​Composite boats, gel coat and spider cracks?

[Guest]

Most of my gel coated boats have spider cracks, but I recently had a curiosity return to the shop after a long absence, a 1991 kevlar Mad River Explorer. Its last 10+ years have seen hard and frequent use, mostly as a rocky river poling boat, enough to have worn through/scraped off much of the E-glass tape I epoxied along the scraped shallow vee years ago.

This ‘91 Explorer has plenty of rock scratches, but the gel coat has few if any spider cracks. OK, I just inspected it, no spider cracks. Zero. What the hell?

I have a similar vintage fiberglass Independence, and other gel coated boats from the 80’s and 90’s, and nearly all of them have spider crack damage, especially along the chines and stems. I know this canoe has not been babied. How is it possible that the gel coat on this boat remains so completely intact and spider crack free?

Seemingly plausible answers (whadda I know):

The stiffer hulls I own definitely have more spider cracks in the gel coat. The sides on this Explorer are flexibly thin, and the bottom a little flexible if I push hard on it (it doesn’t oil can though). The chines seem quite stiff. Maybe some hull flex is actually impact beneficial, especially with thinner gel coat?

The obviously much thicker gel coat on some boats, even very stiff, stoutly built boats, definitely developed more spider cracks. Maybe thin gel coat is better than thick? And weighs less, this ’91 Explorer was speced at 54lbs factory. I presume it is harder to get thinner gel coat done right when building a boat?

This Explorer has rarely been used as a gear-laden tripping canoe. Perhaps that lightly loaded guise helps reduce impact or stress damage?

Or is this was just one of those happy hulls on which everything somehow came together perfectly? How does that happen; just the ideal mix of layup schedule, materials, construction timing and attentive craftsmanship?

I have always though the old Vermont Mad River canoes were exceptionally well made, but for a 26 year old canoe that I picked up as a rotted gunwale derelict 15 years ago this kevlar Explorer is in absurdly good condition. Even with me driving it home an hour on the highway, flopping around near gunwale-less on the racks.

What the hell?

 

[Alan Gage]

The only gel coated canoes I've owned have been two from Bell, a Magic in Kevlight and a Northstar in Black Gold. I didn't keep the Northstar long so can't commend on it but the Magic would spider crack the gel coat if you even looked at it funny. Interesting that you say your stiffer boats have more spider cracks than the flexible hulls as I always heard the reason for spider cracks was the combination of inflexible gel coat and flexible hull.

That seemed to be the case on my Magic and most of the spider cracks were on the more flexible parts of the hull (not so much the stiff bottom). Small point loads seemed to be the worst, even if they were a light hit. A couple times I only lightly bumped into the tip of a pointy rock, like hardly even enough to notice, but it would develop a spider crack around that spot.

I think you might be onto something with the thickness of the gel coat. It would make sense that a thinner gel coat would be less prone to cracking. Isn't a "skin coat" essentially a very thin gel coat?

Alan

 

[pblanc]

Skin-coated boats only have a layer of resin painted or sprayed into the mold prior to the outside layer of cloth.

 

[yellowcanoe]

I have a few gel coated boats. One is a Swift from 1993. The others of that vintage a Curtis Nomad from 1987. A Rapid Fire from more recently.

Neither of the first two have any spider cracks. The Rapid Fire has one and that is fixed. It was from impact. It is a more flexible hull than say my gel coated Peregrine from the same year.( which has no spider cracks)

I prefer gel coated boats for abrasion resistance. I don't find that spider cracks are very common in gel coated boats but then most of my tripping is not on rivers where you might get by sliding over rocks and deflect the boat.

 

[Glenn MacGrady]

Skin-coated boats only have a layer of resin painted or sprayed into the mold prior to the outside layer of cloth.

And that resin coat can be tinted. An experienced skin coat builder can make a tinted outside pretty darn good looking, though never as shiny as a gel coat. Skin coat saves a lot of weight over gel coat.

I have several gel coated canoes, some over 30 years old, and I've never had any problem with spider cracks except when I've damaged the gel coat itself with hard impacts -- once by dropping my outrigger on a concrete dock and once when attacked by an alligator.

 

[sailsman63]

NovaCraft has a video about their 'Tuff Stuff' layup (Basalt/Inegra hybrid fiber) Though the gel-coat was not the focus, it was pretty obvious that gel coat is much harder and more brittle than their standard layup resin. (Not sure what family of resin they are using) I would definitely buy the idea that thin gel coat may be able to flex with the underlying hull a little better.

As a side note, Glen, how big a 'gator does it take to think that a canoe is edible?

 

[Guest]

I think you might be onto something with the thickness of the gel coat. It would make sense that a thinner gel coat would be less prone to cracking. Isn't a "skin coat" essentially a very thin gel coat?

