[Paddlewise] Kayak Volume methods

Chris G Hardenbrook wrote:

<SNIP>
> And now, is there a formula for computing kayak volume?

Since the hull & deck shape of kayaks vary dramatically between different models, a
"formula" that fits all might be a little hard to come by.   Offhand, I can think of
four methods:

1)  Easiest solution would be to call the manufacturer for design specifications.

2)  Another method is to load the boat with something of known density & then weigh
the boat before & after. Let's say your empty boat weight 42 lbs.  Your boat filled
with fresh water weighs 805 lbs.  (805 - 42) / 7.48 = 102 gallons.  (Fresh water
weighs 7.48 pounds per gallon).  So your volume is 102 gallons.  While theoretically
correct, it might be difficult to weigh 800 pounds of water-loaded kayak on the
typical bathroom scale.  :-) 

3)  Approximate method.  Turn a garden hose on and measure the time it takes to fill
a 5 gallon bucket.  Do it again and take the average of the two times.  Now fill up
your empty kayak with water and measure the time it takes to do so - taking care to
evenly distribute the water on both sides of bulkheads as you go.  (A bulkhead could
fail if it has a lot of water pressure on one side, and only air on the other).  You
might want to do this while floating the boat to equalize the pressure inside &
outside of the hull.  When the boat is full of water, check your watch again and
note the time it took to fill up the boat.  

Let's say it took 3 minutes 15 seconds (3.25 minutes) to fill the 5 gallon bucket
and it took 1 hr 10 minutes and 10 seconds (70.2 minutes) to fill the boat.  Then
your boat's volume is 70.2 X (5 / 3.25) = 108 gallons.  I'm aware there's a rounding
error here, but the accuracy of the bucket-filling test is such that our results are
limited to 3 figures.

4)  The Archimedes Principle states that the buoyancy force exerted on an object
immersed in a fluid is equal to the weight of the fluid (WATER, in our case)
displaced.  So float your boat in a shallow sandy area, and fill it with rocks until
it sinks.  Now take all the rocks out & weigh them.  Divide the total weight of all
the rocks by 7.48 and you'll have the boat's volumetric capacity, in gallons.

OK, OK.  I can hear you chuckling!  I *said* it would be easier to just call the
manufacturer.  But for those of us with a scientific inclination, and some time to
spare, these little experiments can be illuminating, as well as amusing!  Although -
I confess I've never sunk my boat with rocks!

Craig Olson
Bellingham, Washington


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