Hi Everyone,
By Popular request - Here is how I replaced my stringers using Vacuum.
Vacuum bagging is not nearly as complicated as it seems on the surface. You really need to decide if you want to go through the expense of getting a (real) vacuum pump. Many will say you can use a vacuum cleaner. However, they are not designed to run for 12 hours and they want air to pass over the motor for cooling in most designs. I am not a fan of that method and I do not think you will get good results.
How does it work?
Standard air pressure is ~14psi at sea level. If you were to vacuum one square foot of material - you would get ~2000 pounds of clamping pressure. (144 square inches * 14 psi). The best part is - the clamp is shaped exactly like the item you are clamping since the clamp is made of air..
The *bagging* comes in when you encapsulate in plastic the item to be clamped and remove the air (i.e. create vacuum) - the outside air is pushing at 14 psi to fill the void. Hopefully this makes sense...
So - to replace a stringer.. (unfortunately - this forum as older software and these do not render.. so links it is...)
First we had to grind out the old ones and the tabbing.
Layout the replacement
Draw layout lines and be sure you have clean hull for the tabbing
Tack down with 5 min epoxy or something. You just don't want them sliding around..
Using poly sheeting, duct tape a length in place with extra plastic so you can make pleats (you will see why..)
Next - a layer of 100% polyester fabric (peelply)
Epoxy will not stick to this stuff - and the excess squeezes through. What you end up with is a high resin to glass ratio and a perfectly smooth surface. It peels off like Velcro once the resin is hard...
Breather fabric (i.e. quilt batting) This allows the air a path out of the bag. Otherwise air pockets could develop and there would be no clamping pressure in those areas. It also helps soak up excess epoxy.
Fold back and ready for tape
We precut to length 6" glass tape, then roll and fold in epoxy in a paint tray.
Roll each layer in place
We missed a picture of tucking in the fabric, but you want to get it into the corners so that there are no *bridges* as best you can. It will bubble on the top a bit, but the vacuum should take care of that for you.
The lay the parting fabric smoothly over the glass, and then the breather. You never want the breather to touch the epoxy directly.. Just because it makes a mess and gets glued down..
Tape everything in..
Use some clay weather stripping material to plug leaks.
Here is what it looks like fully clamped
You should have plenty of pleats so there is plastic to get into all the corners etc. If it is tight like a drum w/o pleats.. then you are likely bridging the area where the core meets the hull.. and there is no clamping going on there.. the bag is loosely fit when going on..
You can see here I'm pulling up on a pleat - pretty firmly - it barely budges... this means that I have strong clamping pressure holding everything in place.
Many questions I get are about the pump. The measurement I care about is CFM (Cubic feet per minute). This is how many CFM will it pull to maintain vacuum.
Why is it important?
A vacuum pump needs to pull air out faster than it leaks in... just like a bilge pump on a boat. If I have a 1" hole in a boat, but a bilge pump that can pump water out faster than it leaks in... I float! Same here.. If your vacuum pump can't pump the air out faster than it leaks in.. then you will never get a good clamp. So bigger is better in terms of CFM.
That said - this project is pretty simple and a small pump will work fine IMHO. As long as you seal up all the leaks you can hear.
I'm sure I missed some details - I'll post the completed pics later today.