Skin-coated boats only have a layer of resin painted or sprayed into the mold prior to the outside layer of cloth.

Observationally none of our skin-coat boats (mostly glass & nylon) have spider cracks, nor have any of the worn and abused canoes skin-coat hulls I have worked on, so there is some crack-able difference between thin skin coat and gel coated hulls.

I still have no guess to the causes. Again, observationally:

Our fiberglass boats with gel coat all have spider cracks, the gel coated kevlar hulls, not as many, but some. We don’t own any carbon canoes.

Our stiffer boats, at least those with seriously thick 70’s and 80’s gel coat, have more (and deeper) gel coat cracks. Those are old boats that have seen a lot of hard tripping use and abuse, and most of them came used with existing spider cracks.

Our tripping hulls show more damage than our day paddling boats (wraps and pins excluded). That damage difference seems only logical, bouncing off a rock with a loaded boat vs a near empty hull.

And that resin coat can be tinted. An experienced skin coat builder can make a tinted outside pretty darn good looking, though never as shiny as a gel coat. Skin coat saves a lot of weight over gel coat.

If that pigmented resin is applied to the bare fabric it can look pretty darn good. It doesn’t do more than a faint blush of color if applied as a top coat once the cloth is saturated.

I have several gel coated canoes, some over 30 years old, and I've never had any problem with spider cracks except when I've damaged the gel coat itself with hard impacts -- once by dropping my outrigger on a concrete dock and once when attacked by an alligator.

I know some of the gel coat spider cracks on our boats are the result of impact. Oops dang rock in the river, or one wind-assisted dropping the hull off the roof racks. Some are from gawd awful windy & wave swept no-choice landings, with the loaded boat beating on the rocks while I hurriedly unloaded. At least one spider crack is from boat torque and partial roof rack failure in extreme winds

 

[deerfly]

gel coat can be over cured and over pigmented pretty easily which will make it more brittle. Impacts and flexing over time along with oxidation and UV exposure are the usual culprits. Depending on how long its been on the shelf and who blended it can make a difference too.

In general I would say most gel coat is, comparatively speaking, a low cost, low tech coating for a relatively low tech industry(boat building) and as a result isn't made to the same tolerances as modern acrylic urethanes and similar catalyzed coatings. So the potential for mixed results in durability and application over time is pretty high. Its telling that your typical weekend warrior can do a lot of work with gel coat without having to wear a space suit or bother reading the MEK mixing ratio's carefully, if at all, and still end up with a very serviceable finish that could last years and be easily repaired most any weekend without much fuss. Its ham-n-egger friendly but if done right can give excellent results too. YMWV

 

[Glenn MacGrady]

As a side note, Glen, how big a 'gator does it take to think that a canoe is edible?

I was paddling my va'a in a relatively shallow lake above St. Augustine, Florida. The canoe is a 22' long enclosed tube of air, about 14" at its widest point.

I felt a strong bump or thwack about 5' behind my seat, as if I had hit a submerged log. But that didn't make sense because a submerged log would have impacted the deepest part of the highly rockered hull, which is right below my seat in the middle, not 5' behind me.

About 10 minutes later I realized that my stern was filling with water and sinking into the gator-filled lake. I've recounted my self-rescue on this forum previously. Upon inspection, there was a line of about eight small punctures and dents in the hull.

My reconstruction of the event is that I surprised a submerged alligator when I glided over it, and the scared gator reacted with snap of its teeth or maybe a thwack of its head or tail. I'm not sure the bumps on an alligator's head or tail are hard enough to puncture carbon + S glass + gel coat -- and I'm not touching one to find out -- so I'm sticking with the fear bite story. It didn't have to be a big gator to bite at the narrow hull.

I had the hull repaired with epoxy mixed with micro-balloons and some kind of white color.

 

[charlie wilson]

Gel coat can be had with differing amounts of flexors in the compound. The highest flex gel is always a little dull to the eye, so there's the tradeoff.

 

[Guest]

gel coat can be over cured and over pigmented pretty easily which will make it more brittle. Impacts and flexing over time along with oxidation and UV exposure are the usual culprits. Depending on how long its been on the shelf and who blended it can make a difference too.

Gel coat can be had with differing amounts of flexors in the compound. The highest flex gel is always a little dull to the eye, so there's the tradeoff.

Thank you both. Those variables, over cure mix, over pigment, UV exposure, shelf life and amount of flexors in the gel coat make much sense.

Most of our gel coated boats with spider cracks were bought used, as fixer-upper derelicts, so I don’t know how shiny or dull they were from the factory. They were not shiny by the time I got them.

I’ll guess the old kevlar Explorer was made with a properly mixed and pigmented high flexor gel coat. It has certainly seen lots of UV exposure; it was a rotted gunwale backyard derelict when I got it and has spent the last 10 years not on a rack but tied atop a van 24/7/365.