what gives a stringer its strength

i see posts about composite stringers with low streingth foam and such and that you acauly raise the wood off the hull bottom to avoid hard spots so tell me if im entirely way off base here. that the fiberglass creates the majority of the streighth of the stringer. although the fiberglass is probably thiner on a wood stringer rather than an composite stringer so the question is why can you not reinforce the fiberglass with more fiberglass rather than replacing the wood, im sure there is a reason otherwise it seems like thats what everyone would do, i would just like to know why.

Re: what gives a stringer its strength

It can be done and some people do it.
Some boats are made with pre-made stringer grids that are bonded in place, they have no wood or foam in them. They just need to be sized to handle the load.

Re: what gives a stringer its strength

if this is true i may very well go this route. i assume i would want to add a few layers of fiberglass over the stringers and across the hull bottom. or what would the proper way to do this be?

Re: what gives a stringer its strength

Without getting into a dissertation on structural engineering you should provide a few more details on the type, size, weight and powerplant of the boat. That'll give us a good start to try to help out.

For the most part the core serve two functions in a stringer, to keep the opposing reinforcements apart and correctly oriented to one another. This is important to ensure that the "I-Beam" affect of the structure remains intact under varying loads of compression and tension. A void structure w/no core will provide, as you guessed, absolutely nothing to the structural integrity of the stringer. Foam us used in these structures to provide at least some integrity and help prevent sheering stresses from compromising the part under load. Wood provides much better structural support for the part than foam but as you know is heavier and more prone to rot when exposed to water over long periods of time. Most of the void structures I've seen on racing boats are reinforced with carbon fiber or other high tech fibers. They are also totally enclosed as in a pipe section which provides the added structural integrity.

A void stringer will have to be carefully engineered for attaching points for engines and equipment such as high density inserts and backing plates. Yes they will be made up of heavier laminations of glass & resin which besides cost will also add weight. In essense layering on more glass and resin will create a core but not one that spans opposing sides of the stringer.

If this is a small runabout restoration I'd look seriously at making your stringers out of marine ply or any other good grade ply, encapsulated in epoxy then glassed into place on a bedding of thickened epoxy glue. You'll only need a layer or two (depending on engine etc.) of 18 oz. biaxial fabrics over the stringers and tab them to the hull with overlapping layers of biaxial tape. Make nice gentle radii of your fillets from the stringer to the hull to allow the fabrics to curve gently in the transition.

Here's a good primer on foam core construction from the Diab group (mfgrs of divinycell).

http://www.boatdesign.net/articles/foam-core/index.htm

Pictures would help too. Good luck with your project.

Re: what gives a stringer its strength

its an 16 foot glastron trident 15ft with an 60hp evenrude obviously im not set on doing this but it seems to easy to not just throw some a few layers of fiberglass over the existing stringers and call it good. of course as indicated in my orrigional post if it were that easy then why wouldent everyone do it. im not oppsoed to replacing the stringers with wood and re glassing. like every american i only have so much time. the less time i spend on this progect the more time ill have for other things. i havent pulled the floor off my boat yet only drilled several 2 inch holes threw the floors for inspection and found the wood in the stringers to be wet when drilling some holes in the wood of the stringers and found the wood to be wet. so i dont have any pictures that would aid in this discussion. i can tell you that there are 3 stringers and one along the outer edge of the floor one along the one down the middle and one on the other outter edge the floor and stringer setup are only 3 ft wide and sit in the v of the hull where the v flares out in the hull shape the floor ends and uses the outter hull as the floor

Re: what gives a stringer its strength

If the stringers are rotted or otherwise compromised, they probably have water in them. Wet stringers weigh more than dry stringers. Wet stringers will eventually rot all the way. Rot spores will propogate more rot elsewhere - floor, transom, etc. Fix it once, fix it right. May seem like a lot of time, but it's winter - what else were you going to do?

- Scott