Cloth over cloth, no mat

Hello everyone,

I have been building a mold for quite a while and I'm about ready to start the prototype of a 14' sailing dinghy.
I intend to use vacuum bag and foam core in the future but for now I want to produce a prototype with hand laid technique. Instead of foam in the first run I'm going to use 3mm coremat in the hull and I have in mind after the gel coat and a veil glass mat, to use 3 layers of 8oz/sq.y 250gm/m2 0/90 cloth each layer in diagonal and across the length of the hull, then coremat then an additional 3 layers of the same cloth.
There will be 2 bulkheads on the hull and reinforcements in stress areas (bow,chainplate areas). In the future, if the design works, I will be substituting coremat with a 4mm foam core in about 60-70% of the hull and solid all around. I may have to substitute the layer below the foam with a slightly heavier mat to assist the bedding of the foam.

Is there a problem do you think with delamination if I don't use mat? I consider it useless weight on a lightweight dinghy.
Any comments on the adequacy of the schedule is welcomed.
I expect that there may be some surprises and refinishing or possibly alterations to lead to an other mold but I will complete the whole dinghy and rig as planned and hope to reduce its weight in the future.

 

why dont u use a stich mat with chop stran witch would be around .5 oz u are probally going polyester for a prototype i wouldnt just use cloth bad for delaminating if u use epoxy it would be allright but will cost a lot more

 

I am not sure for coremat, but for PVC csm-mat doesn't help much. It all depends on the glue (resin) use a designated resin to glue the foam to the glass. Apply the resin generously to the foam and "work it in" with a rubber pad.

When using epoxy use thickened resin to glue the foam.

Make a trial piece first, if you pull the skins of, the core should fail not the glue. Else try again!

 

i agree

 

Some canoe and kayak makers striving for low weight use cloth on cloth layups with polyester all done at one time i.e. 2-3 ( I was told up to 6) layers of 10 oz cloth layed in a mold and then resin applied. If you are going to make a bunch of these dingys, I would make the prototype as "flimsy" as reasonably possible and then go and abuse it badly. That way you have the least amount of materials into it, it will show you the weak areas and you can always repair it. You will get to the optimum layup quicker by adding to rather than taking away from prototype layups, especially if weight is a major consideration. Sam

 

With hand lay-up to apply so much resin to wet 6 layers of cloth I think one can easily run into problems. From one hand you have the risk of non-uniform impregnation, excessive resin will sag to lower spots and insufficient resin in higher areas, or too much resin all together.
On the other hand you have air that becomes impossible to assure that it will be rolled off through 6 layers of cloth.
And there is heat, which causes all kinds of problems and makes the mold's life shorter.
I think vacuum is necessary to wet many layers of cloth together otherwise the purpose of using cloth is lost to the excessive weight of resin. Some will argue that they can reduce weight with this method and more resin is lost when each layer is allowed to gel separately.
Nobody can argue with the reduced labor cost though!

 

Koz,

Perhaps you could use a light veil instead of mat as a resin-rich layer to help with bond-strength. Make a few test strips with two layers of cloth and whatever you want to test in between them. (mat in one, veil in the second and nothing in the third) Put a piece of waxed masking tape along one of the short edges as a separator between the two layers, then test by peeling them apart after curing.

Peel is usually the weakest link in composites, so it makes for a good reality check.

Yoke.

 

Thank you all for the help and advise.
The trap in making small samples of sandwich to repeat over a larger non flat area is that you can easily control how bonding goes for the small piece, the temptation to exert some pressure with ease, and visible evidence of imperfections, which you can not repeat without the assistance of vacuum.
I'm more than 90% confident that laying glass cloth over cloth by hand turns out good results, but adding a core somewhere in between by hand I've been unsuccessful.
Lack of experience has resulted in a mold that doesn't accomodate vacuum bagging, and I simply haven't found a way to even do it in a smaller area than the total mold. I can't get the bag to seal over what's laminated in let's say 60-70% of the mold's width. It leaks!
If I make a hull designed to be about 180-200lbs and make a prototype of about 250-280lbs (estimates of area of laminates, core etc, bulkhead) it will not sail and float the same! So inevitably I will be making a mock for producing a new better mold but it has no real reason of using cloth instead of mat.
Frustrating stage of trying to jump stages I guess

