Thursday, October 21, 2010

Is Yours
Stiff Enough?
I know Derrick has raised this issue before, but I am unclear as to its significance. I obviously am referring to kayaks and, more specifically, their hulls. I and others have noticed how many of the hulls on new boats flex. Simply pushing with your thumb will cause slight buckling of these somewhat lighter hulls. I suspect this has occurred as a compromise between weight and strength. The question for me is, "Is it safe?"

I have never heard of any of these hulls failing or becoming distorted after falling off a wave. Still, it can be disconcerting to have something that feels flimsy between you and the water. This, in turn, begs the question of whether or not it is wise and worthwhile to beef up the hull.

I have heard that some folks are putting narrow strips of fiberglass along the inside of the hull at a few random locations. Apparently this stops some of the flexing, in which case I have to wonder if the now stiffened hull has taken on a new characteristic that will, in some way, over stress it.

I would welcome any suggestions, ideas or references related to this situation (I cannot, at this point, call it a  problem).

Paddle safe...


Susan said...

Hmmm...well, if you think about the durability of the most flexible boat coverings we see (skin-on-frame using nylon, polyester or animal hide), deformability doesn't seem to be a major issue as long as it "boinks" back out to the original shape. I think I'm more worried about hull failure because of light layups, either from puncture on rocks or splitting from blunt trauma.

SherriKayaks said...

Composite kayaks (fiberglass & Kevlar) are generally made from woven fabric impregnated with a resin that stiffens the fabric. Multiple layers of fabric and the resulting increase in resin gives you the amount of stiffness in your hull. Hull shape will also affect stiffness. The relatively large expanse of flat space at the bottom of the cockpit is more prone to flex than the area a tight curves near the bow and stern. A total failure of the hull is unlikely under most paddling conditions unless you are impacting hard on rocks or being tumbled by serious surf. In some of these cases, damage is almost certain regardless of the stiffness of the hull. The major problem with flex in a hull usually is the appearance of spider cracks in the gelcoat since gelcoat is not particularly flexible. I'm sure that there is some mathematical formula that will show an infinitesimal difference in hull performance based on the stiffness of the hull, but which would be mostly imperceptible to the paddler in the real world of wind and waves. The question of greater importance to most paddlers is "how much weight do you want to lift and carry to your car at the end of a long day of paddling?"

Michael said...

As a sailor you probably remember the days when hulls were being made so thin they'd 'oilcan' at sea in a scary way, but I don't recall mention of serious failures. Certainly SOF boats almost need to flex in a seaway. In a glass boat, I'd watch for seam failures at the bulkheads first, assuming you've already got one with a decent hull to deck joint. There are some shoddy joints out there...

Tony said...

Another good reason not to lift a fully loaded boat by the toggles.

Tony :-)

John F - U.S.A. said...

Are you by any chance referring to the Cetus boats? My Cetus LV flexes terribly in the flat spot beneath, approximately, my knees. This is also the approximate locations where it rests on the foam pads on my car rack. The amount of flex I am seeing kind of scares the hell out of me, considering the rough conditions we paddle in, and the distance I tend to solo-paddle offshore.

So far though, no signs of cracking. Perhaps the flexing will not lead to any issues, just psychological stress...

Silbs said...

Thanks for all the fine comments. I guess John sums it up well. We will see, in the long haul, what happens.

gnarlydog said...

My experience is a bit different.
I have holed kayaks that had a too thin lay-up.
And that was with minimal force too.
I am over the eggshell kayaks.
Lightweight is not a priority to me.
I paddle in surf conditions and a lightweight boat generally gets damaged to easily.
Most of my current kayaks (high end carbon/Kevlar) have been reinforced in the cockpit area after they started to show stress marks that could lead to potential failures (a stitch in time saves nine :-)
The only kayak that really needed no beefing up is a Valley and a superlight Chinese kayak.
Yes a Chinese kayak!
Constructed with a core layer is very stiff and strong (carbon/Kevlar outside skin, Kevlar inside).
There are several manufacturers now that have started to offer that type of hull construction, Zegul (Tahe Marine) being one I just tested a few days ago.
Light can be strong, just NOT in the traditional lay-up.

Silbs said...

All great comments. Gnarly, do you have a post up on this subject? Plan to do one?