Wood is a composite too

[Techno]

I've decided to replace the triangle stiffener(this had failed) in the center of my pad with 2 or 3 Walnut I-beams. About 3-4" wide and about the same high. This should be more than enough. The small bulkheads will also be lammed in one direction from walnut. No cloth on either. Coated with epoxy and bonded in with thickened epoxy. The bottoms of the I-beams will be tapered to prevent a "hard" edge.
The pad on my boat is single skin thickness of Kevlar-3/16"? and about 14" wide. It cracked From something but kept going from flexing.

So while everyone else is trying to get rid of all the wood I'm putting it in where it will do the best job.

The red '1' circle is the piece I ripped off by hand, see the diagonal strips from the glass. My best guess is this is what caused the failure. It just happens to be located in between the cracks.

The blue is the areas of the cracks. The brass shim stock is sticking through it.

Right is front of boat.

I think I finaly have the temperature to continue the fix, almost done. But right now I hear a T storm rumbling in!

[Jeff_G]

I'm not sure I understand all that you are trying to do. If the bottom broke on both sides of the pad it is from the flexing. The pad must be 100% ridgid. I would NEVER use a solid piece of wood for structural support. It doesn't have much. Is the boat all kevlar? No glass at all? Kevlar works differently from glass. It creates more of a mechanical bond than a chemical bond like glass. That's why it should always be vacuum bagged.
You need to stiffen the bottom. If you want call me and I can possibly help. 843 844-8739

[Techno]

The little brass shim spot is where I think it first cracked. I didn't know I had any problems and was working up to a speed in funny waves, lots of attempts. I later noticed water in both sponsons and realized that it shouldn't be there!
I think the first failure caused the others to go. Once it did crack, for whatever reason it caused another 1.5" over and the same duplicated on the other side.
My best guess is that triangle section with the hole was too weak and allowed the flexing to begin, when it did, it fatigued the first crack. That led to the entire pad width in this area flexing- I think.

The solid wood- My stringers are solid as I believe all stringers are. I have to strongly disagree with you about wood not having strength. I am using thin slats assembled in an I-beam. (in a way it's not solid, 3 parts) Every other shape I have seen uses fiberglass formed over something, except for a box they all are compromises. The original triangle support only has the peak as the seperated skin to give strength. The sides do little. A semicircle is much the same.
The I-beam places the apposing skin parrallal from the bottom skin and has the i for the attaching web, better than foam.

As far as I know the hull and deck are Kevlar. Some areas have glass but appear to be exceptions.

Glass as a chemical bond? I never heard of any of these having a chemical bond

[Techno]

While writing this I realized this bay may be the problem. Ahead of itand behind it are tiny little frames in the pad only. This bay didn't have any and is pretty long.
The pad forward is an inverted Vee shape and becomes flat in this bay.
This picture shows the situation wherre the first extreme closeup kind of confuses whats happening.
The white on the right is gel coat. I have already sanded it down and started the Kevlar repair. The kevlar is almost done, one more layer. This pic is before any repair, only the prep.
I've taken all the small frames or bulkeads out. Its these little tikes that are going to be laminated wood. The triangle in the center that makes up for the inverted Vee is going to be an I-beam instead.
Now I think I'll add a couple frames in this failed bay.

[Jeff_G]

The only wood I have ever seen in a glass boat is plywood. It is the different plies and the change in the direction of the grain that gives it it's strength. I would not use solid or even laminated wood. I would use ply. If you insist on using wood use something strong like oak. Sitka spruce is used too for strength and weight. Walnut is great for furniture. If you encapsulate the wood in glass it becomes that much stronger.
The number one cause of failure of some light boats is the absence or reduction of reinforcing materials. That is why manufacturers use plywood, balsa, foam to core a hull. It isn't the materials alone but how they are installed that makes the boat strong.

