Friday, February 26, 2016

Bullet Proof Hulls

I am a follower and fan of Chris Morejohn and his Hogfish designs.  His boats are planked with three layers of 1/2 inch marine ply glued together with epoxy resin.  They are bullet proof by virtue of the heavy fiberglass layers that go on over that, consisting of 4 layers of 1.5 ounce chopped strand mat, followed by a layer of 10 oz fiberglass cloth.  They are set in polyester resin.  I believe that the mat is also fastened to the hull with ring shank nails.  The combination makes for a very tough hull.

I have considered this but the polyester just gives me the heebie jeebies.  It is not quite as waterproof as epoxy.  Water WILL migrate inward through polyester - that's why fiberglass boats blister. I would prefer to use epoxy but epoxy is not compatible with chopped strand mat.  It can wet it out eventually but the styrene soluble binder in it makes it difficult to absorb the epoxy.  Plus, it would use an enormous amount of epoxy.

Rueul Parker uses Xynole fabric and epoxy, using multiple layers in high wear areas.  This is purported to be a very impact resistant combination.  If I were to consider it I would have to import the Xnole as Defender Industries is the only supplier.  If I were to consider Dynel, another contender, I would need to import that as well.

An option for me, if I want to buy locally and supply/re-supply myself with short drives to Vancouver is to go with 10 oz fiberglass cloth and Aqua Set epoxy.  I'm thinking two layers of cloth, wet out together using the 'dry' method.  A square yard of double layer 10 oz cloth will require 20 oz of resin by weight.  I figured about 12 gallons to do my hull... Aqua Set comes in 15 gallon kits for just under $1800 Cdn.  E glass cloth is just under $11.00 a running yard for the 50 inch wide stuff.

So it won't be cheap any way I go, but after having re-visited copper cladding many times and ultimately rejecting it - for me - I am committed to using epoxy.  I want that water barrier.  But which cloth to use to get the impact and puncture resistance I want is still up in the air.


  1. Polyester is going to long outlive the rest of your vessel; what you might buy today is not what your father may have bought decades ago. Today's chemistry is different, and largely problem-free. Throw on a couple of hundred dollars worth of epoxy-based barrier coat after your polyester based shell before bottom paint and you're well set.

  2. Thanks for the comment Mimi! Jury is still out...

    1. I've only got three hundred [polyester] boats over two decades under my belt without a blister to be seen, so no biggie ;)

      Polyester has relatively poor tensile strength on its own, which is why laminations typically have traditionally had layers of mat between layers of woven fabric; modern chemistry and the tendency to use unwoven reinforcement largely eliminate the need. Epoxy has a much more favourable tensile strength and has always been happy without the intermediate layer. The result is that chopped strand mat is really only produced with polyester-friendly binders, which are the chemical issue... so use binder-free fabric.

      Besides that, a blister neither stops you from walking, nor might it stop you from sailing.

  3. A few thoughts:

    * Only as an alternative to consider, I planned to suggest what Mimi did, but she was quicker on the draw! ;-) Read something the other day during my seemingly endless hull building investigations about combining polyester and epoxy layers. Though I can't put my fingers on it at the moment, as I recall, the sequence was important -- polyester first, epoxy last. Apparently epoxy will adhere to poly fine, but not so the reverse. That needs confirmation beyond my statement.

    Also, maybe in your blog, someone recommended the spray-on black coating like in the beds of pickup trucks. I suspect it is what's generically called "polyurea". I contacted one of the several companies doing vehicles, etc. and got some info. Roughly speaking, for an area about 200-300 sq ft, if you want to stick with the basics (black; minor prep work; dry wood, etc), but still get a thick-ish coat (1/8"), in can be done in the U.S. for about $8-10 USD per sq ft. If it had previous layers of some resin/cloth, then it would likely need to be primed first.

    Not sure yet how that compares with epoxy in price, considering multiple resin/fiberglass layers, but the black gunk price might be favorable. And only about 1/4-1/3 the cost of copper. Important caveat: I have not progressed far enough along to have any technical specs or answers about that material under constant submersion. However, there is some evidence on the web of it being used on boat hulls.

