Stringer placement... structural difference?

Excellent advice, thank you for the response

 

The test for considering the wood as structural or not is very simple. The wood is rotted, if it was not structural, you would not be needing to repair anything.

 

Gonzo, again, I get not understand you. You mean that wood rots or not depending on whether structural or not ?. What's, according to you, the relationship between these two concepts, structural - rot?
Precisely what I am suggesting, contrary to your opinion, it is that it might not be necessary to place new wood.
But I think it is best not to get dizzy further the OP, which is already happy with the response of PAR, who does give good advices.

 

If you don't want to use potatoes, you can make a simple form, like a 2x? wrapped in wax paper, then make a very light laminate over it. Remove that light laminate, trim to fit the hull, tab in place and then laminate the required amount of fiberglass over that to create an all fiberglass, foam less stringer.

 

SamSam, very good, and cheap, solution.

 

That is an interesting idea. So you're saying basically use the 2xWhatever as a mold, pop off the laminate, tab it in, then glass over that several layers and create a hollow stringer?

 

It's much simpler just to leave the "mold" in place, rather than remove it and secondary bond it back down. Again, if you use laminate as the sole stringer material, it needs to be nearly the same weight (thickness) as the hull shell in that area. Typically this means a lot more fabric, than just tabbing in some Douglas fir (or whatever).

Simply put, if you use an appropriately sized wood core then you'll need a couple of layers of 12 ounce biax to tab it to the hull shell. If you use foam or honeycomb, you'll need several layers of biax. So, which do you want to do (and pay for), a couple of layers of tabbing on a wooden stringer or several layers of biax over a sacrificial/mold core?

 

I'll repeat, but I will not say more times, the material used as the core, does not serve as resistant material. If you want to use wood reinforcements, do not use fiber (except to seal the wood and in this case need not worry about tabbing). If you want to save weight and money, do not use wood or material for the core.

 

This will depend on the core height, not the core material. The core provides shear strength itself, and contributes to bending strength almost entirely by separating the skins.

It's quite common to calculate bending stress without even considering the core itself, as its contribution is usually insignificant.

Again, the core material is insignificant in terms of strength. The height it gives between skins is virtually it's entire contribution to bending.

Core material choice has to consider its resistance to compression for sharp localised impacts on the skin, weight, and cost.
Wood is cheapest, and most popular - not a coincidence.

The is terrible information. It is wrong. Using a core other than wood will NOT mean you need heavier laminates.

Again, that will be determined by the height of the core, not the material.

 

TANSL, you're incorrect. Pleasure boat cores usually are structural in nature, particularly wooden ones, which tend to be much stiffer, for their weight than the traditional laminates usually employed. Sandwich structures absolutely rely on the cores modulus participation in the structure.

Your understanding of pleasure craft may need to be revised. Technically you're correct, in terms of stringer dimensions being the driving force, but in most pleasure craft, costs are based on unit production, so the first thing that gets cut is materials. Tabbing is usually just barely adequate in pleasure boats under 25'. This coupled with insufficient room in the bilge of a small boat, to have very tall stringers simply means they cheat, in the direction that's most economical.

It would be nice if these boats were actually engineered, but in most cases they aren't. They are built to what they know will get them through a basic warranty period, which means they have a couple of layers of roving for tabbing, high resin to fiber ratios (because cheap labor, doesn't do it very well) and 10 years later, the boat owner comes to a site like this to find solution to problems that shouldn't exist.

A wooden stringer, properly tabbed to the hull is a lot easier and cheaper to do, than a hollow stringer of sufficient height and thickness. This is the very reason it's still done this way by many of the manufactures (cheap labor, cheap materials). Some manufactures have learned about transoms and soles and some are even stepping up, to much better fabrics and resins, but for the most part, the pleasure boat industry is just barely out of the 1970's engineering and material application techniques. This isn't the case with higher end production, custom and semi custom builds and of course commercial, but the pleasure boat industry (92% of which is 15' - 23' powerboats) is clinging to old school methods, materials and techniques like a bad haircut.

