Just an update. The product I am looking at is called Urefil 15 Micro-balloons filler. The info says it can be used with Epoxy. 

No reason not to think so if the manufacturer states it’s for use with epoxy. Just be sure to add any fillers to the combined and well-mixed resin + hardener recipe you know to be proper for your building project. Might even want to make a small test batch first too, see how it mixes & hardens before venturing a large batch on a project step that might end up tricky to back away from if you should get caught by surprise.

   It just so happens that I did use micro balloons in the endpour of my Shearwater Sport, so plenty of similarities here.  I don't remember the brand name, but the microballoons worked out just fine.  I used quite a bit of filler in the mix, as the application is non-structural, and even then the stuff hardened up like a good hard, white, light plastic.  The finished stuff is quite a bit more brittle than unadulterated epoxy, and more brittle that cabosil or wood-flour filled epoxy.  I think microballons are suitable only for skim coats of epoxy used for fairing/filling a surfaced prior to sanding and painting (NOT for bright finish), and for things like end pours.  That's the only two applications I can think of - I wouldn't try to save weight anywhere else.  And in the first instance the application is really just to make the sanding easier (and maybe save some pennies by saving epoxy).  As noted in the earlier post, get a good mixing of raw epoxy done prior to mixing in the balloons.

I'm writing to pass along a lesson learned.  I did my first endpour on a hot (90+ degree) day in TX, boat leaning up to a convenient Y in a nice big oak tree in the yard.  I did expect a big cup full of epoxy to cook-off pretty warm, but hadn't fully anticipated the amount of heat generated.  It got so hot so as to "speckle" some areas of the glass cloth on the exterior boat finish.  I think the combination of very warm/hot softened epoxy, combined with very small amounts of residual air/gas withing the wood (but extra hot and thus expanded gas) shoved some miniscule gas delaminations out into the glass cloth.  It is an interesting look (think special race-car/jeweled metallic finish), but I would have avoided it if I knew it would happen. Now it isn't a blemish on the boat mind you, just a "feature."

So, for the second pour I stood the end of the boat in a bucket of cool water, and swished the water around a little with my hand during the 5-10 minutes of the hottest part of the cook-off.  That prevented the overheating/speckling.  

Sandy,

on micro balloons....while i am not a chemist, my perspective is that you should go with something that is explicitly identified to work with marine epoxies such as West Systems, MAS, etc.   they are going to typically cite Phenolic Microballoons.  the microballoons you are citing does not appear to be the same thing and its reference to epoxy does not appear to be marine epoxy.

my general rule for boats i am going to put a person in, is to stay with known marine-grade ingredients.

on buildling light, i have a bit of experience in this area and for a stitch and glue, there are not a lot of options to significantly improve the weight as advertized ....but lots of ways to get an overweight boat.  this is becuase the okoume plywood is relatively dense and heavy and a large percent of the overall weight....and you just can't do much in that type of construction to make that portion any lighter than it is (unless you swap out 4mm okoume for 3 mm).   so to get it 'on weight' you need to be very careful with your epoxy work.  for this type of boat,  after the 4mm okoume, the second heaviest ingredient  is going to be the epoxy. 

so the 'tricks' to get a light build in my book are as follows: 1)  diligence and care with the layup so that you have no more epoxy on the boat then is necessary to encapsulate the glass.  you can go with 4 oz cloth rather than 6 oz. in a lot of places which takes less epoxy for the glass work.  2) care with all the fillets...no bigger than they need to be; 3) endpours - do them with the deck off so you carefully control where the material is put and only have the end-pour covering the hole you put in the end for your toggles, so you do not create an opening into the hull.  the least weight endpours can be made with a piece of cedar or balsa wood epoxied into the corner.  4) use of fillers like microballoons on non-structural fairings. in a shearwater for example, the inside of the hatch structure gets faired into the inside of the hull...microballoons are great in this area.  5) just a lot of attention to clean work...particularly on the inside (like when you seam the boat) where it is easy to build up a lot of epoxy that is not necessary.  

i have built both the shearwater 17s and the Arctic Hawk....and they are beautiful boats and, if reasonably built, will be much lighter than almost any comparable commercially built boat of the same size.   that said, .i would consider hitting the design weight a very good accomplishment.

if you want to get into the 30 lb space or better for these boats....you need to adopt a different core material such as cedar.

i hope that helps

h

Thank You

H

I will stick to Phenolic Microballoons. I have just begun reading the Manual for the Artic Hawk Kit, and thinking ahead to maintain the strength and keep the weight closer to the specifications. While building the Chesapeake 17 LT, I did take every step to minimize extra weight that was recommended. I taped the seams prior to filleting, trying to keep my fillets down to 3/4" total. I used a roller to apply thin coats of Epoxy. My endpour before adding the deck started out really bad when my cardboard dam failed. I probably added extra weight there. I ended up elevating the opposite ends as high as possible in my 11'6 garage. I will take your advice and permanently Epoxy in a Balsa wood or Cedar dam, prior to attempting another Deckless Endpour.

