RE: [Paddlewise] Two-up on the FC Java

Paul Ash AshP_at_sundaytimes.co.za asked:
>>>>>But it was a salient lesson in boat stability and usability. While the
Java can take a second paddler comfortably (and I'm 6'3"), it alters the
stability of the sit-on-top quite dramatically.

My question: is this purely a function of waterline length coupled with a high
centre of gravity? I would guess that Feathercraft's Gemini (a proper double)
would be a more stable boat given its greater waterline length, but I would
appreciate any thoughts on this issue.<<<<<<


As long as the center of buoyancy moves to the side further than the center of
gravity the kayak will right itself. Once the center of gravity moves out
further to the side than the center of buoyancy the kayak capsizes. Therefore,
hull width and center of gravity height are the main factors determining a
hulls stability. The shape of the hull underwater and the length of the kayak
that the kayak's maximum width is carried along the length of the hull are
also factors. The longer the kayak's width stays near maximum the more the
center of buoyancy will shift when leaned. The relatively blunt ended Java
benefits from this factor. More rounded hulls are less stable because they
don't move the center of buoyancy of the leaned hull to the side as quickly as
a flatter hull does. In fact, a perfectly cylindrical hull shape (sunk to half
its width) would have no static stability at all because no buoyancy shifts to
the side when it is being leaned.

There is another reason that double kayaks are usually much wider than single
kayaks. That is because doubles have two independent paddlers so it is much
harder to anticipate what you need to do next to make compensations to
actively keep that center of gravity inside the cone of stability because you
can't always anticipate just how and when the other paddler will react.
Sometimes the two paddlers balance each other out, but it is also likely they
will sometimes move together in the wrong way, or even fight each other
unknowingly, thus intensifying the other paddler's reaction. More paddling
time together with your paddling partner will help here.

So with the Java you have a kayak that already has a relatively high center of
gravity (like other sit-on-tops, in order to make it self-draining).  As a
single its 27.5" width is adequate for normal sized paddlers. Even though a
big paddler would sink the kayak deeper in the water (lowering the CG some),
unless their body weight is carried very low, the center of gravity will still
likely be higher when compared a lighter single paddler aboard the same kayak
(and any weight shift the same distance to the side will have more effect on
the kayak's stability when the weight shifted that distance is larger). Two
paddlers paddling in the same kayak will have about the same effect on
stability as putting a bigger paddler in it that weighs as much as both the
paddlers (given their centers of gravity work out the same). However, the
double paddlers aren't likely to be as coordinated in actively keeping their
center of gravity inside the cone of stability as the heavy single paddler
will be.

The smaller Feathercraft Uno sit-on-top is 30" wide. It floated so high that I
felt I had to reach way down to the water to paddle it and it also emphasized
initial stability so much I found it hard to lean to turn (and therefore also
much harder to balance it when I had it leaned because I had to move my center
of gravity over so far to one side to get it up on edge I was teetering on the
brink). The 2.5" narrower Java leaned to turn rather nicely so was the only
sit-on-top Feathercraft we sold. Although I don't think I ever paddled one,
the 30" wide Feathercraft Gemini sit-on-top double is going to be much more
stable as a double than the Java is unless the CG is way higher. A three inch
difference in a kayak's width is huge when considering stability. So is a 1"
change in CG.

I suspect having the Java as fully inflated as possible will help with the
stability because it will act more as a unit (like a hard shell kayak). But
mainly it will shift the center of buoyancy over better than with a softer
side tube (that is being collapsed some by the water pressure it is being
pushed into when leaned).
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