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AeroE

Design of flotation for kayaks (and canoes)

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The primary question I have is -

 

How much flotation is recommended in a kayak?

 

In other words, "What is the design weight or load?"  Is it 100% of the gross weight (vessel plus crew plus equipment) or design displacment, 50%, or some other value?

 

Does the Coast Guard provide guidance?

 

 

Kayak and canoe flotation goes into the ends (first), because that is the volume that is available and the need to displace water from the ends to ease rescue.  But that is also the most inefficient place to displace large volumes of water.

 

Foam flotation adds weight.  A light weight boat of 300 pounds displacement with 100% flotation requires 4.8 cubic feet of foam that weighs something like 2 to 3 pounds per cubic foot, so about 10 to 14 pounds of weight is added; that is significant in a boat that weighs 30 or 35 pounds before adding foam.  Foam that is allowed to get wet will absord water, too, unless a conscious effort goes to drying the boat out and storing it in a dry place.

 

Flotation bags are light weight, but it's hard for me to warm up to welded seam pvc or bonded neoprene assembly.  The question is not whether the bags will leak, but when.  Foam is dead reliable, cheap, and heavy.

 

I intend to build a skin on frame kayak and I have no relevant experience in kayaks.  I will use the boat on Missouri rivers.  I'm concerned about the safety of these boats due to the low sheer, plus I'm interested in understanding the underlying design and engineering.

 

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Here's my capsize test of the Morten Olesen Day Tripper canoe with increased sealed flotation at the ends .

I eventually put inspection hatches so they can be used to store small items.

No mathematical computations here though, just eyeball design:

 

http://www.youtube.com/watch?v=_IaqBQ1BSUY&list=UULi7iAwq_WrBC__F4s9qLzg&feature=player_detailpage

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Either the Coast Guard has no recommendation or requirement, or the guy who inspected and passed mine is not as informed as he should have been.  A SOF kayak will float with no floatation. The purpose is really to displace water taken on capsizing to make rescue easier.  So the more the better.  The only down side to more is less storage space.

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Home builts aren't subject to this level of inspection, but manufactures are. If the build quality is good, this is a good selling point.

 

The amount of floatation you need is enough to keep the rails above water (usually just barely), with a full crew compliment aboard (usually trying not to move too much). This is the swamped condition the boat is expected to tolerate. In this condition the boat can often be easily tipped (especially if the floatation is mounted low), but with some bailing, she'll gain stability and you can sail off. In small craft it's often difficult to justify the volume and weight of some forms of floatation, but on the other hand, you might want it. Air doesn't weigh much, so consider compartmentalization, possibly with bags, though without works too. Of course a deck plate or some type of water tight hatch is necessary to ventilate these spaces, which can be handy for storage too.

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This afternoon I figured out that "where" the flotation is added is at least as important as "how much" due to the effect on the stability of a filling boat.  (Besides preserving basic flotation and reducing the volume of water that has to be removed.  I already understood the need for flotation above the CG, that's more or less instinct, but hadn't considered the benefit of flotation along the sides.

 

I like the idea of air bags, I don't like the idea of leaky air bags, and even with a "pre flight" to check whether the bags are holding air, they're most likely looking for an excuse to leak.

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Most production sea kayaks have bulkheaded compartments which provide air tank floatation.

 

Hard thing to get with a SOF Kayak...

 

Having crew compartment bulkheaded off from fore and aft compartments ads a lot of security.  If the cockpit gets flooded it is a whole lot less water to removes rahter than entire vessel.

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Air bags would be my first choice for flotation in a SOF kayak, but what I actually used in mine (because I'm cheap) was lots of 2- and 3-liter soda bottles tucked into the bow and stern where they wouldn't bother me. I can't remember how many there were, but there were enough of them that they didn't rattle around.

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Aero:

Dave has it right. Whereas the goal of flotation for most boats is that it floats with the gunwales above water with engine and crew in place, the goal for kayaks is more stringent. You want to be able to get back in the boat after a wet exit, pump out the water that got in, and paddle on. Depending on conditions, anything less can be fatal. So, a skin-on-frame kayak without float bags will float full of water but you cannot get back in because it will be completely unstable. All you can do is hold on to the boat and wait for rescue. More careful paddlers of skin-on-frame boats use seasocks in addition to float bags to minimize the volume of water that can get in the boat. It's a really good idea to take your boat to a beach where you can play with it under controlled conditions and see how it acts full of water and to practice rescue techniques.

Fair winds, Andy

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