    Couple of references (WARNING: Mute volume before watching # 2)
    1. Overview:
    2. Obnoxious but crudely effective example:

  4. They call that spray on stuff Rhino Liner around here. It is very expensive - as much as epoxy and two layers of 10 oz cloth. I would not try it on a boat as big as mine but as an experiment I may build a little dory and have it sprayed.

    My biggest concern - and this seems to be universally shared - is how the polyester sticks to plywood. I thought at one time I could put an interfacing epoxy coat on first, but it turns out that this would make it worse. The amine that comes off the epoxy inhibits the cure at the interface. Even though the polyester appears to be cured it is not where it meets the epoxy and delaminates over time.

    I'm still in epoxy mode so far...

    1. "They call that spray on stuff Rhino Liner around here. It is very expensive - as much as epoxy and two layers of 10 oz cloth. I would not try it on a boat as big as mine but as an experiment I may build a little dory and have it sprayed."

      An experiment sounds good. Getting a bunch of data from somewhere on existing hull applications sounds even better.

      From what I've seen, it might be much stronger than epoxy and two layers of fiberglass. I doubt those could stand up to a shotgun.

      Also, rumors only, but I've heard that polyurea coatings are often used on working boats in Alaska. Maybe some random Alaskan will wander by this post and help us confirm or deny that. ;-)

  5. One more thought: Fiberglass cloth choice

    While investigating the differences between fiberglass and carbon fiber, I came across something interesting to me about differences between different fiberglass cloths: E-cloth (standard woven cloth) and S-Cloth. No high quality references to pass on this time unfortunately.

    In any case, estimates seem to vary, but with S-cloth over E-cloth, I encountered information claiming 50%-200% tensile strength increases, but with a similarly low tensile modulus (flexibility). S-cloth, though about 2-3x more expensive than E-cloth, seemed to offer some of the benefits of carbon fiber without most of the cost of CF. Might be worth investigating.

    In terms of sources, one stuck out as being very informative with excellent prices, at least compared with West Systems: U.S. Composites*. They do ship, but in your case, I'm not sure how much the shipping cost would negate the lower prices.

    There are at least three FAQs on their website: General, Epoxy, and Foam. Ironically, still trying to find the one for fiberglass cloth.

    (*No, not owned by my cousin ;-)

  6. I can get 6 oz S cloth locally and yes it is very expensive. I'm presently researching the advantages in my case. Thanks Yoda...

    1. U.S. Composites: 6oz S-cloth is USD $7.80 per yard (27" width) or $8.10/yard for 30" width. 8.9oz S-cloth is USD $14.25 for 38" width. Both are cheaper progressively starting at 10 yards. Not sure how that compares.

      Speaking of S-cloth, I did finally find a few comparative strength references, though dated:

      * Sheer Deformation: Old NASA test document shows S-glass to be the most resistant to sheer deformation:


      * Comparison----Tensile Strength---/---Modulus of Elasticity
      - E Glass----------------2.3 GPa-------/----------74 GPa
      - S Glass----------------3.9 GPa-------/----------87 GPa


      Fiber Selection - S-Glass:


      Comparison Chart:

  7. Once used pressure treated 1X4 (dbled) as a external chine on a boat. The cloth and epoxy stuck well to the plywood but delaminated after about a week in the water on the pressure treated. If the chine had been internal there would have been no problem. I would caution a pre-test for whatever woods you are using at the chine.... and elsewhere for that matter. Also, as I recall, Chris MJ reported NEVER having problems with his bottoms delaminating, absorbing water, etc and he commented that he routinely sat down on gnarly bottoms with no problems. This is on an extremely well braced sharpie hull with probably little flex to begin with. Daves trilo bottoms are meant to flex quite a bit as part of their strength regimen so tough to say if a large, relatively unsupported flat bottom might flex too much with Chris MJs thick poly bottom treatment and eventually crack. Daves robust metal bottoms aid in this regard big time. I suspect though that a Morejohn hefty bottom would forgive a lot of piloting errors :) And I mean a SUPER Morejohn bottom on a flattie. Here's a relative mindblower to end this comment: Bolger experimented with a series of thick steel plates (1/2" as I recall) added to the plywood bottom on his second generation AS39. Drilled holes in the plate on about 1' centers and bolts up through the bottom with 5200 donuts on each bolt and washer facing. The builders actually did this but I never read a report on how it worked out long term. Such a armor plated bottom would ground out on probably just about anything. If I went this way I'd use roofing tar as a well slathered on bedding. Lots of options................

