Pre-Preg and Balsa...

[AppleNation]

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_...alsavsFoam.pdf

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

Thanks.

[marshmat]

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.

[Herman]

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.

[masalai]

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...

[tinhorn]

Originally Posted by Herman

...installing hardware requires more care than usual.

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

[masalai]

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....

[masalai]

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 wanker who says he knows but lacks multiple build experience....

[Herman]

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...

[AppleNation]

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?

[masalai]

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....

[Herman]

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)