. Here's some more thoughts on your questions:

Regarding penetration, I think a good precaution for a double layer might be to precoat...not only to assure penetration, but to reduce the amount of epoxy you have to put on with a double layer. I would be less concerned with penetration than I would be with starving the layers. I never precoat with a single layer.

On tight weave vrs loose weave, the only experience I have had is with trying to put on a layer of 9 oz tight weave. I simply could not do it and get a clear coat despite heating the epoxy and perfect working temperatures. I can't recommend tight weave at this point to anyone despite the weight of the cloth.

I had noted that I used MAS slow the last time. I really like that epoxy because it has a lot more open time than West. It also doesn't break me out. In the heated garage, I've found that you don't really even need to worry about that epoxy foam that you get from working the projects...they simply dissipate. I'm not sure that West is that forgiving, even though West is a very, very good product.