Hi all, I am scratch building my first footy out of 1/32 balsa (make that scratch building first rc boat) I am at a good point to glass if needed. Do you typically glass over balsa? what other hull reinforcement do people do? Things are progressing very well, I am basically following the “Cobra” plans. Any help is appreciated.
Thanks
Mike
Mike, you can learn an awful lot by using the search function on this forum.
Your question has been asked recently and answered by many on here.
Find it, read it, and if you still have questions they will be answered.
Best wishes.
Hi Mike,
There are several schools of thought when it comes to balsa construction, some say fiberglassing negates the weight savings of using balsa in the first place, others feel that without some protection, the balsa is too coarse grained and fragile to risk sailing under racing conditions for fear of damage to the hull that could actually sink the boat. I tend to agree with the latter opinion, particularly if the balsa is only 1/32" thick. My boat carries several foam insulation blocks in the bow for flotation, just in case, and was built using 1/16" balsa (actually, the hull sides are double layered 1/16" sheet due to damage suffered from an accident during construction). Other options besides glass & resin include covering with dope & tissue or silk, like a model airplane, or possibly using water-based MinWax clear Polycryllic with silkspan or ladies nylon stockings as lighter weight alternatives. Some say all you need is a couple of coats of varnish to seal & harden the balsa’s surface under a light coat of enamel for color. I’ve never tried glassing anything with epoxy, in the past I’ve always used polyester resin, which finishes out reasonably well. I understand epoxy is a little tougher to use (but it might be slightly lighter), since it doesn’t wet the cloth out as well as the polyester, and the cloth seems to want to float more, rather than lay against the surface of the wood. Whatever method you use, it will likely add considerable weight to the naked balsa hull, especially if you are looking for a smooth, grain free finish. Bill Hagerup’s original Cobra hull was designed to weigh in at about 450-500 grams, but it’s capable of supporting heavier construction. I know, because I built one, and mine came out quite a bit heavier at over 600 grams (I used a cigar shaped fishing sinker for the ballast bulb that weighed just under 8 oz.), yet it floated only slightly below the designed waterline. Even with the limited 305mm high storm rig, it sails reasonably well.
Regards,
Bill Nielsen
Oakland Park (Ft. Lauderdale), FL USA
AMYA #0835
Footy #835
Nice summation Bill.
On my boat Tanto, also of 1/32" balsa, I lightly wiped over the hull with a barely damp “lintless” cloth (the type that comes in a box from home centers) to raise any loose balsa fibers. Once dry, a light sanding (220 grit or finer) with the grain produces a velvety surface. After brushing the surface free of dust I applied West System 105 epoxy to the outside of the hull and before it started to set I wiped it off the surface. Enough epoxy soaked in to harden the balsa without adding much weight to the hull. I gave the epoxy treatment a couple of days to harden (I had other things to do anyway) and gave the hull a light sanding back to a nice smooth finish. Now balsa has pronounced grain seams that are the main concern of Bill Nielsen and others, that are hard to eliminate completely.
I use an abbreviated version (without the sanding and pre-wetting) of the method above to water-seal the inside of the hull as well.
If you planning a bright finish then the method I use involves another West Systems product, 410 Microlight Fairing Filler. I mix enough of the 410 with the 105 epoxy to a thick, peanut butter consistency and apply it, spreading across the grain with a squeegee (an old credit card will do) in a very thin coat. You don’t need much, ideally you want to fill only the grain seams and see the wood through the coating. Once the application has set completely its back to sanding the hull so that the filler is only in the grain seams. If a couple of spots seem devoid of filler you can repeat the 410 process again locally. Be careful doing this as its very easy to go overboard and end up with a mess. The key is to err on the stingy side.
Once the hull is filled and sanded to your satisfaction you can varnish or paint the hull and you should have a nice smooth surface. One thing to keep in mind (besides the weight gain) while doing all this is that the underwater part of the hull is the part that you should be most concerned with. So on the Cobra I would put most of my effort into the bottom panel, the bilge panels and perhaps the lower part of the side panels. When you get to the upper part of the sides once those see water while sailing you are going real fast anyway.
One other flotation idea that I’ve used before is to partly fill a Ziplock sandwich bag with air and stuff it into the bow. Weighs almost nothing and will keep your Footy afloat in case she gets flooded.
I have had success on another technique.
Instead of going with the million-dollar West Systems stuff, I used a readily-available and convenient acrylic resin called Polycrylic (about $10 a quart, compared to $100+ for West Systems.) I first sealed the balsa with a light coat of the water-based Polycrylic, then a second light coat, and covered the still wet hull with light fiberglass while wet. I like 1/2 oz, but you might use 3/4 or 1 oz cloth, or something altogether different (light stocking hosiery or cloth?) the cloth is applied and smoothed to your desire, then more Polycrylic is applied to soak the cloth. You can paint on more Polycrylic as you desire to fill the weave or build up the finish.
When fully cured you can then apply any kind of paints you like.
Tests showed that 1/32 or 1/16 balsa was much stronger, even when punctured, and the resin seals and waterproofs the hull well, inside & out.
thx
Wow, thanks guys. I think I am going to forgo the glassing part and try an epoxy/waterproofing system. I will try to use the search more, I just wasn’t having a whole lot of luck. Shooting for the 500 gm range and seem to be doing just fine getting there even with a big steering servo, I may breakdown and get a small one that better fits my needs.
Mike