Benefits of composite boats

[josko]

What are the advantages of composite (carbon and/or kevlar) boats over fiberglass? I know they are lighter and harder to repair, but how much of an effect does their additonal stiffness have?

[Brian Nystrom]

First, a little clarification, fiberglass boats are composite boats. They are composed of layers of fiberglass bound with a resin (polyester, vinylester or epoxy).

As for the advantages of carbon fiber, Kevlar or combinations thereof, there's a difference between the theoretical advantages and the way they're commonly used. Kevlar could produce a more durable boat and carbon fiber could produce a stiffer boat. However, what manufacturers typically do is to use these materials to produce lighter boats, which may not be any more durable or stiffer than the same design in fiberglass. There's nothing wrong with that, it's simply a design decision. There's also an expectation from the paddling public that higher-cost boats will be lighter, which may help to drive these decisions.

There are also compromises involved. Kevlar is a tough material (it's used in body armor) and it's very light weight. However, it tends to get fuzzy when abraded and it will float on the resin if it's not vacuum-bagged, resulting in excess resin in the layup, leading to excess weight and diminished strength. Carbon fiber is very stiff, but has poor abrasion resistance, so it needs to be protected from contact with abrasive surfaces. Like Kevlar, it needs to be vacuum-bagged to achieve optimum strength and weight.

In racing boats, you may see either or both of these materials used with little more than a light clear coat for protection. This results in really light, stiff boats that aren't very durable, but they're expected to be handled carefully to prevent damage. In a sea kayak, protective layers of fiberglass and gelcoat should be used in order to protect the carbon fiber and/or Kevlar layers from the inevitable contact with rocks, stone beaches, other boats and such. Consequently, they'll be somewhat heavier, but still lighter an a comparable all-fiberglass layup.

If you're considering such a boat, it's a good idea to find out how it's constructed and determine if the layup is optimal for your intended purpose. If you really baby your boats (I've seen paddlers who wouldn't even rest their boats on sand), you can get away with a really light layup. If you expect your boat to endure the bumps and scrapes of more typical handling, look for something with better protection. If you like to play in rock gardens, you probably want to stick with an all-fiberglass layup.

[Melissa Meyer]

Given two identical boats, same weight but one is stiffer (say one fiberglass and one full-carbon), would the stiffer one be advantageous or easier to paddle in some way?

[leong]

7 hours ago, Melissa Meyer said:

Given two identical boats, same weight but one is stiffer (say one fiberglass and one full-carbon), would the stiffer one be advantageous or easier to paddle in some way?

Stiffer is better as less energy is wasted as the hull flexes. So the stiffer boat will go faster for the same effort (power). However, it’s not that noticeable unless you’re racing.

-Leon  

 

[Brian Nystrom]

22 hours ago, leong said:

Stiffer is better as less energy is wasted as the hull flexes. So the stiffer boat will go faster for the same effort (power). However, it’s not that noticeable unless you’re racing.

 

That's only true if the flex in question absorbs your paddling energy. That's typically not the case. In some circumstances, a more flexible boat can be more efficient. The classic bidarka kayaks of the Aleut were often specifically designed to allow the keel to flex, as it was more efficient in waves.

[leong]

 

2 minutes ago, Brian Nystrom said:

That's only true if the flex in question absorbs your paddling energy. That's typically not the case. In some circumstances, a more flexible boat can be more efficient. The classic bidarka kayaks of the Aleut were often specifically designed to allow the keel to flex, as it was more efficient in waves.

Certainly flexibility contributes to performance. But I'm not convinced it increases average speed.

Nevertheless, I was mainly thinking of racing style kayaks. They will run the fastest if the hull is as streamlined as possible, not flexing or deforming from the intended design. Ocean surf skis have a very stiff construction. The keel is not designed to flex. The Epic V14 GT is Epic's fastest surf ski and it has the stiffest construction.

[josko]

Interesting. Under what conditions would a flexible hull be faster or more seaworthy than a stiff one?

[leong]

15 hours ago, josko said:

Interesting. Under what conditions would a flexible hull be faster or more seaworthy than a stiff one?

From here about Baidarka kayaks it says “His book is very interesting reading, and provides basic theoretical background to support claims about their speed and seaworthiness with its forked bow and transom stern.” So perhaps it's the bow and stern and not the flex that accounts for their speed in waves. I don’t really know. Perhaps Brian can comment?

 

 

[Brian Nystrom]

My understanding is that the flexible keel helps keep the hull in the water in swells, effectively increasing the waterline and reducing wavemaking drag. It may have been developed accidentally (a cracked keel or failed joint), then put into common practice because it worked.

As for the bow and stern, there has been a lot of debate about them. I tend to subscribe to the theory that the bow design is an homage to the sockeye salmon and simulates the hooked jaw the males develop during spawning. The fact that it may improve performance is likely a fortunate accident, but has resulted in the design's longevity.

I don't know of any real explanation for how the stern design was developed, but it apparently works.

[josko]

9 hours ago, Brian Nystrom said:

My understanding is that the flexible keel helps keep the hull in the water in swells, effectively increasing the waterline and reducing wavemaking drag.

Is there any evidence, data  or reason to believe this is actually true?  That a flexible hull actually reduces drag in seas?

[rfolster]

All I can picture is a rubbery kayak slinking along the surface of waves like a snake.  It is an amusing image!

[leong]

47 minutes ago, rfolster said:

All I can picture is a rubbery kayak slinking along the surface of waves like a snake.  It is an amusing image!

Ha, ha. This video compares the Epic (a stiff hulled, low-rocker and plumb bow/stern kayak) vs. the opposite. Perhaps we need a similar video of the first (the Epic) vs. a Baidarka.