Epoxy precautions and Construction tips

[Bushwacker]

A couple of weeks ago Skip, Carla and I were fortunate enough to spend a couple of hours talking boat restoration with a neighbor of mine who is a structural engineer and world renown consultant on composite materials and boat construction with over 30 years in the business. He’s a real nice guy and has a very well trained black Lab who likes my Lab/greyhound mix, Shadow! I've gotten to know him over the last several months as we walk our dogs at about the same time every night.

He grew up on Thunderboat row in Miami during the 60's and 70's heyday of offshore powerboat racing and knew Don Arranow, Moesly and others in the business. Said he ran Donzi for a couple of years and has consulted for all the major boat builders as well as the US Navy; he even built a boat for the Navy out of carbon fiber! Said that turned out to be not a very good boat building material, as it's electrically conductive and has all sorts of electrolysis problems! He and a partner have just finished construction of a 2nd 90' Yacht-fisherman that he oversaw construction of in the Cheoy Lee yard in China. He said this boat is all composite construction so it’s much lighter than conventionally constructed boats. Even with a full load of fuel (5000 gallons!), it’s lighter than a 70’ Viking! It’s essentially the Ferrari or Porche of the sport fishing world!

He owned a SeaCraft 20 CC built in the early 80's by SeaCraft Industries and has a lot of respect for the innovative stuff that Moesly has done. When I told him about the old 21’ ex-raceboat that Skip and Carla are restoring, and mentioned the difficulties they were facing with installing the Seafari cap on it, he volunteered to advise them on it for free, so we took him up on it! We spent a couple hours aboard the new 90’ boat behind his house and came away with our heads seriously overloaded with information! Carla took lots of notes, so may be able to chime it with stuff I forgot, but here are a few of the highlights:

1. Safety: Epoxy is EXTREMELY TOXIC because the hardener contains many carcinogenic chemicals! (He speaks from experience because he’s been around it so long that he’s become extremely allergic to it!) It’s much worse than polyester or vinylester, even though it has much less odor, so you MUST take the following precautions:
a. Use NITRILE gloves, not latex, because the in the chemicals will go right through latex! (You can get them at Walmart for about same price as latex.)
b. Remove the gloves by pinching them and pulling them off, don’t stick a finger under the wrist-band to remove glove because it probably has resin on it that you’ll get on your skin!
c. Wear a respirator with the NIOSH approved filter cartridges, not just a dust mask.
2. When laminating fiberglass cloth with epoxy over old polyester, as in filling in a transom cutout, overlap the new cloth/epoxy at least 2-3” to insure a good bond.
3. Go from SMALL to LARGE when adding additional layers of cloth! Reason is that, although it creates a slight kink and stress concentration where the larger layer overlaps the smaller one, IF there is a bond failure between the epoxy and polyester, it will tend to STOP when it hits that step, i.e., it’s like a rip-stop. (We did just the opposite on the 21’s transom on the advice of an experienced fiberglass repairman, but he doesn’t have the depth of understanding that my neighbor has! Wish I had gotten to know him a couple of years ago!) He said these separations tend to be cleavage type fractures, with very high stress concentrations at the end of the crack which make it propagate. If you’ve gone from large to small in laying up the laminate, and there is a separation between the new layer and old polyester, the whole thing will tend to peel off! I had enough exposure to the science of fracture mechanics analysis in the jet engine development business to realize that what he says certainly does make sense.
4. Fiber orientation – for transoms, the strands should run 90 degrees to one another, because primary stresses on the transom tend to run in horizontal and vertical directions. When wrapping cloth around a corner or fillet, the strand orientation should be 45 degrees to get the maximum number of fibers crossing the corner.
5. He also says you don’t need mat with epoxy; it’s only needed with polyester. His preference is for stitched e-glass instead of something like 1708, which is 17 oz cloth and 8 oz mat. Said if you do use something like 1708, just make sure that it’s fully wetted out and that there are no white areas in the laminate.
6. For replacing a core in a transom, he prefers stringers that run all the way to the outer skin with the core cut-out to fit over the stringers. That way the stringers are trapped by the core and more rigidly attached. He also prefers 45-degree braces or knees from stringer to transom.
7. He also likes the idea of a horizontal beam or shelf about 1x4 or 1x6 across the inside of the transom, as it provides significant stiffness to resist the bending loads that exist in a transom.
8. He said to be careful about creating hardspots between hull and internal structures like bulkheads and cockpit sole; said cockpit sole should rest on a cleat that runs along the hull. Bulkheads should have a trapezoidal shaped piece between them and the hull sides to spread out load from bulkhead into hull.
9. He also prefers composite foam core material for decks & cockpit sole (he and a partner started a company that produces foam core material), and says it’s real easy to create a hatch in them – just set the saw depth so it doesn’t cut thru the lower layer, then flip it over and cut thru lower layer an inch or so inside of the top cut, which will create lower lip to support the hatch.

