Inflatable Boats & Kayaks: Squishy vs. Firm


 

SailboatsToGo sail kit on better inflatable dinghy
SailboatsToGo sail kit on better inflatable dinghy. Note hull is not twisting or bending much under stress.

Skin Deep

In the world of inflatable boats,  beauty is indeed skin deep.  If the skin of the boat is stretchy, the boat will be soft and squishy no matter how much air you put into it.  But if the skin resists stretching, then the air pressure inside makes the boat firm and rigid, so it feels and moves more like a boat made of solid materials and less like a water bed.  
  
At  SailboatsToGo, we make sail kits for inflatable kayaks, canoes and dinghies. We try out every inflatable boat we can get our hands on, so we can  design and test sail kits for them.  Even if you don't plan to sail, our experience can help you understand your inflatable boat options.   Our sail kits make inflatable kayaks and boats into real sailboats, with a big enough sail for some rip-roaring fun, leeboards for tacking upwind, and a steering unit to give you good one-handed steering control.  Please visit our Ebay Store (click) .  Also find us on the web atSailboatsToGo com. 

 We don't make inflatable boats; we enable you to sail them.  Since we make the sail kits and not the boats, we can give unbiased guidance on choosing a boat, whether you intend to sail, motor, paddle or row. 
  
Here's some of what we have learned about the differences among inflatable boats : 
 
 
 
Supported skin has a middle layer that won't stretch.
Supported skin has a middle layer that won't stretch.

"Supported" Skin is Best

The least expensive inflatable boats have a skin made of a single layer of vinyl.  Vinyl is stretchy, so these boats remain squishy and flexible when inflated.   Better inflatable boats have a skin made of 3 layers, with the middle layer being a non-stretchy fabric or mesh.  The non-stretchy layer resists the air pressure and results is a firm and rigid boat.  When there is a middle layer of woven material the boat is said to have a "supported" skin  -- the "support" being the threads of the middle layer. 

Video of sailing supported-skin inflatable kayak:   Click Here  Note hull rigidity. 

 
 
 
Boat at left is a single-ply Sevylor Caravelle.  See hull shape distortion where man sits.  Boat at right is supported-skin Intex Sport 400 (like today's Mariner 3).  Note how hull retains uniform shape despite stresses of sailing.
 
Boat at left is a single-ply Sevylor Caravelle.  See hull shape distortion where man sits.  Boat at right is supported-skin Intex Sport 400 (like today's Mariner 3).  Note how hull retains uniform shape despite stresses of sailing.
Boat at left is a single-ply Sevylor Caravelle. See hull shape distortion where man sits. Boat at right is supported-skin Intex Sport 400 (like today's Mariner 3). Note how hull retains uniform shape despite stresses of sailing. The bow is able to lift out of the water.  The hull makes a straight line from front to back, despite all the strain from wind on the sail and weight of the sailor.
 
 
Cross section of drop-stitch air chamber.   Connecting opposite sides are thousands of fine threads.
Cross section of drop-stitch air chamber. Connecting opposite sides are thousands of fine threads.

Drop Stitch Construction is Super Firm

When inflatable boat makers want to make part of the boat even more rigid, and perhaps give it flat rather than rounded sides,  they add thousands of threads inside the air chamber that go from one side to the opposite side, adding further resistance to the air pressure.  This is called "drop-stitch" construction and is used most often for inflatable floors in better kayaks and boats.   But in a few models, the whole boat is made with drop-stitch construction.   Inflatable paddle boards have been made possible by using all drop-stitch construction.  Nothing else would be firm and rigid enough to make a satisfactory inflatable paddle board. 
 
 
 
The Sea Eagle Razorlite kayak is entirely made with drop-stitch construction.  Note the flat contours and thin air chambers that would not be possible any other way.  At left, a cut-through illustration.  At right, the kayak with SailboatsToGo sail kit.
 
The Sea Eagle Razorlite kayak is entirely made with drop-stitch construction.  Note the flat contours and thin air chambers that would not be possible any other way.  At left, a cut-through illustration.  At right, the kayak with SailboatsToGo sail kit.
The Sea Eagle Razorlite kayak is entirely made with drop-stitch construction. Note the flat contours and thin air chambers that would not be possible any other way. At left, a cut-through illustration. At right, the kayak with SailboatsToGo sail kit.
 
 

Air-Bladder with Separate Skin

So far we've mentioned single-ply, supported skin 3-ply, and drop-stitch construction.  There is a fourth approach, and that's to have  single-ply air bladders encased in a separate fabric skin.  The air bladder (or air cell)  is the air-tight part but it's stretchy, just like a single-ply boat.  The outer skin is the unstretchy part which contains and protects the air bladder and doesn't need to be air tight.    This technique can also result in a firm and rigid boat, if the outer skin is truly non-stretchy.   The performance of this technique depends crucially on the characteristics of the outer skin:   There are a lot of mid-grade boats that use this technique and cheap out on the outer skin, using nylon that is somewhat stretchy instead of polyester which resists stretch much better.   An air-bladder-plus-separate-outer-skin boat made with a nylon outer skin will not be very firm.   The best (firmest) examples of this type of boat use a polyester mesh coated with PVC, urethane or another rubbery substance.   
 
