Fiberglass Hull Flex???

Just out of curiousity on this one and nothing more……I recently got a Greenlander Pro and really love the boat.

The layup seems good, but is a little flexy in the center of the hull. I did not really see this as an issue; however, I did notice something interesting when paddling in rough water lately…something I had not noticed in the past with other glass hulls……I can actually feel the hull flex underneath me in really bumpy water.

Anyone else experience this with their glass boats? Any comments? I just found this rather surprising. Maybe just because I have been paddling boats with heavy layups in the past.


NDK Boats
You mean that lovely thump and shudder you get when the boat lands from having gone airborne over a wave? It’s an NDK thing from what I can tell - and after a few years of this I have yet to see any indication of spider cracking or whatever from it.

Maybe an NDK thing…
My Romany does that, but it is an Elite layup - one less leaver of cloth and only partial keel reinforcement.

My Aquanaut does not seem to flex in this way.

The thudding Celia refers to, which I have experienced in Romanys and Explorers, I’ve always associated with the rather flat bottom on these boats. I don’t recall if the Greenlander Pro has a flattish hull bottom.

I know the launch and thump that that you mentioned…this is different. When recently paddling in very choppy conditions (20-30 knot winds with steep and frequent wind wave) I noticed that the hull slightly oil canning when on the peak of a wave…or maybe it is when in the trough after a wave passes under you. It’s not the result of a crash landing though.

Never felt this before with any other boat.


Did you change the seat?

– Last Updated: Apr-18-07 9:11 AM EST –

One reason that manufacturers hang the seats with struts from the deck is to disperse the load. Believe it or not, a fiberglass panel is not that stiff. It gains it's stiffness from the rounded shape and a few other tricks.

Many times people cut out a hung seat and plop in a foam one and now the load is totally carried by the 2 ft square of space on the bottom. In trying to keep the boat as light as possible, manufacurers have to use the hung seat system. If you (or others) have cut out a seat and sit on the bottom, the boat is not going to explode or come apart. But there will be more flexing than a hung seat.

BTW: the stiffest of the conventional building methods is the wood strip built method.

The seats on the
Romany LV and Explorer LV are not hung from fiber glass seat hangers. They come with a foam seat that the paddler can find the right position for. I don’t know about the Greenlander Pro.

GP Feex
I have the GP Elite lay up and experience the same as you. We also have Explorer and Romany and do not have this problem. Beach Landings with GP make me very nervous.

no flex here
In my regular greenlander. Mine feels very stout and cobble beach landings don’t faze me any more than usual.

Older ones are
My Explorer LV seat is glassed in. The one I demo’d was also that way, but that may have changed a year later. And mine was somewhat of a custon job anyway.

GP Flex
I experienced the thud and vibration that Celia and Jim described when going over waves in my standard lay-up Greenlander Pro but not any flexing. The totally flat bottom between the keel and the chine makes this boat more susceptible to vibration than an Explorer or an Aquanaut. Elite lay-ups are subject to more flexing than the standard lay-up. Jay makes an excellent point about more flex if the seat isn’t on hangers. If your boat is an elite lay-up, Nigel says to treat it more carefully on surf landings than the standard lay-up. Either way, I don’t think it’s anything serious to worry about.

Enjoy your new GP!

airborne …
Justine Curgenven will have to get that on film for This Is The Sea IV

depends on the hull and layup

Flat panels and hard chines…
… makes for a less rigid structure in composite. Our Kevlar Sparrow hawk also has a bit more flex in it’s panels.

Basic geometry and physics. The panels just span straight from edge to edge with little or no arc for support. This is why you see some arc to suspension bridges, why feet have arches, and why eggs have no corners or flats (also makes it easier for the chicken to pass!). In addition to making for weaker panels, more stress is also concentrated along the edges/corners.

As has been said many times here, flat panels do not take best advantage of composites. Curves do. People like the hard chine look, and ascribe some magic to the shapes, so people design and build them. Boats like the GP and SH/AH are good kayaks, but could be better structurally with no performance loss (and some gain due to added stiffness and slight reduction in drag).

Structural mechanics
I think it’s at least partially explained by the geometry of the hull. A single curved surface will flex more easily than a double curved surface. I assume the Greenlander is rather flat between the sharp chines, compared to a more rounded hull?

I have an old all-Fiberglass canoe
that flexed significantly even though it was laid up competently, used wovern roving for the final inside layer, and had a longitudinal wood strip laid into the bottom, covered with roving.

The only solution was to use keel support uprights between the three thwarts and the longitudinal rib. Racers at that time used telescoping supports with engine valve springs, to keep the bottom where it belonged while allowing the hull to flex upward if the canoe was run over a hidden rock or log.

“Glass” whitewater kayaks and decked canoes often rely partly on the interior minicell walls to keep the bottom where it was designed to be. The only composite boat I own where the bottom would show absolutely no flexing, even if I took out the walls, is a 22 lb decked slalom boat built with S-glass layers outside and carbon layers inside. It can be flexed by rocks, but not by water.

It doesn’t sound like you need to do anything, but if you decide otherwise, I would recommend using epoxy to lay in a single layer of carbon cloth, or hybrid Kevlar/carbon cloth, with the usual procedures. I did something similar to stiffen the foam seat area of a very flexy Noah kayak, and it worked very well.

If you like the boat, forget the flex,
it will only give you unneeded aggravation.

I had a different boat in the past with the same

mid-flex. Ending up selling it, and now wish that

I had it back.

Had an older AH
that flexed a bit, bot big deal really, long flat glass panels are not going to be stiff with out some addition of either a foam for or say carbon stringers.

Good points
Thanks for the info. The fact that the hull is largely flat on the bottom makes a lot of sense that it would be the cause.

I am not really worried about it…just was curious.