  8. Well thanks to everyone for contributing and giving this some thought! I'm doing my best to keep an open mind about things here.

    What I am thinking - if I were to go with a polyester layup - is that I would take Mimi's advice and apply a barrier coat of epoxy prior to bottom paint. I know that it is SOP for a lot of fiberglass boats out there and that it works well.

    So my layup might go like this: Two layers of 1.5 oz mat and polyester resin, and then bronze ring shank nails through those two cured layers on down through to pick up all three layers of ply. So they would be 1 1/2" nails. I would space them on say an eight inch grid. Then glass on two more layers of CSM and a final layer of 10 oz E glass cloth. I would fair that up with polyester based filler, and then apply the epoxy barrier coat. Bottom paint and Bob's yer uncle.

    What do you think? BTW, I haven't any contact info for Chris Morejohn - I would like to ask exactly how he did it with his boats. I'm thinking that nailing through a couple of cured layers is a good idea because the grip on the laminate would be better than nailing the dry mat. Resin would not penetrate under the nail... does anyone have any light to shine on this?

    I think that the hull I've built might be somewhat stiffer than Dave's because I have a longitude stringers every two feet...they are 2 by 4 structural Douglas Fir doubled at the chines - and they will be picked up by all of the interior furniture to boot.

    1. Chris M. is out sailing right now apparently but is really good about replying and a very nice guy. E-mail contact is listed on his site. spankthemermaid at something as I recall.

    2. Thanks Robert! I have found it and have sent him an email.

  9. Just mulling here ...

    If you nailed only the first layer of mat and poly, would the rest of the poly layers adhere sufficiently? If so, one less layer would be penetrated by the nails, and so could better contribute to watertight seal. SOP or not, you could even put some type of thin compatible washers on the nails to give them larger surface area contact and so holding power. It would all be covered up with layers anyway.


    So, it seems your hull would go like this:
    * 3 layers of 1/2" plywood
    * 4 layers of fiberglass and polyurethane
    * 1 layer of fiberglass and epoxy

    I will plainly state that I don't know how much is enough or what is customary. However, for a wooden box boat, which is basically a big floating girder with many nested smaller girders, allowing the wood to provide the strength, and the fiberglass the watertight seal, is 5 layers of fiberglass necessary or typical? Strikes me as a lot, but maybe not.

    I'm puzzling along the lines of necessity first, then budget, time, effort, etc. Plan to apply some of these ideas myself. On a similar hull, I was hoping to get away with one or two layers of fiberglass cloth & epoxy.

    I'll wait and see what those with more experience have to say.

    1. 3 layers of ply
      4 layers CSM set in vinyl ester resin (somewhat superior to polyester)
      1 layer 10 oz cloth set in vinyl ester
      Barrier coat of epoxy resin only - no cloth

      The thickness of the layup is not for strength. It is strictly for bottom protection.

      If I put nails through the first two layers there will be not problem with the subsequent layers bonding - both are vinyl ester resin and are compatible.

      BTW I have some experience with vinyl ester. It is very similar to polyester but is stronger. San Juan swimming pools (I've installed a few now) are made with vinyl ester resin instead of polyester. Chris Morejohn has used polyester in some boats and vinyl ester in others I think.

