How Much Stronger is Epoxy really over Poly ?????

[HVAC Cruiser]

OK here is the scenario, I have been doing my entire restore using epoxy. On average I have been doing 3 layers of 1708 Bi-axle with epoxy and going through an obscene amount of epoxy.
The other day I was reading someones post on the great debate between Poly and Epoxy, someone wrote that seeing epoxy is stronger you can build lighter, which makes perfect sense. Yesterday doing my final layer of 1708 I went through a gallon of epoxy . Last week doing the final layers on the inside of my transom and a 12x30" repair on the core of my hull I went through 3 gallons.

I had bought another 3 gals and am now down to 2 gals to finish off both projects. I still have to finish the glass on the splashwell as well as the platform for the fuel tank on my 1st project and install the motor mount stringer on my second. I am now worried of running out again .

Im I doing a lot more layers than needed????? HELP!!!!

 

[oops!]

Re: How Much Stronger is Epoxy really over Poly ?????

welcome to the great debate !.....is it worth it !

if you are repairing a large hole in your hull.....3 layers of 1708 is correct...3 layers is as thick as your hull...

however...if your hull is still there....and you are covering it or your stringers or transom....3 layers of the big bi ax is awwwwwwwy too much......1 layer with poly is fine.....3 with epoxy ?

i think you may be going way overboard.

1708 really drinks resin.....huge...it is thirsty stuff.....you could also just go with a 17 bi ax material this does not have csm on the back of the woven cloth. the reason for the csm on the back is for poly resin, as the poly needs the csm to do away with the resin rich layer between the substraight and the material....epoxy does not need that.....so you can save on the csm.

 

[sprintst]

Re: How Much Stronger is Epoxy really over Poly ?????

I'm a fan of epoxy so I'm a bit biased. Here's a link going over todays poly resin compared to yesteryears and todays epoxy. The author also goes over weight issues from past builds.

http://www.epoxyworks.com/22/fiberglass.html

The second link is from the US Naval academy with an into on Marine Composites design and goes over their testing and potential savings, etc.

http://74.125.95.132/search?q=cache:bFZ78LRhHoQJ:www.usna.edu/Users/naome/phmiller/Session%2520404.ppt+epoxy+poly+resin+psi+comparison&cd=9&hl=en&ct=clnk&gl=ca

I used some excess of a batch I mixed to repair and friends tailgate ramp off his skidoo trailer. He races on lakes and has a studded track that eats the wooden ramp. I coated the area in thickened epoxy and near no damage since. The excess I let kick off on a mixing stick to form a blob. The guys was so impressed with the epoxy he tried beating the blob down like a hammer on his heavy steel welding bench...and it left dents in the table with no marks on the epoxy. Tough stuff..

You should be good to repel torpedo strikes

 

[HVAC Cruiser]

Re: How Much Stronger is Epoxy really over Poly ?????

Thanks Opps,

Drinks resin is an understatement, it drinks faster than I do beer lo, based on what you said I have been doing way to much which is what I kinda thought.

Question, how about when your splicing things back together? My one boat the splash-well was glassed in with the transom. I already have 1 layer of roven woven + 3 1708 on the inside of it. Now I am going to re-attach the well to it, you think I could get away with 1 layer of 1708 for the splice/tie-in?

EDIT: Now im going back to making my stuffed artichokes (waiting for the 1st leaf to fall off )for Easter dinner Yum Yum anyone wanting the recipe, that my grandma originally gave to me 40 yrs ago I will be posting it in the culinary zone

 

[oops!]

Re: How Much Stronger is Epoxy really over Poly ?????

yes.....easy you might want to add small tabs of 1708 on the stress areas...(underneath in the corners at the joint)...the splash well is a major structural part of the cap.

in the hull thread.....i took some wet bi ax cloth off the boat that i could not get to form to the shape....(instead of leaving a bad job and grinding it off...i just removed the wet piece and started over with a different plan)

after the wet fiberglass cured....i took that one piece of cloth and hit it with a hammer as hard as i could....it didn't even dent it.

remember.....fiberglass is as strong as steel.
what you are trying to do....is get a good bond between the old glass and the new....that's why we take the air out....air means no bond. since you are using epoxy...you are already twice as strong as poly in the bond.