As far as My Shearwater Sport goes, I bypassed the end pour, due to cold weather.I have completed both boats, with the exception of the end pour.

I plan to weigh both when completed. Prior to adding the seats and other rigging, as well as Varnish, they weighed approximately 43lbs each.

Already I am wondering about the method of building, compared to the Chesapeake, and the Shearwater sport.

All the parts are to be coated with Epoxy and cloth, prior to any assembly?

 Am I reading that correctly? I understood that Epoxy coating would make the panels more difficult to bend?

you are reading that correctly.  on the arctic hawk the insides are pre-coated and glassed in advance.  don't worry, they will bend just fine.

i do want to ensure you understood my point about a cedar or balsa end pours.   the purpose of the endpour is not really structural...there is plenty of structure in the ends.  so the functional purpose of the end pour is to allow you to drill a hole in the bow or stern for the rope for the toggles without  making hole that would allow water to get into the hull (because the hole goes through the end pours - not into the hull cavity).

so basically what i do is mark where i want the holes to be prior to putting the deck on and then put a little block of cedar in that area and then drill my hole. of course the balsa or cedar needs to be coated with epoxy. after putting it in place and when you drill the hole....you need to coat the hole with epoxy.  so i am not suggeting using the wood as a dam....but as a substitute for the traditional end pour.

there are a couple other techniques i wanted to mention for light epoxy work:

• use skim coats.  a skim coat is basically taking a squeegee and  little bit of epoxy and just quickly rubbing it across the surface of the okoume and letting it set  before doing your glass work.  its pretty amazing how much wood you can cover with very little epoxy...you are not trying to get it to soak in but just to wet the surface.   you will still be able to see the grain...it will not be shiny...but you can tell that you wiped epoxy across it.  the purpose of this is to plug the pores in the wood with a very tiny amount of epoxy.  when you do your heavier epoxy coat with the glass, now that you have blocked the pores, the wood will not absorb any more epoxy and the result is lighter than not using a skim coat and has the added advantage of helping prevent problems like outgassing and dry weave.
• don't fill the weave on glass inside the hull.   for glass inside the boat, while it needs to be fully wetted out, you don't need to fill the weave to create the same type of smooth surface you do on the outside of the boat.  this will save some epoxy weight...especially on boats with a lot of internal glass work in the fore and aft sections and under the deck (fwiw, i do fill the weave on the cockpit floor to make it easy to clean).
• use care on beveling the pieces of okoume so they fit tightly.  if they don't fit tightly, you will end up filling the gaps with epoxy...which is heavy.  if i have a seam like this that i can reach prior to glassing, i will fill it with a microballoon paste so that it does not fill with raw heavy epoxy.
• to the extent you use metal fittings, once you have fitted them, cut off any extra length in the bolts.  stainless steel is very heavy.  this has the added benefit of creating less 'snagging' hazards in the fore and aft storage areas.

the perspective i am getting at on the okoume-based boats...is to hit the target weight is about all the little details....there is no single thing that you can do that will make a big difference other than swapping 4mm for 3mm. 

based on my check on weights...btw...if you are at 43lbs without the rigging you are not off target weight by that much.  fwiw....a long time ago, at least on the chesapeakes, they did offer a lite build option with 3mm okoume. 

an interesting perspective on the role of okoume thickness in weight is looking at the shrike which is about the same length as a shearwater/arctic hawk....but if spec'd out with a build of 3mm okoume..http://cnckayaks.com/project/shrike/ ..they have a completed weights in the mid 30 lbs realm.

Thanks for clarifying the end pour. I will probably use that on my next build. It is too late for my Shearwater, so I will be adding phenolic micro balloon filler to my endpours.

I will also be referring to your tips for reducing weight, starting with my Artic Hawk build.  

I use a mixture of phenolic micro balloons and colloidal silica for just about everything and my boats are just fine. I think West System do a similar mix (I get mine in bulk locally). I sometimes use fibreglass powder but that's quite heavy - I used it to glue up the akas on my sailrig but it's nasty stuff to work with.