vacuum bagged stringers

I have searched everywhere i can to find the info im looking for but have not found it yet, so if i missed it sorry to bring it up again. Im working on a 21' glasply hard top, the transom is in and now its on to the stringers. my question is how thick should the glass be to make foam filled composite stringers that will be vacuumed bag? I am using polyester resin, I have 1708 and the form will be two 2x12 put together to make a 3x12 stringer. Is 1708 ok to build with or do I need something else or something plus the 1708? I know most will say just use wood but I just feel like thats a thing of the past and the transom is a piece of coosa so why use wood now. any help would be appreciated and here are some pics of what im working with.

 

Havent seen any replies to this site yet ! have you managed to find the info you were looking for >
The sizes you were quoteing 3 wide x 12 high ? thats a girder not a stringer !! is the motor mounted on or between it any where ?? .
What about tanks and other heavy things like batteries and the like ??
Tell and send some better pictures if what you are wanting and see what we can come up with .

 

Thanks for the reply ill try to explain in more detail , and no i have not found a good solid answer. My main concern is the thickness of the glass. I was told 1708 is what I should be using so I'm fine with that, but how many layers or what thickness? I plan to use a outboard on a bracket so there will not be a motor on the stringers but a bit of stress from the outboard. The tank is in the center and mounts to the side of the stringer which i might change and foam it in place. I will set the batteries on the floor and at most run 2 batteries one per side so not too much weight. As for now, unless some one can give me a better answer, I will use 2 layers 1708 and bag them only because it take too long to wet them out to do more layers. Then tab to the hull and add two more layers of 1708, each extending further onto the hull then the previous. I also plan to lay 1708 down on the hull before I place the stringers to give them a chance to bond better to the hull.

The original stringers were 2x12 cut to fit and a small amount of glass. I'm only using a 3x12 mold to try to allow the glass to roll over the edges easier and make more room for foam. I have order Dave Gerr book that most people talk about just waiting to receive it. If anyone has any insight to fiberglass stringers or sees a bad flaw in my design please let me know, I don't want to finish this boat only to take it apart again if i did something wrong. Thanks and here are some pics of ones that have been done for this boat. In the last picture you can see the back bulkhead, that is where the fuel tank sits from there back toward the stern.

 

If you are wanting to make stringers theres a better shape to use than what you have but they woild be only 4 to 5 inchs tall and the foam is just a former to make a shape for the glass to roll over !
Bagging is of little to no advantage !!.


Like i asked are they just bearers for stiffness or do they have anything attached to them ?? ontop ? through the side? or anywhere ??
Tall structures like the bearers get a lot of there ridgid strength from the 2 sides of the vertical panels BUT when it bends over its total length or between frames its the very top that can and will take a huge amount of strain as it wants to streach !! This has great effect on the tall side panels and under enough load they can and will simply buckle out of shape (inwards or outwards in a rippled effect) to the point where they fracture , break and even destroy them selves !!

Just think about all of those things for a few minutes .

 

They have the floor attached to them and that's about it. So what you are saying is I should build smaller stringers that nothing attaches to and then build bulkheads and hull stiffeners to support the floor? My goal was to replace what the factory did but without wood. I am open to doing it in any manor as long as i know it is strong and will outlast me, I don"t want to do this again!! You are addressing my main concern, the stringers collapsing or buckling under the load. Thanks for your help I need as much knowledge as possible.