Pre-Preg and Balsa...

Just reading something from goetz.

THey seem to be implying that with pre-preg balso is the optimum core! (well better than foam)

IS this correct... or am I missing something..

(I did just sort of skim to the bottom)

http://www.goetzboats.com/resources_services/whitepaperpdfs/BalsavsFoam.pdf

What about things like core-cell compared to balsa with pre-preg ?

Thanks.

 

In most respects, good end-grain balsa is superior to any of the foam cores.

I wouldn't even consider foam core for use with prepreg. If you're going aerospace-style with the structural skins, you generally go aerospace-style for the core as well; that means honeycombs or balsa. It also means elaborate, time-consuming engineering analysis.

Many of the foam cores on the market are only suitable for light-duty non-structural parts. Only a handful are actually stable and durable enough to use in a boat hull. They're basically just a filler material to thicken (thus stiffen) a fibreglass panel. If you're putting a lot of effort into engineering the skins- as the use of prepreg implies- you may as well go with a better quality core, lest you leave a major weak link in the system.

 

Stiffness-wise, a balsa core and carbon skins (wet laminated or prepreg) are unequalled. Balsa has a nice shear modulus, and good compressive strength, which helps the skin to stay in column, when in compression.

Balsa is not used in aerospace (satellites), because of the continuous freeze-thaw cycles, but for high-tech boats, why not? Thin skins can be a factor, but for racing boats not too big an issue. And besides that, ABN 1 (Now Delta Lloyd) is racing the Volvo for the second time, with 3mm skin thickness (and honeycomb core...)

Balsa has a bad name due to rot-issues, but a well built boat with balsa core is something to be treasured. It is just that installing hardware requires more care than usual.

 

works for me if you follow the instructions of the manufacturer - then, no rot and no leaks of "venting" - mainly from the mouths of builders who cannot read the flippin instructions

Result is light robust and durable - see my link and ask Bob Oram, He will put you right...

 

When I used 3/4" balsa as a core, I'd create "hard spots" from oak where hardware would be attached.

 

Everything is glued - no screws or bolts - EVER - - - - for things that penetrate the hull cut a hole with a hole saw then put a hex-key in your electric-drill and remove a further half inch of balsa but leave the pre-glassed bit then epoxy up a bit of woven tape making a bit of a sausage shape and carefully and diligently insert that in the area you removed the balsa (and leave NO AIR BUBBLES) and then insert your through hull fitting (plastic/glass that will adhere to the epoxy) and let that set before adding some internal epoxy-wet-tape to further fix the item on the inside.... I haven't done that yet so I hope an expert comes along to clarify this

Chain-plates are made of glass/epoxy and each of the 6 will more than carry the weight of the entire boat (NO BOLTS, NO SCREWS - JUST GLUE "epoxy" and various compatible tapes with no air bubbles or holes that are not filled with tapes and stuff - this is all defined in my build instructions from Bob Oram - which I haven't got yet... Do it right and you have a light, strong and waterproof boat.... - If you must bolt something on - - bolt it to a piece of pre-glassed and sealed ply that will spread the load and any through holes are re-sealed so NO water can ever get in to the ply EVER, then glue that ply in place - easy to do when done right the first time, if not your boat will rot within 2 years, cause YOU did not do it right when you built it....

 

Balsa cored panels work very well if your designer knows how to do it properly, - - **** it up and you will kill someone sooner or later, - - because of your careless approach... Get advice from someone who builds and knows how to do it right first time, - - for there is no repair facility when you are mid ocean in a rotten boat that your negligence allowed to happen.....

I spent time looking carefully at several different builders work using Bob Oram designs before I committed, and I hope his son will be on the build team.... Learn, observe, understand why and PAY for competent expertise - not a ****** who says he knows but lacks multiple build experience....

 

Oh, one thing I forgot: To get optimum results, use weight selected balsa, in the range of 100 kg/m3 (sorry guys, imperial drives me crazy, but useless to discuss here). Unfortunately lead times on 100 kg/m3 balsa are about 12 weeks here, so planning ahead does not hurt...

 

Thanks guys.. very informative.... and confusing...

What if I adapted tinhorns approach and had honey-comb or core-cell in the places where I am placing winches and inhull fittings etc.....

How would that impact overall stiffness and performance?

 

Go and get a small sample of each from your supplier (4" square - just to see it), then you will understand honey-comb etc., - never on external surfaces.... - - ATL (Australian supplier), explained the reason to me, and your supplier should be able to advise you - otherwise they are too incompetent and know "sweet-f-all", to be considered a supplier....

 

No honeycomb under fittings or winches. Compression strength is way too low.

You could use Core-Cell in 200 kg/m3 or heavier under fittings, or alternatively use Airex T90 (in 240 or 320 kg/m3, and at half the price)

If the fitting is really heavy loaded, I would opt for a single skin laminate, and a generous backing plate, bedded in epoxy paste (so no point loading from uneven laminate thickness)