You need to cut open a 15 year old center console and have a look, at the materials and engineering most of the folks coming here, want to talk about. I can't tell you how many times I have and just stood there saying to myself, "how could they think this would work . . .". I'm sure I'm not the only one.

 

Absolutely not the only one......

 

The possibility to build a stringer without any core (or without consideration of the stability of the core) means not, this would be the only way to look at it.

A Stringer is (in terms of statics) a beam, a structural member which is mainly loaded by bending moments. A bending moment creates a tensile force and a compressive force at the opposite side. Wood (along its grain) is a wonderful stuff and able to deal with both forces.

Inbetween a hollow glass fibre beam an a wooden beam are many mixed solutions (wooden core with more or less glass lamination). It is the question of actual requirements, which solution is to prefer.

And it is not bad, to know about and to consider the developments in boat building, as PAR does.

 

PAR, all your comments are very valuable, based on practical experience and, I assure you, I will try to incorporate them into my back ground, aware of his worth.
You'll notice that I speak mainly about how reinforcement works, where to strengthen it and what material is not useful from the point of view of the resistance. From that point of view I say again that the material used as the core for fiber stringers is very unhelpful and therefore from that point of view should remove it.
Construction practices, not in project engineering, may be as you point out. I'm not talking about that.
The controversy over solid or hollow stringers is not correct, imo. A wooden stringer should not be hollow. A metal stringer can not be hollow but, to save weight, we usually place lightening holes on it. A fiber stringer, from various points of view (not all), ought to be hollow. A wooden mast is solid, while a metalic one is hollow. From the point of view of engineering, it all depends on how each element works and its material. We know that solutions, for the same problem, are different for different materials.
In any case, these are my opinions based on my experience as a designer of structures of ships. But, for much experience I have, I can also go wrong or misinterpret the resistance of materials (although I do not think so).
Thank you for the time you've had trying to tame my stubbornness

 

Yes. And it doesn't have to be totally tabbed in, just held in a few places until a first layer of glass holds it in place more solidly. Hot glue can hold it in place.

But, as you've noticed, there many different ways to do this. I go along with the theory that a stringer core can have a structural function, and that is to hold the two sides together and to hold the two sides apart. That stops the stringer from buckling. If it can do that adequately, then less fiberglass materials can be used. If it is only there as a form and doesn't contribute structurally, then more fiberglass has to be used to prevent the sides from buckling. Wood is a very structural core, sometimes they are held in place with some tabbing on the hull and that is it, the sides aren't covered and the deck is just screwed and/or glued to the bare wood. Pressure treated wood is a wonderful thing. Different types and densities of foam can vary from having no structural abilities to having a very big contribution to the structure of a stringer.

And I go along with the idea that it's probably easier to just stick some foam down with bondo, level off the top to where it needs to be for the deck and then glassing it all up.

One thing I always try and mention is make sure the hull holds it's shape while you take all the structure out of it. Once you take the structure out the hull itself is somewhat flimsy, once you put new stringers in, the shape is locked in for good or bad. You have to watch out for bumps introduced by whatever is supporting the hull (trailer rollers etc), you have to watch out for the hull getting twisted, climbing in and out can do that easily. You have to watch out for the hull sagging and you have to watch out for the sides spreading out. Also, you have to watch out for your own bodyweight distorting the hull while you're glassing it back together.

If there were problems, like roller dents etc, you can do a lot to fix them when re-installing the structure.

Also the deck is a very important part of the structure and is not just something level that is handy to walk on and keep your feet out of the bilge. The deck, the stringers and the hull bottom are sort of like an upside down roof truss, all 3 parts work together.

Although they are very similar, sort of relevant here is I think a cored stringer and a cored skin are different in that they take the majority of their loads differently. A skin takes them mostly on the faces of the laminate and a stringer takes them mostly on the edges of the laminate. So I'm just talking about stringers here and not skins.

 

Cores play a very minor role in bending. They exist only to separate the skins far enough to be able to take the required bending stress, and to resist shear.

As I've said before, it's not uncommon to simply ignore the core completely when calculating bending stress.