Builders' Forum Archives
Posted by LeeG on Aug 29, 2007
If you and your wife land on sharp surfaces with weighted boats having thicker layers of epoxy on the outside or inside really doesn't help much compared to having glass within that epoxy.
My first s&g kayak was a CLC Patuxent, the first time I ground over some rocks I was sure it went into the wood. Only went about 1/3 the way into the glass.
Built as instructed it had a layer of 6oz glass on the outside and two layers of 3" 9oz tape on the interior keel and chine with another strip of tape on the exterior keeline. With no interior glass it developed lengthwise cracks through a thick interior layer of epoxy. Those cracks didn't have a corresponding ding on the outside.
Subsequent kayak constructions with interior glass developed no lengthwise waterstaining cracks. On some kayaks that were made with only 4oz glass there could be a whitening of the interior hull glass where a hard impact occured on the outside but none of the random waterstaining cracks. Large flat panels of plywood will flex, without glass on one side it'll flex enough to crack the epoxy seal coat.
For example I built an eight panel Coho with 4oz s-glass with doubled layers on the bottom panels. After a few years of rough use there were numerous dings that dented the exterior and whitened the interior glass where there was only one layer of 4oz. on both sides. There were no lengthwise cracks anywhere. Where there was 8oz of glass on both sides there was no interior whitening but the outside had major dings and scrapes. An interesting example was an impact that occured right at the edge of the transition from 8oz to 4oz of glass. The whitening of the interior glass radiated out from the impact about an inch on the one layer of 4oz and about 1/4" on the doubled 4oz glass. That was a pretty clear message that I needed more than 4oz on the exterior and interior where the loaded kayak can hit something. Having just 6oz on the outside may provide lots of abrasion resistance but it won't keep the interior seal coat from cracking.
There's no way around it,,more glass is more durable. A panel of 4mm okoume glassed on one side is pretty tough for a vertical side panel but there's no reason to use it on the bottom of a loaded kayak.
My $.02 would be to order up three yards of 4oz s-glass from RAKA or Sweets composites. It comes in 50"-60" widths. You could glass the interior of the compartments as is comfortable to reach. It'll take some power sanding on those rippled tape edges. Then turn the kayak over and re-glass over the 6oz hull glass with a "football" piece of 4oz s-glass that goes from chine to chine and tapered to the ends. More of a diamond shape with the sides cut off. S-glass has a higher tensile strength than E-glass and is more abrasion resistant. 4oz s-glass isn't a sufficient straight replacement for 6oz e-glass in exterior applications but is noticably more durable than 4oz e-glass. Without glass in the aft compartment you'll be doing similar repairs in the future with the same kind of paddling.
In Response to: Re: Oops! by Eric Mattison on Aug 28, 2007