Fabric Type

Years back 6 oz cloth was the norm. A while back the norm became 4 oz. I have both on a bunch of strippers. The 4 oz holds up fine and I beat on my kayaks. So if you haven't purchased your cloth yet, I would strongly look into 4 oz. Less epoxy needed for wet out, and its easier to wet out all equals less weight.  I use a double football shape on the bottom.  I have never found any need for extra reinforcement around the cockpit opening. Others can chime in, but what I do is fairly typical.

If you bought 6 oz, use it.

I like 4oz S-Glass. I do a single layer inside and out, no football layer, no cockpit reinforcement. I do put in a bulkhead behind the seat though, and  a forward bulkhead too. Many first timers underestimate the strength of these boats and over build them.

Et.

You may pick up some useful info from my photo galleries...

http://etiennemuller.com

I'm with Jay, anything heavier than 4-Oz is not necessary. The area around the coming is generally well stabilized and strong. Take it from me, I'm 220-lbs and have sat on my coaming many times...no damage done. In fact, I never consider using 6 oz at all because it's overkill and adds weight due to the need for more epoxy (weight increase). I will use 6-oz strips to reinforce any chines (S&G) but otherwise I use only 4-oz. I've built 6 S&G kayaks so far, never had a problem.

Robert N Pruden

Hey, Bug, yes, I used a single later of 4 oz E-glass, inside and out with thickened epoxy fillets on all inside seams. I did this for 4 S&G kayaks ranging in length from 16' to 18'. My expedition kayak, a 17' Waters Dancing Lightning 17, was done with 6 oz E-glass with brass keel strips, came in at 52 pounds. My 17' S&G Night Heron, with a single layer of 4-oz E-glass inside and out, came in at 32 pounds. All kayaks are strong and handle well. Heavier weight glass means heavier finish weight due to the increased need for epoxy. Now, I abuse my expedition kayak, use it for running through and over ice floes during the cold season. I do not abuse my Night Heron, have used it a lot without damage to the bottom. All of my kayaks have a graphite/epoxy antiabrasive coating on the bottom because of the gravelly bottom of the lakes and rivers I paddle on. I choose a little extra weight to protect the hull from abrasion. I can sit on the deck of my boats without risk of cracking the structure and I'm about 220-lbs. Hope this helps.

Oh yah, you can add protective keel strips afterward if you think it's necessary. Know that running the keel over gravel will chew the keel up over time, sand is not as bad for the keel. Nothing will stop rocks from breaking through the hull with a hard landing, say if the boat rides on a wave and drops the hull down hard on a rock, been there, done that...but the antiabrasion coating will save the hull from modest scraping from gravel and sand. If you're paddling on the ocean, watch out for barnacles on the rocks, their worse than the edges of ice pans for chewing through glass and wood

Robert N Pruden (without the T)

Just jumping in with a few quick ideas/comments:

Glass sources- I've had good luck ordering (from Canada) with CST (TheCompositeStore) but you need to pay attention to the specs and make sure you get a 'soft drape' fabric. I bought a bunch of glass that was too stiff - designed for the circuit board manufacturers or making flat panels.

Thayercraft is another place that has a big selection and quite good variety, but I've never bought from them. I've heard (?) of people getting glass from MAS - the epoxy company.

WEST (Gougeon) graphite powder is used as an additive for epoxy by quite a few people. It's charcoal-grey, not jet black. Another black additive is black iron oxide which you can get from pottery supply places or eBay. You can combine those with fumed silica (Cabosil anti-sag) for a harder finish.  I've used all of those;I'm not convinced that any of them do a lot more than straight epoxy, and there are some tests online which tend to show that.

http://www.canoetripping.net/forums/forum/paddlecraft-construction/29052-abrasion-resistance-in-epoxy

If you want 'blacker' colour, use SystemThree black epoxy pigment. If you live in a hot climate, beware of black hulls getting too hot in the sun  during transport/storage.

For clear-finished boats, usually the black hull coating (or paint) comes up to the waterline. On a lot of S&G boats, the waterline isn't defined by the hull panels, so you'll need to get advice on how to locate it. On strip boats the waterline is usually shown on the plans, and you can run a contrasting colour wood strip along that line to use later.

For 'normal' abrasion (not running on to rocks while paddling, just abrasion from pulling the boat up on the beach) the aft couple of feet along the keel line (depending on the rocker) gets the most wear. Extra glass there (or Dynel if painting the hull) would be a good idea. Some folks also use KeelEzy tape along the keel, but I find scratches all across the hull on most boats I see.

Greg, take a look at the images here, one shows the graphite coating intact, the other shows the gray area where I sanded it off. These should give you an idea of where to put it. I can't remember exactly how much graphite powder I added to the epoxy but it would have been enough to create a completely black, non-transparent coating. I usually mix the two and keep adding the graphite powder until it looks black on the stir stick very black. I would say start with about 4 oz of epoxy and add two heaping tablespoons of powder to start. If you apply it to the kayak and it looks translucent, add more powder to the next batch. I will generally add two to three layers of this coating to the bottom. If you want a white bottom, use hexaboron nitride (HBN), it performs well as an antiabrasive but it's harder to mix into the epoxy due to lumping of the HBN...if it goes on lumpy the final finish will not be uniform in color. 

Robert N Pruden

The black bottomed image is for the Night Heron (center) and the other image I posted is for my Lightning 17 with graphite coating sanded off. The gray shading on the Lightning is where the graphite coating used to be. I will replace the graphite coating exactly where it used to be once it gets warm again.

Epoxy is quite hard when fully cured (depending on the brand).