    2. This comment has been removed by the author.

    3. With Chris Morejohn's permission, here is the text of his email to me:

      Your boat sounds great.
      My suggestion and advice is this,
      Do everything as you describe except the epoxy barrier coat.
      If your fiberglass coating was all done in epoxy then fine.
      Using vinylester resin, then fairing your hull with this resin using glass balloons or Q-cell type fillers will be enough. I normally fair the hull and then just roll on a light coat of the resin I'am using.
      If you use an epoxy coating it will need to be applied a long time after the vinylester has finished gassing off. If applied too early it will bubble down the road after you have launched your vessel.
      I strongly discourage mixing resins.
      Epoxy is highly overrated as a barrier coat in my experience. I have epoxy coatings on my daggerboard and rudder that were given to me by the manufacturers reps for free to see how they performed. Since launching in 1999 they are to only things on my outer skin that have faired in blotches and chipping. Everything else is as new.
      I put on a first coat of hard bottom paint and then have been using ablative paint since with no build up. But I can scrub my bottoms when needed as I cruise in warm waters mostly.
      Try and find a book by Alan Viataes on boat building in his shop in the 50-80s. It's where I got the idea to nail my skins to the hull.
      I hope this helps and don't hesitate to talk.
      I wish you well, Chris Morejohn

    4. And some further advice:

      Here goes,
      I divide the boat in half using the centerline as the middle.
      I then measure off starting at the stern sheer going up to 3" over the chine onto the bottom using 60" wide material.
      This means the measured Cloth will be 60" wide x the measurement to the top of the chine in 3".
      This also means as some parts of your sheer and chine will have rocker you will have two different lengths.
      With this info you can pre cut all your cloth.
      I use a felt pen to number each section and place an arrow sign to show which direction it needs to be laid out.
      With all these pieces cut out and labeled and placed in order you can then start glassing.
      Of course you would need a dry shop setup and the room to have this amount of cloth laying around.
      With the bottom it's the same thing.
      I like a 3" lap starting at hull side going up over the bottom to the centerline.
      Now to start glassing on doing it alone I use 1 gallon buckets with maybe 1/2 gal of catalyzed resin in it at a time. I use a 4" short nap roller cover on a standard carriage using a short handle to reach across the cloth.
      I start by rolling on the resin on the bottom of the boat first. I do one section at a time. So I would pour out enough resin to cover 1/3 rd or so of the area. I then carefully place the pre rolled out pre cut cloth in place with its number and arrow in place to show me I have the right piece. I then roll it out far enough over the already pre wetted out area. From here I start applying resin to it.
      Once it is covered I can then either roll out with a hard roller and then continue to the rest of the area.
      What really happens is I roll out the whole area with resin and then roll my next layer on top and then wet out on top. Because the bottom layer is already wet it takes less resin. Once my whole layer sequence is done I then carefully hard roll out the entire area.
      Now when starting the next section I then wet out as before and then carefully align the next pre rolled up and numbered piece and align it so it is butting up against the previous wet area.
      When you have wet this area out and start rolling it out with a hard roller the two sections will meld together with no hump or overlap to have to grind off latter.
      With this system you have control over your resin and cloth as it cures. If something happens you can quickly stop and start with just the edge left to finish.
      I have done all my boats in temperate climates in the summer and in coltish weather.
      I will assume you will have nice cool temps so will have lots of pot life.
      If everything is precut then it all goes along smoothly. If not its a nightmare.
      With the size of your project I would dived the project up into quarters or maybe more like doing the bottom first then the sides.
      Once the whole boat is glassed I then grind all the bumps off and the the skin ready for the fairing putty process.
      This is another process that I can explain in my next email.
      I'am in St. Martin today to race in the Hieneken Regatta.
      I hope this helps.
      In a few days I could draw out and send you a sketch.

  10. I made up my mind this morning... I am going with the Chris Morejohn method. To summarize, I will fill and fair first with vinyl ester resin and micro balloons or similar thickener, then I will laminate on 2 layers of 1.5 oz chopped strand mat. Then, I will use 1-1/2" silicon bronze ring nails on an eight inch grid to nail through those two laminations and the two underlying sheets of 1/2 " ply, and on into the third. The ends should not penetrate to the inside...

    After that two more layers of the mat and a final layer of 10 oz cloth. I will then fair with the resin/thickener mixture and as Chris suggests, roll on a thin layer of resin. Bottom paint after that and she's ready to flip.

  11. That should be plenty. After all, we're talking about a DIY plywood box barge. How fair does the bottom really need to be? America's Cup fair or practical fair? I'll be going with the latter.

    1. For sure. It boils down to personal aesthetics. But it WILL give me an opportunity to get my chops back for when I do the top sides. I want that looking nice :-)