Hull flex...good, bad, indifferent?

On poly and ABS long boats, it’s an accepted reality. on poly WW and surf boats, it’s an obstacle the manufacturer must over come for safety. With SOFs, it’s an attribute. I know for composite racing boats, it saps speed and glide. What about composite canoes and sea kayaks? I can see where some flex may absorb shock rather than cracking. We-no-nah, Riot, Current Designs, and Impex to name a few, I’ve noticed lighter layups and definite flex. P&H, NDK, and VCP all have very stiff layups. Is this simply different trains of thought or are the North American crafters more focused on weight than durability?


Back in the days of the dinosaurs, we used to call it “oil canning” and was considered a bad thing. I’ve paddled a few “flex-boats” and find very little to say positive about it, other than I’m positive I don’t want a boat that does it.

The sides of my Kevlar canoe
flex because of the light lay-up, but the hull is heavier and rigid.Seems to be a good combination.

My perception as well.
The few We-no-nahs that I’ve had the privilege to paddle were kevlar. The sides flexed quite a bit, but they had some sort of foam core floor and ribs with kevlar overlaid. These areas were solid. I assumed it was a weight savings measure, but what about rocks and debris? Wouldn’t the sides need just as much protection?

adds exponentially to stiffness, and is a great way to create a stiff lay-up while keeping weight low. Surf kayak hulls are an excellent example. Very common in race lay-ups etc… Kevlar, being weak in compression really needs to be combined with other materials if a stiff lay-up is required. We’re gonna see some real cool composite technology in the industry I think. Looking forward to Valley’s new lay-ups.

It’s what makes SOF so great to
paddle in rough water. (Which you aready mentioned.) The boat will flex instaed of getting pushed over.

Do you mean flex you can see and
feel under normal paddling conditions, or just a lot of flex when you push at the sides? Anyway, I have two relatively short ABS canoes, a MR Guide and a MR Synergy, and neither shows noticeable flex during normal use. The Synergy is just too narrow and round to flex much, and the Guide’s center is firmed more by a pedestal seat under a thwart.

In decked boats, weight and stiffness are not mutually exclusive. I have a 21 pound Dagger C-1 with Kevlar/foam sandwich deck and S-glass/carbon hull which is extremely stiff in spite of large flattish areas. I also have a Millbrook Wide Ride cruising C-1, S-glass outside, Kevlar inside, which is 27 pounds, quite stiff, and takes abuse quite well.

Re Saltys post … ASSuming infusion
… cool way to go and environmentally nice too. Absolute ightest, stiffest boats ( kayaks, canoes and other human powered vehicles) will never be be built using it though.

Still room for Valley to have globs of resin in the ‘Vs’ and filling open weaves if they are still stuck on roving.

Intended paddling environments
The Brit boat layups are designed in an area where the shorelines are rocky and the seas very big. A lot of paddling in the US happens on inland lakes or where there are sandy beaches. So the Brit layups have traditionally been quite heavy to handle their paddling situation.

For boats to paddle in Maine, with tons o’ rock, we have preferred the Brit layups. If all of our paddling was in Florida, maybe some different dicisions on minimal layups.

Not really oil canning
I can push in on the flat surfaces of these boats and see about a half inch of travel. I don’t see it so much in the curved surfaces, but it’s still there. I demoed an Outer Island in fibreglass and a Solo Plus in kevlar last year and didn’t feel any oil canning. The hulls definitely flexed enough from simply sitting on their midsections to be visible though. I haven’t demoed the other mfrs. listed, but have noticed the flexing on the showroom floor.

Gotta hand it to them for a splendid job of selling people on anchient, heavy, technology. Owned many of them and they are no more tough than my North American boats. In fact it is they who are changing to catch up to our build standards. Oh gosh I’m gonna get it now!

In solo canoes, center flex can be
reduced considerably by putting your weight on the floor with a pedestal seat, or for legs-out sit and switch, a little tractor seat on a pedestal. If the hull is so light and thin that it wobbles under the thwarts and gunwales, that is too flexy. Otherwise, good thwart placement and a bit of support to the bottom at the center of the hull is quite sufficient.

Regarding Celia’s comment about British paddling environments and their heavy layups, the standard S-glass outside, Kevlar inside layup used for whitewater is more than sufficient for rocky conditions. It results in a hull which is plenty stiff enough for efficiency, and both tough and flexible for when the hull has to bend.

You’re right
The heavy, stiff layups of traditional Brit’ boats are NOT an advantage. A layup that will flex on impact with hard objects will be more durable than one that won’t flex and cracks or holes because of it. Companies like P&H and more recently VSK (formerly VCP) have seen the light and are using more advanced layups with a reasonable amount of flex.

This is not the same thing as floppy plastic boats that are insufficiently stiff to hold their intended shape.

Please explain what you mean. If you’re insinutating that an SOF will flex when hit by a beam wave, that’s simply not true to any significant degree. Greenland kayaks are quite stiff laterally due to the “ladder” construction of the deck where the ends are rigidly attached to each other. Some SOF designs - baidarkas in particular - are designed to flex vertically, but they are pretty stiff laterally, too.

The only advantage…
…of the ridiculously heavy layups of traditional Brit’ boats is that the thick gelcoat doesn’t wear through in a hurry. While their layups are stiff, it’s often due to the use of inferior materials such as chopped strand mat, which is stiff, but weak compared to woven fiberglass cloth. These stiff, weak layups don’t distribute stress well, resulting in cracks and holes from impacts that a more flexible layup would simply absorb.

The bottom line is that the lore surrounding these boats is exactly that, mythology that has no basis in fact. It shows a lack of understanding of basic engineering principles and material science.

