light weight lay up

I have a mold of a 16' 8" rowing boat that I took off a boat that I designed and built. I pulled one boat out of the mold. The lay up is 3 layers of fiberglass mat and polyester resin. I didn't use any gel coat in the mold, and had a lot of pin holes to fill before the hull was painted ( a lot of work ). the resulting boat preforms well but is quite heavy ( about 180 lbs.) I race in open water events, and weight and strength are both important. Any help on a lighter lay up that is not too technically demanding would be a great help

 

One layer of 1810, one layer of 1/2 inch nida core. one layer of 1810 glass. light and strong.

 

To jimslade, I'm an absolute beginner at this. actually, I was amazed I even got the mold done and a boat out of it, let alone a pretty good boat that rows fast and wins races. To get to my point, I have no idea what 1810 or nida core is, and how difficult they are to work with. If I can figure how to do it, I'll post a photo of the hull for general info. thanks

 

Nord, there are two reasons I can think of why your boat is so heavy. One is chop-strand mat, the other is that you're new to composites.
CSM is weak, porous, absorbs a lot of resin and is by far the heaviest fibreglass for a given weight. Cloths are far lighter for the same strength.
New builders almost always use far more resin than needed. If you find yourself using more resin than glass by weight, you're probably adding a lot of extra resin that isn't making things any stronger. The trick is to use just enough resin to fully bond the cloth layers together, without any excess resin.
Core materials are tricky. They can be really beneficial, but I'd suggest you get some practice with simple laminates and vacuum-bagging before you go all gung-ho on coring.

 

Marshmat, Thanks, I think the lay up was pretty good on the resin to glass ratio. We rolled it very thoroughly as the mat and resin went in. the final thickness was just over 1/8". Cloth, maybe a more sophisticated variety, was what I had in mind. Coring and vacum bagging are something I need to learn a lot more about before I try It. The real culprit on the weight issue was the extras aded to the hul:flotation tanks, floorboards and thwarts. I built them all much too heavy. That part is easy to fix, especially if I start with a much lighter hull.

 

1810 is fiberglass that has 18 oz of woving glass with 1 oz of mat. nidacore is a coring material. google nidacores website. they have alot of info. I use nidacore for floors, almost anything that used wood.

 

You could come close to cutting the weight of the hull in half. I would even try 1208 or maybe just DB1700 (17 oz. biax with no mat). Either way a composite sandwich is going to be a big upgrade

Joel

 

Nord, it's perfectly normal to overbuild the first boat... all production builders do this from time to time. Once you see how she handles, then it becomes a lot easier to see where weight can be lost.
A cored sandwich will give you a stronger and lighter hull, yes, but definitely practice on small stuff first. Vac bagging isn't hard, just tedious. The bagging materials themselves will only be a couple hundred bucks for your whole boat; the cost is in the labour it takes to form and seal the bag. (High-elongation films like Airtech Stretchlon can cut the bag-makig time by half, if you want to give bagging a try.)

 

I would think that for a one person rowboat, any PVC foam core of 10mm and one layer of 13 oz biaxial fibreglass outside, and one layer of 13 oz. double bias inside should be enough. Use epoxy resin and definitely vacuum bag with bleeder cloth to absorb excess resin. It is worth while using the slurry technique to seal the foam core first, which should save a few lbs of resin also. There is a thread on slurries in the forum. Thwarts, floorboards etc can be of selected very light pieces of Western Red Cedar with one layer of woven carbon reinforcing on the under surface to resist bending of the timber under load.
Could weigh in under 80lb depending hull dimensions, (beam and freeboard), and stronger than your first attempt.

 

If you're interested in vac bagging, which I think could be really beneficial if you want to build more of these boats, check out http://airtechonline.com/ They're busy, but they have a huge range of bagging supplies and are very helpful in selecting appropriate films and breathers for any particular project. Bagging supplies aren't expensive; a 200-foot roll of a really good bagging film or release film might run you $150 and bleeder cloths are around $50 per 25 yards. It's amazing how much stronger and lighter parts can be with this kind of technique.

 

better lay up

Ok, we did the new boat ( see designing a fast rowboat ) in a lighter layup and cut a lot of weight out of it. Vacume bagging still scares us. how about something like bi-axial cloth as some one suggested? any way to do a cored hull without bagging it? I'll be doing another varient of the boat next year as a 17' sail boat (battened sails, ketch rig?) so there's another opportunity to stretch our skill level. That will be the fifth varient of the original stitch and tape 9' plywood dinghy we built 12 years ago. Keeps the family tree growing.

 

Vacuum bagging is an option, more than a necessity when using a core.

I like JR-Shines idea. Biax, no mat, with a PVC core.

And since you said your new.....Fully wetting out heavier cloths can be a little time consuming.

What I like to do is lay down some painters plastic on the floor. Wet out the plastic with resin. Lay the cloth on top of it. Then add more resin to the cloth. Roll the air out of it fully. Then peel it off the plastic and form it into the mold. This will help you keep tons of extra resin out of your layup (and prevent puddling). Its easier to get the air out of a 17, 18, 24 oz cloth when its laying flat on the ground.

 

Vac bagging is not mandatory, no. If done properly (not actually that hard), it will result in a lighter and stronger part with minimal excess resin and very few if any air voids. Air voids between core and skin are what really kill a lot of cored boats, hence why bagging is so helpful.
A light biaxial (two unidirectional layers stitched together) is easy to wet out and gives you more engineering options than woven cloths or rovings, plus it's a bit more efficient on the resin. Heavy bi- or multi-axial cloths, though, are a real pain to wet out and the technique JRL describes will make the work easier and more reliable.
Wetting out from the bottom- lay down resin, press cloth into it, let resin soak in on its own, then once it's nearly saturated squeegee the last few drops in from the top side- almost always results in a more thorough wet-out and better resin penetration into the yarns than you get by squishing gobs of resin in from the top. Once an air bubble is trapped under a top-wetted cloth you'll never get the entire bubble out; better to let them float out on their own as the resin wicks up from below.

 

lay up options

food for thought, still need a lot of info about bagging, ie: how do you hold all the laminations in place in the mold through the lay up and bagging, also, how do you retard the resin to give enough time to get everything done?

 

If you're working on a horizontal surface you have gravity on your side. If you're working on a vertical surface it's a little more tricky. Pre-coating the surface and pre-wetting the fabrics will help hold them in place. If you're working upside down well that's a whole other story. If worse comes to worse there's always Raptor Staples.

Use a slower curing resin or do your lay ups first thing in the morning when it's cooler. You can chill your resins before mixing to give yourself more time. A large boat hull will require more hands. One or more mixing and one or more spreading.

Vacuum bagging is pretty simple but as you noticed you have a shorter window with which to lay up your fabrics and get it bagged before the resin kicks. It can be done though. Look up resin infusion which has alot of similarities to bagging but has the benefit of a virtually unlimited lay up time before you have to mix resin.