 
 
The rigidity of this Sea Eagle 420X kayak makes if fun to sail
The rigidity of this Sea Eagle 420X kayak makes if fun to sail

Air Pressure (PSI)

 Drop-stitch air chambers may take 5 to 10 pounds per square inch (PSI) of air pressure (follow manufacturer recommendations), which is high for an inflatable boat.  The pressures recommended for many boats' air chambers that are not drop-stitch are in the 1 to 3 PSI range.   One PSI is about what your lungs can do with some straining and a little turning red in the face.  You won't routinely inflate a boat by mouth, but if you don't have a pressure gauge and want to approximate 1 PSI, then use the pump just enough to get the boat to take shape and then top off by mouth, just to learn what 1 PSI looks like and feels like in your boat.   For a single-ply boat, that's all the pressure you want.   For an inexpensive supported-fabric boat, the recommendation might be 1.5 PSI, so a few pump strokes beyond what you can do by mouth will get you there.  A supported-fabric boat will be much firmer than a single-ply boat, even if both are at 1 PSI, because the supported fabric boat will resist stretching when the weight of a passenger or other stress is applied to it.   Better-quality supported-fabric boats may come with a recommendation of about 3 PSI.   If you don't have a pressure gauge but want to aim for 2 to 3 PSI just inflate it until it feels quite firm when you sit on an air chamber.   CAUTION:  Never use a source of air intended for tires.  Tires run at much higher pressure. 
 
 
 
With just 2 PSI in this premium-quality supported-skin inflatable boat, it is firm beneath the fanny!
With just 2 PSI in this premium-quality supported-skin inflatable boat, it is firm beneath the fanny!
 
 

Why is Firmness Important?

A boat that is firm and rigid feels like a boat, not a water bed, as it rides the waves.  It takes less energy to move it through the water, so it paddles or rows with less effort and goes faster for a given amount of motor power or wind power applied to it.   A firm boat is easier for you to move around in because your feet and your fanny don't sink in to the air chambers beneath them.  Also, the firmness of a supported-skin boat or drop-stitch boat allows the manufacturer to give a better-contoured shape to the hull.  The bow will generally be upturned a bit where it meets the water, to help ride over waves.  The stern may also be upturned to reduce hull drag (the effort required to move the boat through the water).  In a firm boat, the bottom may have a center ridge to help it knife through the water and hold a straight course.  
 
 
 

Skin Chemistry

The most popular material for inflatable boats is PVC (vinyl).  The chemistry of the PVC can vary quite a bit from one model boat to another.  Some PVC will retain its suppleness for many years.  Other PVC ages more quickly, becoming harder to fold up compactly and feeling more stiff and brittle when deflated.   There is nothing in the specifications that will allow you to predict this.  You'll have to rely on the reputation of the manufacturer, length of the warranty and other external indicators in this regard.  In general, the higher-priced boats have better chemistry.  
 
 
 

Denier

On some boat specifications you will see "1000 denier" or "500 denier" (pronouced "Denny-Er").  This refers to the size of the threads in the support layer of a multi-layer skin.  The bigger the number, the bigger the threads. We  have not found this number to correlate well with anything meaningful about the boat's performance or durability.   Bigger threads are stronger, all else equal, but all else is not equal.  The denier number says nothing about the material the threads are made of and the denseness of the weave (how many threads in each direction per square inch) and these important considerations are not measured by denier. 
 
 
 

Our Experience

Cheap, single-ply inflatable boats have their place.  We've had a lot of fun with them.  We successfully turn them into sailboats by adding one of our folding strap-on sail kits, and they work.  Yes, they flex and flop -- but they go!  And they are the most lightweight and compact-when-deflated boats.  Also, least expensive.    But we far prefer a boat with supported skin.  A drop-stitch floor is also great, but we stop there -- making more parts of the boat drop-stitch adds weight and cost without a commensurate increase in performance in our opinion.  We don't see any virtue in the air-bladder approach with separate outer skin.  It's heavier to lug around on land, and harder to dry when you want to pack it up. Yet, we've used some nice boats done this way. 
 
 
 

The Great Variety of Inflatable Boats

The different construction techniques discussed above -- single ply skin, supported 3-ply skin, drop-stitch and air bladder with separate skin -- make possible a tremendous range of variety among inflatable boats and kayaks. 
  
The single-ply boats which often sell for less than $100 offer an amazing value.  And they do work.  They are soft and floppy, but they float and you can even put a motor or sail kit on them.   Pay a little more -- or a lot more -- and you can get inflatable boats that will perform to an amazing level.  Supported-skin and/or drop-stitch construction allow inflatables to be firm and rigid and allow them to be designed with streamlined contours.   Put a SailboatsToGo sail kit on an inflatable, and you have a sailboat that can travel in 2 duffel bags, store in a closet, and fly in checked baggage on an airplane. 

 


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