Lest you think I’m a “Brit’-o-phobe”, I’ve owned four British kayaks and still have two of them. They’re my favorite boats, despite their flaws.

Not arguing stiffness etc

– Last Updated: Feb-23-06 11:45 AM EST –

Thicker gel coat will take more loss from sharp edges before you have to repair it. It makes it easier to live with on a three week vacation when I'd rather be paddling than patching the gel coat. The remaining characteristics of chopped strand vs other approaches is a specific piece that has been argued about a lot in other threads - flex is a different matter and may or may not matter depending on the type of impact.

One of the things that got me to thinking about this is Pat’s video:

Very impressive, and made me take a step back to reevaluate my thinking.

Thanks all.

Several already are!

Nick Schade’s post addresses this…
To go along with Brian’s post regarding the British kayak myth that weight = strength, here is a great post that Nick Schade wrote up a couple years ago on Connyak regarding this posted in its entirety…

Posted on Connyak by Nick Schade on Tuesday, 6 May 2003:

Excuse me while I rant. If you are particularly partial to the heavy construction please don’t read this.

The NDK description of their layup is here: and here

First gelcoat: Gelcoat is not very abrasion resistant. It is hard and brittle. The hardness makes it somewhat hard to scratch, but glass is significantly harder. The gelcoat gives no strength. The reason NDK gives for the thick coat is: We hand lay the Gel coat as we believe the Gelcoat needs to be thick in order to withstand heavy scratches without breaking through to the glass cloth. This sounds good, but what exactly is the problem with scratching into the glass. If they reduced the thickness of the gelcoat and replaced it with the equivalent weight of resin and glass they would have a boat that is stronger and more resistant to scratches. Yes, the glass would get cut sometimes, but the result would still be stronger. Scratching glass is only a problem if you don’t have much glass in your layup.

And about the glass. The standard Explorer layup is Hull: 2.5 oz C.S mat plus 8 oz cloth. 2 inch keel strip. Double thickness reinforced bow and stern. and Deck: 2.5 oz C.S mat plus foredeck rescue patch of extra 1.5 oz mat.

If this were one of my products I would be embarassed to let anyone know that is all I used.

My standard layup for a sea kayak includes 18 oz of glass on the hull and 12 oz on the deck. Since my boats are wood there is also a 1/4" core between some of the layers. The resulting boat is approximately 15 to 20 lbs lighter than an NDK of similar size.

C.S. mat stands for Chopped Strand. This stuff is dirt cheap. NDK is right to point out they are not using a chopper gun, because mat is slightly better than that (shower stalls are made with chopper guns), but using C.S. mat is not something to brag about, it just means you are using the second cheapest reinforcment material available.

Mat absorbs a huge amount of resin relative to the amount of glass fibers. The result of using mat is a very heavy boat that is brittle. The mat can assure a smooth exterior finish, but with the thickness of gelcoat used, that is not a problem. They do add 8 oz cloth over the mat on the hull. This is good stuff, but it is too little too late.

If NDK was interested in making a seriously strong boat, they could use 3 layers of the 8 oz cloth and eliminate most of the gelcoat and the layer of mat. The weight of the boat would not change much but it would be much stronger. With a skilled layup, they could cut the weight by 10 lbs and still have a stronger boat.

The Explorer is a great design. It is a really nice boat, and for most paddlers the layup is plenty strong enough. But please don’t be under the impression that it is strong because it is heavy, it is heavy because of inexpensive materials. It is generally strong enough, but probably not stronger than other boats that are lighter but use more expensive materials. I would recommend an NDK boat for anyone looking for a nice kayak. I just wish they would use material consumate to the quality of the design.

Most paddlers that I know could care less about these “best materials vs construction method” arguments. Few of us are properly educated with regard to the complexities of the properties of various materials and construction methods. My education was in chemical engineering so I hear and appreciate both sides of the argument but I have no expertise on the failure modes of the heavy Brit hand lay-up vs the NA vacuum-bag vs resin-injection.

As a paddler I buy and paddle hulls that do what I want and need in a boat. Come the day when the boats I prefer are offered in various versions of the construction methods then this argument might interest me. Until that time, all these discussions do is attempt to inflate the egos of those that so forcefully present their opinion at the expense of other paddler’s egos.

It’s not the Brits that have convinced people their way is best. People buy a particular boat for lot’s of reasons: aesthetics, function, level of acceptance by the pro’s, fit, finish, durability, ease of repair, color, price, availability, material, construction, etc. Each of us rate these traits differently so each of us will have a unique combination of logic that leads us to our decision.

Once we put down our money, our boat is a source of great pride and anyone that attacks our choice prompts us to defend our position and our choice. Those of you that insist on jamming your construction preferences down the throats of others will never convince people you are right but will only press them to assemble a different defense for why they chose the boat they did. I would venture to say that many people buy Brit boats because so many high-end paddlers use those boats. I own the boats I own because I like them, I could care less about how some new construction technique could make them stronger. I only care about learning to become a more capable paddler and no construction technique that I know of can help with that.

What I want to know is what do you guys gain from making other people feel defensive about their choice of boat? What does boat construction have to do with paddling anyway? Among glass boats the performance differences due to material and construction methods are marginal at best. I run my boats pretty hard and rarely have to do any repairs. I own Brit, Euro & NA designs, glass & plastics and they all work, differently to be sure, but they all paddle and all perform well as kayaks.

You are free to have and express your opinion, but there’s precious little to be gained by making others feel regret because their boat isn’t constructed with space-age materials and/or cutting-edge construction techniques. That’s all marketing bullsh*t and you guys seem to have bought into it.