Foam core damage

-- Last Updated: Oct-24-09 11:53 AM EST --

How many have actually damaged a foam cored kevlar/ carbon canoe? I have read how difficult it is to repair a foam cored boat, but do they really get damaged that often?

I have
flat panel UL Layup head on into a rock. Missed a portage that is (typically in the North) less than a canoe length from rapids.

Come to think of it that boat had kevlar skid plates (dont know why) and the impact knocked a one inch dia hole into the bow plate.

And sixteen fracture cracks in the interior foam core. None went to the outside hull. Impossible to keep fixed as now the separate pieces seem to flex and break down the epoxy holding them together.

Yes we had the breaks professionally done but they are not permanenly fixed.

I have never done in a boat through the foam core from the bottom.

Pock holes in Kevlar skid plates used
to be common in whitewater boats. A switch to more flexible resins has helped. But better to just skip Kevlar skid plates as they are a crude solution.

Chethro, one place you will seldom see
foam core hull construction is in whitewater open canoes. One goal of a foam core bottom is a reasonable degree of stiffness, but that stiffness means that a point-source force from a rock is very likely to penetrate at least the outer part of the foam core, because the hull can’t flex inward with the blow.

Millbrook uses Spheretex swathes to add stiffness to their thin S-glass over Kevlar hulls. Spheretex allows some flexing, but it is not as board stiff as conventional foam cores, and not as light per volume either. I have a Millbrook with a Spheretex bottom, but after a year of use, it has not been hit hard enough in the Spheretex zone for me to say anything.

I’m not sure why kayakmedic’s foam core repair has been unsuccessful. Swathes of bias-cut Kevlar should have been enough to weld the cracks, though admittedly that fix would have cost a few pounds of weight.

We have a Bluewater Chippewa with foam core. It has seen very little whitewater use, partly because I know the foam core area can’t bend with big impacts, even though Bluewater deliberately used a slightly more flexible foam core construction. I have heard from other Bluewater owners that damage can occur in the transition zone between the foam core and the curved bilge areas.

An alternative to foam cores is to design boats with just a little more arch in the bottom, to use a couple of layers of S-glass and/or carbon on the outside of the hull, and to use flexible keel supports between thwarts and the bottom. This used to be quite common in USCA marathon boats. They actually used telescoping keel supports with internal auto valve springs.

I don’t plan on any whitewater, but in the lakes I paddle hitting a submerged stump and occasionaly crossing a floating log is very possible.

I’m sure my Bluewater can take an
occasional floating stump, and can take being forced over a log… unless heavily loaded. When the load is heavy, there may be a risk of the kind of internal splits kayakmedic reported.

If a stump punches a hole in the outer layer of the hull, it is mainly necessary to let the foam core dry thoroughly. Then some sort of foam can be added to replace any that was crushed, and finally, a classical 3 concentric layer patch can be done. Largest layer first!

To put you at ease.
I own three boats all ultralight with foam core, and for the paddling you describe, the most damage we have ever done is gouging the outer epoxy, which is a easy fix.

We have slid over many logs and rocks, and many time I have cringed when I hear the scraping, but we have never hurt the inner core.

I think you would have to come directly down on a sharp pointy rock or a jagged tree stump to do more damage than I am describing.

We once punched a hole in the bow of our competition cruiser in a down river race when we were going 7MPH.

and hit a submerged log.

I patched it with a piece of fibre glass and epoxy, and it was good to go again the following week and the patch is only noticible if I point it out to others.

I have been using my ultralight foam core Jensen for the past ten years in a class I-II down river race, and have careened over many rocks without any more then bad scratches.



Thanks for the feedback, Jack. I’ve
been thinking of using my Bluewater for poling in class 1-2, and it’s nice to know that damage is likely to be minor and easily fixed.

I just wouldn’t advise coming off…
the top of a three foot drop onto a solid rock.

I put as nice crack in the inner shell of my Old Town cross linked poly boat doing just that.



Yeah, there are some moves amongst
water and rocks that will inflict forces that NO boat can withstand. It gives me a laugh sometimes to hear of WW kayakers taking drop-boofs off rocks who are surprised when the bottom of the boat splits open.

Repair Patch Sequence
I’d be interested in the paper on reversing the stepped repair sequence.

Lazarus and Raybold in Pro. Boatbuilder, Feb/Mar 08 suggest smallest patch first. Pfund, Pro Boatbuilder Aug/Sept 07 suggests scarfed patches, smallest first.

This is done to minimize cosmetic repair efforts.

Greene, in Composites Manufacturing, Oct 07 suggests largest first and Pfund, in Pro Boatbuilder June/July 07 quotes a couple lamination engineers recommending largest first. The theory is the larger patch provides a better bond with all parent layers and minimizes the edge effect resin build up around each step sized patch.

But, all four articles are referring to deeply scarfed or sanded out damage areas with often 20 layers of laminate. The canoes we are repairing are two or three layers thick in the sidewalls, three to nine layers thick on the bottoms so we aren’t going to grind a tapered patch scarf. All four articles have the repair guy or gal starting with an oval outline and a router!

In terms of surface condition, largest patch last gives less sanding to smooth if using glass or carbon. A last patch of Kevlar requires peel ply as the edges cannot be sanded, and, using peel ply solves sanding and smallest last cosmetic issues on carbon and glass patches too.

Anyway, with my literature divided on the issue and with the rather serious caveat that none of our hulls will allow the deeply tapered patch scarf, is their a definitive paper out their?

Largest patch first was already the
rule when Charlie Wallbridge’s “Boatbuilders Manual” came out. It is the best method when the goal is maximum strength.

Let’s take the example of a 5 layer hull and a hole clear through. One tapers the edges of the hole so that the patch will sit a bit down in and so that the overall stiffness of the hull in the repair zone will be something vaguely resembling what it was beforehand.

Putting down the largest patch first means that the patch fibers cross the beveled edges of the hull layers and bind them together. This is less likely to happen unless the smallest patch covers the entire dished out zone.

When the concentric layers are put down from largest to smallest, the edges of each layer are up on the surface where they can be easily tapered. If patches go on smallest first, then the edge of each patch (except the last) forms a hidden step that constitutes a possible zone of weakness.

I think you can see that “largest first” is not ABSOLUTELY the best method, but it is a bit better method when the goal is strength rather than easily attaining a smooth result.