[Techno]

As far as I know if a boat has wooden stringers in it they are solid wood. Have you seen any made from plywood?
Plywood is both an engineering tool and a compromise. In the case of stringers the vertical grain would serve no purpose. For a transom it helps to stabilize a rather large area, stringers don't have a large area to stabilize. It also distributes a load verticaly in a transom.
Like you said it depends on what is being replaced. The triangle shaped reinforcement was foam filled and this foam was saturated. It no longer was a foam filled support and relied on only the shape for the stiffening(it also was defective). This triangle shape uses the most glass for the least stiffness. The peak is really the stiffening skin and is minimal for the amount used. A semi-circle would be next, then a box and the best is an I shape. the side surfaces don't contribute that much to this stiffening
An I-beam is The stiffest shape for the least material. They work on the skin seperation just like cored hulls do. Seperate 2 surfaces a set distance and the total is stronger than only a thicker surface. This is a squared system. A 2" seperation is 4 times as strong as a 1" seperation. Along with this is how wide the top surface is. On a the triangle it is maybe 3/4" if that. The I shape can be several inches wide and dramaticly increase the stiffening. A box is weaker since it uses 2 sides the same material would make it thinner. The 2" seperation would become 1". Squared rule, rules! The sides don't contribute since they are in both compression and tension. Glass has no compressive strength, wood does so you can see the comparision between the two.

Walnut isn't only a furniture grade wood but an excellent building material and is weather resistant. Most "furniture" woods do have excellent strength, thats why they are used. They usually also have a natural beauty. Oak walnut sitka In this order heavier to lighter. Strongest to weakest. Hickory is one of the strongest but isn't considered a furniture wood because it is rather plain. Maple although extremely strong is also extremely heavy and has no weather resistance.


Those are the reasons I'm doing what I'm doing. For the glassing of the wood. It would increase the strength but eventually you reach a point that no more is needed. Along with this is the difficulty of skinning an I shaped part. If I coat them with epoxy they'll have the strength and the water proofness needed.
The small frame members I may get some marine ply, if I can get a small enough amount. If not then laminated wood in one direction will be strong enough. Again, there isn't a flexing- top to bottom on these 4" high x 14" wide parts Plywood would have vertical grain and it doesn't do anything for this part. By laminating it I'm mostly just using the stabilizing factor more than increasing its strength.

For plywood transom knees these are only a building expediant. They are actually compromised since the cross grain contributes only to buckling and not any distribution of force. But to make them from laminated wood would cost too much. Solid wood probably would warp. So plywood is used. Even though its weaker its more than strong enough, plus being skinned is a cored composite.


Today I found 2 more of those triangle stiffeners. They run on either side of the cockpit. Not only can I push these with my hand but both the deck and the triangle surface deflects. Which leads me to believe that these are crap. Since they're mounted under the side deck area they shouldn't be saturated. Still have to drill a hole and find out.
another thing people don't think about is the cyclic loading. This leads to fatigue and there goes your strength, that might be what happened to these glass triangles. The boat is 12 years old after all, probably used half that but still...

[Michael Martin]

Most boats (especially mass production) have (had) plywood stringers, transoms, and floors. Most were covered with glass to help prevent water saturation.

Mike

[Techno]

Never saw plywood for stringers but then again I haven't seen a great deal of stringers.

I've posted these before but it makes sense to include these next 2 just to show what I've done.

The white in the hull is gel coat. I sanded the little strip completly away since it was cracked thru on both sides, thats the red marker

[Techno]

And the kevlar.
What Jeff_G said about vacuum bagging sure would have been usefull. I couldn't do it but the small sharp edge was hard to get it to lay down. The plastic was used because I ran short strips diagonal to this problem area after I did the regular fix. I was attempting to put kevlar strings across the area that had cracked. The thing had turned into a stringy mess and the short pieces were trying to strike on me, kevlar doesn't like sharp bends. I put the plastic down and smoothed and "clamped" it all down this way. You can see them on the left lower, not very wide.
Cold weather and work made me stop.
I have at least one more layer possibly two to cover the entire area in a single sheet. That and the stiffener and frame I'm adding.

BTW Kevlar puts up a good fight with scissors. You have to use a good pair or it won't cut at all, just folds.

[crazy horse]

My Viper and the Image tunnel I'm rebuilding both use plywood. I've never seen solid wood use in fiberglass boats. I wouldn't use walnut in a boat for stringers, 1/2 plywood on edge covered in glass is amazingly strong.

[Jeff_G]

Techno give me a call 843 844-8739. I have built or repaired everything from a 6 foot glass dingy to F1 all composit boat. I believe I can help.