 

[HVAC Cruiser]

Re: How Much Stronger is Epoxy really over Poly ?????

yes.....easy you might want to add small tabs of 1708 on the stress areas...(underneath in the corners at the joint)...the splash well is a major structural part of the cap.

Thanks Oops,
Excellent, you just saved me from overkill again . Today I will start with tabbing the 2 corners from underneath, then just do a 8" wide strip across the entire seam to tie it in.

I value your opinion, 2 last questions if I may? If you get a chance could you look at my transom turned rebuild thread? I am not sure if I should completely seal in the fuel cell bulkhead that I'm adding, or leave it short so air could circulate.

Last question is how long should I let the epoxy cure before I start priming?


Thanks,
Bill

 

[erikgreen]

Re: How Much Stronger is Epoxy really over Poly ?????

There are a lot of numbers for different properties of resins out there, but they all come down to "epoxy is stronger than poly". But that's actually not the main reason you should consider epoxy for a home boat project.

Here's a list of advantages epoxy has over poly for a boat project, from this doc: http://www.c-cyachts.com/images/com...91-785DC064755F/16cf4_Adv_of_Epoxy_v_Poly.pdf

• Better adhesive properties (the ability to bond to the reinforcement or core)
• Superior mechanical properties (particularly strength and stiffness)
• Improved resistance to fatigue and micro cracking
• Reduced degradation from water ingress (diminution of properties due to water penetration)
• Increased resistance to osmosis (surface degradation due to water permeability)

The biggest one for me is the first one... epoxy is a much, much better glue than poly resin. It's possible to screw up horribly on prep and application and still make a strong joint with epoxy, where poly would fall off. That's not to say you should skip prep to save work or something... epoxy simply has an "insurance" value to it.

Poly is great for a lot of things, I use it to cast new parts and for non structural items that don't need a ton of adhesion or strength.

It sounds like what you're doing is using too much resin - can you post pics of the repairs you made, along with any drips, etc and we'll see if we can tell what's happening? Tell us about your application technique too.

I have just about finished the interior structure of my boat, which is a 22 footer, using epoxy and 1708 glass. 1708 "drinks" about 2x as much epoxy as 1700 without the mat, and 4 as much as eg. 8 oz plain fabric. To do 2 stringers, two bulkheads and 10 deck supports including laminating each side, fillets, and tabbing them in place took me about 6 gallons of epoxy.

I could actually have used less resin if I'd taken time and been more careful, probably about two gallons less total.. but I had to re-do one part four times due to mis-cuts and sizing, some epoxy dripped off and puddled, and I could have saturated the fabric with less epoxy and more work.

Erik

 

[ondarvr]

Re: How Much Stronger is Epoxy really over Poly ?????

In most applications epoxy is considered to be about 1/3 stronger than polyester, but this is just a general statement because there are many different types and formulas used to make polyester and VE resin. This goes for epoxy also, not all of them are created equal, low cost epoxy, or epoxy that has been thinned with solvents, may be much weaker in every aspect than a good high quality product.

The problem is none of the companies that sell these products to the retail market actually make their own resin. They buy it in larger quantities, repackage it, put their label on, then markup the price. This means you don't really know what it is you're getting.

For the most part any epoxy will be stronger than a polyester, what you don't know is if the extra strength is needed.

When an engineer designs an item, then decides on the products to make it with, he looks at the stresses and loads the item will be exposed to, plus at the target cost and weight.

If the item needs to stand up to a 1,000 lb load at the bond line and polyester will provide 5,000 lbs and epoxy is at 8,000, then the additional 3,000 lbs is of no value, although it is much higher.

In a typical repair done on a transom and stringers like so many people on this site do, the difference in the end product will be very little, if any.

Here are few of the reasons.

The failure is typically not associated with the resin, its in the workmanship of the original build and care taken of the boat by the owner. Unsealed holes, thin glass, no glass, bare wood, no drains, poor designs, etc, and then being left outside uncovered with the drain plug in, all lead to rotten wood, it makes no difference which resin is used.

The prep work for either resin is the same, you should grind it with something around 24 or 36 grit and clean it. While epoxy may survive a little better if the prep work is poor, it still has a very good chance of failing.

It is possible to make the repair a little lighter with epoxy, but with the small amount of resin being used it won't add up to much when you're done and there will be no weight savings if you use the same amount of resin and glass as you would with polyester, which is what's done many times.