Carla, feel free to add anything I missed. Ihink I've only hit the high spots! Denny

[hermco]

6. For replacing a core in a transom, he prefers stringers that run all the way to the outer skin with the core cut-out to fit over the stringers. That way the stringers are trapped by the core and more rigidly attached. He also prefers 45-degree braces or knees from stringer to transom.
7. He also likes the idea of a horizontal beam or shelf about 1x4 or 1x6 across the inside of the transom, as it provides significant stiffness to resist the bending loads that exist in a transom.

Why would you do this when the original transom design did not have or need it?

[Bushwacker]

You have a good point Don, it's definitely overkill! Sometimes "better" is the enemy of "good enough"! Where he's coming from is extremely lightweight high powered boats that are stressed right to the limits of the material capability. The boats normally discussed on this site really don't fit that category, although that lightweight 90 footer he built with 4800 horsepower definitely does! Same goes for his concern about delamination between polyester and epoxy with the large-to-small vs. small-to-large laminate lay-up . . . given the bond strength of epoxy with proper prep, I think that concern is a nit and can't see that being a problem on Skip & Carla's boat.

I did think that his warning about toxicity of epoxy resin was worth sharing with this group however, and I now understand why you use mostly polyester or vinylester resin! He said you could probably swim in that stuff and not have a problem! Although I've used nitrile gloves for sometime because they seem to be tougher than latex, I had never heard about latex not being acceptable for epoxy. I also suspect that most guys don't wear respirators around epoxy when they probably do with polyester.

Next time I see him, I'm going to ask him what happens to that foam cored hull if he hits a floating hatch cover offshore at 30 kts! A SeaCraft, with it's heavy solid layup, would just bounce off of it, but not so sure about that fancy hi-tech hull.

[hermco]

Next time I see him, I'm going to ask him what happens to that foam cored hull if he hits a floating hatch cover offshore at 30 kts!

If the outer skin gets damage that allows the force of the water to hydraulicly delaminate it the hull will most likely fail. I have seen it happen on some cored hull boats.

[Islandtrader]

Bulkheads should have a trapezoidal shaped piece between them and the hull sides to spread out load from bulkhead into hull.

Since I am getting ready to finish up my floor...the tried and true method is to leave a space and put in a fillet and then glass hull, fillet, and floor.

I guess I don't see what a trapezoidal piece will offer

[Ryank]

As for the 1x4 or 1x6 The owner of american boat works in Tarpon, did this to mine calling it a "strongback" he said this adds 50+% strength to it. He sent me complete pics of the entire transom process. They even did this to large trawlers they have re done.

[Bushwacker]

I guess I don't see what a trapezoidal piece will offer

Hi Terry- my neighbor mentioned this when discussing bulkheads, which create a narrow and very stiff hard spot against the relatively flexible hull sides. The hull panel will tend to bend around that hard spot and create high tensile stress on outside of panel. A trapezoidal shaped piece between hull and bulkhead will create a gradual increase in stiffness as you get closer to the bulkhead, very much like a fillet, so it would reduce the stress concentration on out side of hull. However if the fillet is cabosil type material, it's probably harder and more brittle than the foam core material, so think I'd prefer the tapered foam in that case.

A cockpit floor is slightly different situation, especially if it's laid on top of a longitudinal cleat running along the side of the hull. The cleat will act more like a miniature stringer but won't be near as stiff relative to hull panel as a bulkhead. As long as there is a little space between floor and side of the hull with a large radius fillet between them, I'd think that would work fine. In either case what you're trying to do is transform a narrow very hard spot into a wider and slightly softer area so that any bending stresses around it are spread out and not concentrated in one spot. Tying the floor to the hull sides with glass across a generous fillet should do exactly that!
Denny

[gofastsandman]

I saw a 25 Hydrasport that hit a telephone pole on the way back from West End and delaminated 2/3rds of the side of the hull. If not for the Kevlar, it would have sunk.

30 years ago an old surfboard shaper told me to always go small to large when fixing dings, but I remember you saying the opposite when you guys were doing the transom on the 21. Sometimes old surfers know more than the experts. 6 oz cloth will tell you a lot too.
I had no idea on the epoxy, but I use poly. I`m still not convinced of the merits of epoxy. A boat is supposed to flex after all.
GFS