 

Post some pictures, explain your process and materials. To fix the problems (leaks) in what you have has got to be easier, cheaper and faster than building a hand layup plug in your present mold, making a mold from that, and then making another (vacuum bagged) shape exactly like you began with. It might be the right weight but you still won't know if the shape is right, and you might have to go through the whole process AGAIN. Sam

 

If you want to make a lighweight hull you should use thick core and thin skins. 4mm is quite thin and your core might fail in shear. I would use at least 6mm better 8mm. But it also depends on the type of boat. I had a core shear failure in a 16ft dinghy where the core was 8mm.

To stick in the core you have to mix a paste. I think it's made from polyester resin plus micro boubles. There are special epoxy products around if you want to use epoxy. Your core has to be drilled about every 4 inches. After you applied the lighweight paste you put in the core and put a plastic sheet over it. If you have air leaks use a big industial vacuum cleaner. It can pump enough air volume to produce a reasonable vacuum.
If you did it right you should see past comming through the holes in the core. This is a indication that your foam is bonded in well.
The beauty with a thick core is that you only need one layer for the inner skin and one layer plus mat between the gelcoat for the outer skin. No additional mat required.
If you design your boat well an epoxy hull shouldn't be much more expansive. The cost of the resin is usually small compared to the cost of the total boat. It's not worth saving at this end. The epoxy laminate can be flexed twice as much before cracks in the resin appear. This saves material and therefore material costs in other areas. And things you don't have to stick in save labour hours as well.

Cheers,
Karsten

 

I would make the prototype as "flimsy" as reasonably possible and then go and abuse it badly. That way you have the least amount of materials into it, it will show you the weak areas and you can always repair it.


What would you consider the least amount reasonably possible for a dingy prototype?

 

Sorry I don't readily have any sketches or pics at this moment and the molds are covered with a few layers of cloth already. My original plug has been scrapped due to lack of space. I also had a few problems with the mold where it cracked after kids got in my work area and vandalized everything. Although repaired I expect some blemishes and warps to come out.
I haven't really found epoxy in low enough price to compare to isotropic polyester that I'm using. I know alot of things become much much simpler with it and I give it a good chance of using it in the future.
One of the problems with bagging an area of let's say 10 sq.f (3x3) with polyester is that by the time the paste/resin has been applied to the time the bag is sealed and pump is plugged in it is begining to gel.
Since I've already laid some material on the mold with polyester it is too late to switch. It has gotten too cold to work right now in my shop so I'm just sitting around trying to make a decision on whether to try sticking the core by hand, or use something like coremat and stick to the original planned schedule, or trying to vac.bag really small pieces at a time.
I guess it doesn't take much with free time to drive one's self mad, and if I was at this stage in a different season I would have plunged to the impulse of the moment
I really do appreciate all the feedback and hopefully I'll have some pics soon.
The dinghy is nothing radical, something along the lines of Laser 4000 or RS400. My deck is rather different from anything I've seen and it is intended design is a training springboard between a Laser and some skiff like an I14 or 49er.

 

What Karsten suggests sounds reasonably flimsy. What weight and type of cloth are you talking about, Karsten? With polyester, I would use mat. Gelcoat,cure. 3/4 oz mat (for print-thru and to stick the gelcoat to the cloth), cloth,cure. If you're going to bed the foam in mat , use a 1-1 1/2 oz layer,foam, cure. 3/4 oz mat, cloth. I think you could switch resins if you want, epoxy sticks to poly but not poly to epoxy. I attached a chart that might help you decide a layup schedule. I used coremat once to bulk up a mold. It was pretty lumpy stuff and seemed to absorb a lot of resin, I'm not sure you really want to use it in a lightweight project. If you use foam that's cut into small squares with a scrim backing that holds it together, you might be able to apply it without vacuum. Sam

 

The chart didn't attach. Here it is in another thread, let me know if it gets big enough to read. Sam
http://boatdesign.net/forums/showthread.php?p=58654#post58654