If the person uses good practices and eliminates the problems I listed above, the chance of it failing anytime in the near future have been drastically reduced and this is normally the case.

Poorly done repairs will fail again with either resin for the same reasons.

If you are looking to actually reduce weight and increase strength for specific reasons, as in putting on a much higher HP engine for serious speed, or making a very light kayak or canoe, then it may be worth it to use epoxy. Using it to repair a typical runabout like ones discussed here won't make a noticeable difference.

 

[erikgreen]

Re: How Much Stronger is Epoxy really over Poly ?????

Or if you're like me and are expecting to do gratuitously stupid things with your boat once it's finished and on the water, then epoxy is the way to go

But you can work with poly just fine.. many people have done full refits on their boats with it.

An alternative between the two is vinylester, which has strength and price between poly and epoxy. Ondarvr can comment more on that choice.

Erik

 

[HVAC Cruiser]

Re: How Much Stronger is Epoxy really over Poly ?????

Thanks Erik & On the River for your input, I was wondering if I was going overboard with my layers, if by using epoxy I could have cut back some. I rebuilt most of the boat out of roven woven and 1708.

Erik based on what you used I am probably right on the money with consumption, But based on what Ward said earlier I went way overboard with my layers for bulkheads, stringers and such. When I built the new deck I put in 4 extra bulkheads, built the deck out of 2 layers of 1/2" acx with a layer of 1708 in between gluing them. I then did a layer of 1708 on the bottom of the deck and used PB to glue it to the bulkheads. Topping it off is 4 layers of 1708 for the deck surface.

Here is a pic of the splashwell I just finished reattaching to the transom. The transom had already on the inside had roven + 1708, I just added one full sheet of 1708 overlapping the well then another 8" wide running the seam

Pre-fitting the 1708

All Glassed in

You guys think its enough, to much not enough? I think it should be enough, what do you think?

 

[HVAC Cruiser]

Re: How Much Stronger is Epoxy really over Poly ?????

Or if you're like me and are expecting to do gratuitously stupid things with your boat once it's finished and on the water, then epoxy is the way to go

But you can work with poly just fine.. many people have done full refits on their boats with it.

An alternative between the two is vinylester, which has strength and price between poly and epoxy. Ondarvr can comment more on that choice.

Erik

Hey Erik, My one project is almost done I did the entire thing in epoxy. for the entire project 22' CC I have used about 12 gal. That is new deck,bulkheads, repairing and re-glassing the entire subdeck , 2 new stringers and transom . I had to glass both sides, like a jerk I listened to someone before joining iboats and cut the outer leaving a 3" margin. The back corners of the boat where destroyed because it had bounced off the dock in a hurricane so I wound up re-glassing the entire stern of the boat.

Its just that I lately have been going through an obscene amount of epoxy and I had read I forget who posted it that you can build lighter using epoxy and wondered if I was really overdoing her

 

[erikgreen]

Re: How Much Stronger is Epoxy really over Poly ?????

Well, I can tell you that your deck is never, ever going to come apart.

To give you an idea: I built my last deck so it could stand the occasional scuba tank being dropped on it. I built it over-heavy for this reason alone.

The deck I built was a single layer of 3/4" plywood, covered top and bottom with 1 layer of 8 oz regular twill fabric and then glassed to the sides of the boat with 1708 tabs about 6 inches wide. I faired it a bit and added some 8 oz twill patches here and there, but that's about it. That deck never even got a dent in it. And I did drop tanks on it now and then.

From the sound of things your new deck is about twice as thick as a "normal" deck and has about twice as much glass as it needs to have. This isn't necessarily a bad thing, but as you've noticed it uses a lot of resin to build something like that.

The extra weight is something too.. probably it's about twice as heavy as it needed to be, a difference of maybe 300-400 or so lbs for a 22 footer, which might affect your performance some. But since (I presume) you glassed the deck to the sides of the hull, your boat is now far, far stronger than when it came from the factory.

By the way, when using epoxy you can glue between layers with thickened epoxy, and you don't need glass... actually glass between plywood layers doesn't get you much with epoxy. You need it for poly though, and in that case use mat only.

For the transom repair you posted pics of, I'd say it looks ok as far as thickness... maybe add a layer of light cloth before you fair to make that a smoother transition.

Erik