For the gentle usage you are planning, I'd just use epoxy without additives. When it gets scratched, do a repair. You could paddle a wood core boat for weeks, day after day, on a trip with the outer glass scratched down to the wood, and not have any leak problems. Dry it out later and repair - it's been done before.

When builders have done actual tests (dragging samples over concrete and rocks) the various additives haven't made a great difference, in the examples I've read. Adding graphite to epoxy was first developed by the Gougeon team (WEST Epoxy) for making low speed bearings for things like rudder shafts, IIRC. From there, people who had only seen graphite used as a lubricant (lock lubricant, etc) started using it to 'reduce friction' on hulls. That gradually morphed into 'anti-abrasion' .....

John, you are causing me to rethink the antiabrasive concept. That said, adding Cabosil to the epoxy does make the epoxy finish much harder...and a bitch to sand...what are your thoughts on using Cabosil to make the antiabrasive characteristics better?

Robert (always deferring to his elders on a)N(tiabrasive coatings) Pruden

Robert-

I agree that CabOSil (fumed silica a.k.a. antisag) is a 'pain' to sand smooth.

I've used it in the fill coats on hulls (I paint my boats, so clarity isn't an issue).

I'll try to remember to take some pics of scratches on the bottom of my boat(s) - I have a few 'good ones' from barnacles. It looks like the CabOSil epoxy fill just chipped off the glass along the scratch. If it were just wear from pulling up on sandy beaches, the CabOSil might have reduced that.

Here in BC many of the 'beaches' are UK style - rocks and pebbles. If the rocks are covered in barnacles, they will gouge the hull - especially if the boat is loaded. Most of the scratches I get on the boat are from paddling full-speed and sliding up on a rock - tide levels are tricky!

If you are concerned about wear from launching and landing, the simplest 'reinforcement' would be a second layer of glass on the bottom - commonly called 'the football'. 

This would wet out clear.

Any additives (or Dynel) will not give you the clear 'wood-look' finish you want, so you don't have a lot of options.

If, after a few years, you find that the bottom is a mass of scratches and repairs that re-varnishing won't cover up, you can always paint the bottom or try some of the 'additives'. 

Most strip boats can keep going through years of tough use with just periodic touch-ups where the epoxy has been scratched, and varnishing.

Joel White told me years ago, when I was building a sailboat: "John, it's not a grand piano. You are going to toss it in the ocean, so just keep everything to a good workmanlike standard and it will turn out fine. If you get too particular, you'll never get the boat finished."

...where 'bouts?

I'm in the Victoria, BC area - great paddling country (all year round..)!

Ever notice that most glass kayaks have white hulls. Scratches are white.

There is no secret substance out there that is scratch proof. Epoxy is very hard and the best you can do. If you ever do a kayak repair on an older boat, the sander is laboring away to sand it away. Epoxy gets harder over time. Besides, scratches are under the boat. You don't see them. You'll know if you really scrape over some rocks and then you just add some epoxy on the scratch and you're back in business. A normally built stripper can take a lot of abuse. Obviously you don't drag it across rocks when launching which is just common sense. If you use cabosil with epoxy, the only advantage is that you might build up a thicker epoxy layer. But don't bother with that.

I might be on the Island come late summer, John, if I get to Victoria, I'll let you know.

Robert N Pruden

I thought about your comment, John, and then I thought about the 18 years that I have roughly handled my expedition kayak, the VJ. What I have noticed with a graphite/epoxy mixture, as I mix it, is that every time I have had to sand it off the hull to do repairs, I have had a bitch of a time removing it, even with 40-grit on an ROS at high speed. Plain epoxy sands off easily by comparison, so I am left to believe that the anti-abrasive qualities of this mixture is better than a slight improvement over straight epoxy. In my humble estimation, it is worth while for me to add this mix to the hull of my boats. As I stated before, it won't stop a crack or hole from forming after a hard impact but I have never worried about paddling or dragging my kayak across gravel, rock, sand, or a mixture of each, even fully loaded...I've never been able to gouge the hull right through the coating...and my friends know, I ain't gentle with my expedition kayak. Note that the VJ has three coats of epoxy top and bottom: the top has the wet out layer and two more to fill the weave and remove major defects to the final finish. The hull has the wet out layer and then two layers of graphite/epoxy where contact with abrasive forces could occur. 

Robert N Pruden

Just to add my experience, on my boats which are going to be used hard I add 10% by weight of Silicon Carbide to the last fill coat on the football area, and the result is very definitely much more abrasion resistant - like-for like comparison with a friends boat, which doesn't have the additive, has made this really obvious. But it turns the epoxy a translucent grey colour - this is why my friend didn't add it.

You can also add stone dust, which is not quite as good for ant-abrasion, but only gives a  very slight "milky" finish to the epoxy, which is not at all obvious - you can still clearly see the wood grain under it.

YMMV

Justin

If you have someone local making artificial granite counter tops, just ask them politely for a couple of handfuls, they are usually happy to oblige :) Otherwise you can buy it for example on Amazon .- look for quartz powder or quartz fines

https://www.amazon.com/Eastchem-silicon-powder-ingredients-dioxide/dp/B…

Short question of fabric for a light Spray by Thomasson design for flat water? 

160g/m^2 (4,7oz) or 110gsm? 

Will I loose to much strength by using 110gsm? Would if be a good compromise to use 160gsm for the outer and 110gsm for the inner layer? 

I will save about 1kg of fiberglass + epoxy if I use 110gsm on the whole kayak. 

If you want the boat to be light, you'll probably be OK with 110 g/sq meter cloth (3.2 oz) throughout.

If you make it light, you'll have to treat the Spray like a commercial surfski - delicately!