peel ply will make no difference in strength/abraison resistance.  it does not change the epoxy or its properties.  it just smooths the application (and in theory reduces the sanding required) and is totally out of the the equation when it is removed.

so that said, three coats are still three coats.

by three coats, what they are suggesting, is that becuase this is a high wear area, you don't want to sand back into the glass as you are building this part.  you want it well coated with epoxy so when you fair it you are really only taking off excess epoxy and still leaving plenty of epoxy/glass to take the wear.

peel ply, by smoothing, could help in some applications to reduce the number of coats of expoxy required to get a smooth finish.   but, since this is a high wear area, i would just do at least three coats if that was the recommendation.

Peel ply lets you fill the weave with no additional coats. This is great from a strength/weight point of view- you can achieve all the glass strength with half or a third the added epoxy weight.

Abrasion resistance requires a durable surface (glass is good for this) and thickness (from either more glass, or more coats of epoxy). So glass applied with peel ply offers a *slightly* less durable surface, since it is a thinner layer.

But, since you've saved a bunch of weight in areas where you aren't interested in abrasion, maybe there's room in the weight budget to add small layers of glass in the areas you know will receive extra abrasion....

fwiw, i thought i would comment that nemochad is spot-on regarding the concept of tailoring the approach to the particular issues a part encounters.

in the kayak building area....there are often several lay-up schedules that can be used by the builder based on expected usage - for example, a  light, mediium and 'expedition build'.

most of the difference in the layup schedules is not so much about absolute strength but various levels of abrasion protection (e.g., how tough a scratch you can take in the places likely to take an impact/abrading force prior to it getting into the structure).  so for example, an expedition layup will often have three layers of glass along the keel line vs a light build will only have one layer of glass along the keel line.  the topsides may have no difference in lay-up between the light and expedition build.

certain additives and special fibres can also be worked into high abrasion areas if you are expecting rough treatment against specific parts of the boat.  and you can limit and tailor these areas to only be where you need them so you don't pay any significant weight penalty.

in addition to extra glass which is the most common approach, additives like cabosil, wood flour and graphite change the abrasion properties of epoxy as well as other cloth types have unique properties that may be valuable in high abrasion areas.  for example, actually forming cabosil/woodflour rub strips or using dynel with graphite as a rubstrip can provide some pretty good protection if there are edges/areas that you are really worried about getting scraped. 

clc usually does a good job of specifying a reasonable build approach for average use.  and they have even changed recommended lay-ups given owner feedback. it might be interesting to try to get some perspective from other dory owners of 'wear and tear' experience and react accordingly.

Thanks guys for the great comments!  Very good information.Your points above about more than one coat of resin make sense, and the points about abrasion protection aligns with what Joey at CLC told me in general:  "Adding cloth to the rudder and or daggerboard is not necessary by design, but it will add a significant amount of ding resistance to both."

So, here's my plan...

Leading edges of the bow, skeg, rudder, and daggerboard will utilize this rope method (scroll down) for impact damage resistantance.  http://www.duckworksmagazine.com/07/howto/foiled/index.htm

I'll apply 6 oz cloth, resin, and peel ply to skeg, rudder, daggerboard, and maybe do peel ply on the bottom of the dory, then 2 more coats of epoxy, on all surfaces of these items, then prime and paint.

I'll check with some other dory builders, but if I do the rope trick, glass, and at least 3 coats of resin, I doubt I'll need dynel on those surfaces

Thanks!

Curt 830/997-8120

Wow Curt.  I almost feel like I'm underbuilding my dory but just doing the graphite mix on the final two coats of epoxy on the bottom, daggerboard and rudder.  

Not concerned with making her too heavy?

Hi Tim...how is your dory build going?

My main desire is to have high abrasion and impact resistance on the bow, skeg, daggerboard, rudder, and bottom.   The rope on the leading edge of the bow, daggerboard, skeg and rudder would likely add (guessing) 1-2 pounds and the cloth on the daggerboard, rudder, and skeg another 1-2 pounds.   With a working load capacity of 800 pounds, a few pounds doesnt concern me...but, that being said, the graphite is light weight, maybe I should add graphite as you are doing also. 

I've never used graphite, does putting graphite in the last 2 coats of epoxy provide more abrasion and impact protection than a layer of cloth? 

graphite as an additive tends to improve abrasion resistence.   not impact resistence.  think of 'abrasion' as dragging a hull across the sand/rocks...it's very much an oblique action.  think of 'impact' as hitting your hull flat on with a hammer.

the way graphite improves abrasion resistance is by making resultant epoxy graphite coating a low friction surface that is more likely to allow the abrading material to slide off and not actually 'grab' and scrape/damage the expoxy/graphite coating.

from west systems description:

423 Graphite Powder is a fine black powder that can be mixed with WEST SYSTEM epoxy to produce a low-friction exterior coating with increased scuff resistance and durability. Epoxy/graphite is commonly used as a bearing surface, and as a coating on rudders and centerboards, or on the bottoms of racing craft that are dry sailed.

the only other comment i will make on this is that it can also make the surface act like a big pencil (the graphite powder is the same thing that lead pencils are made of....just in powder form).  so it can leave nice marks where you might not want them if handled in an unexpected fashion.

for impact resistance, more cloth is the what you would typically do.

for areas that have impact and skuffing action, like the bow of a kayak, you will often combine the techniques with graphite/epoxy as the final coat (it provides no value underneath a layer of cloth.  that's what is the basic idea of a dynel rub-strip.

hope that helps

howard

Howard,  

Thanks for the great notes.  I'm thinking the rope epoxied onto the leading edges of the bow, daggerboard, rudder and skeg will provide good impact resistance but I get your points about abrasion not being the same thing.  That screech of a pointed rock scraping down the length of the boat coming into a gravel shore is where I hear you saying graphite does well...so, would it make sense to do graphite just on the bottom of the dory, and maybe 2" up the sides, leaving the rope to handle frontal impacts on the bow, daggerboard, rudder, and skeg?

my first advice is to avoid running over a pointed rock at speed.   the screeching sound you aluded to made my fingers curl.  a lot of what we are talking about are within certain margins....and boats, pointed rocks and screeching sounds are usually a signal of bad news to follow. (see story en.wikipedia.org/wiki/Costa_Concordia_disaster ‎) :)

with respect to your graphite question, you dont typically pay any material weight penalty i am aware of for using it.    think of it like printer ink powder.  it's incredibly light and is similar to working with cabosil or microballoons.

once you have decided to use it and wherever you use it, it is also like black paint.  so its now also a bit of an artistic decision as well as using it for whatever properties you are trying to get.

one thing folks like about it is it sands very easily (watch out for black dust) and is very easy to sand a scuff out of and match if you ever have to do a repair over it.  (its easy to match black on black).  also, becuase it is like a paint, you can use it cover areas where you applied a fairing compound or used any technique that you don't think looks great clear/bright.  (e.g., if you don't want to show your 'rope' leading edge on your blades). 

what strikes me as fairly common is folks use it by drawing a waterline on their craft and using this techniqe for all the areas below that line.

a final note on this is that you would want to probably buy the large size of this.  a 12 oz can is $18 and should be more than enough to handle coating the bottom.  the small bottle (2 oz) is $8.00