Re: [Paddlewise] pressurize the boat

On Apr 3, 2007, at 7:55 PM, Peter Treby wrote:
> "A small leak can  become a big problem... It is
> surprising how much water can come in through a
> small hole under such circumstances. It is like a
> pump. Usually this is a nuisance, but the hole
> does not have to be that big to become a problem
> on a rough day."
> Have you ever seen this become a significant
> problem? I have drilled a 1 millimetre hole at the
> top of each bulkhead and each compartment remains
> dry, no matter how much rolling, surfing or
> bashing over waves the day involves.

I mentioned the small hole in the bulkhead only to say that the goal  
of that hole is to keep the pressure in the bulkheaded space equal to  
atmospheric pressure when you are taking your boat over a mountain  
pass or when the temperature is changing.

It works great for that.

But it is not a big enough hole to maintain an equilibrium pressure  
in the setting of the bellows effect of paddling a boat in rough  
water with the hull being alternately compressed and released by wave  
pressure every few seconds.

The water is not entering via that small hole in the bulkhead. There  
is no water up against that hole most of the time. The problem is  
some small leak in the seam or hatch or deck attachment point that  
normally does not admit much water but will admit water when the boat  
is flexing etc when traveling in rough water. The "vacuum" phenomena  
will suck the water in and alternately puff little bubbles of air out.

Rolling is a different situation. The "pressure" in the bulkheaded  
space is likely never much different than the outside and small holes  
(such as that deliberately put in a bulkhead) do not admit much water  
between the fact that there is no pressure difference and that that  
hole only for short times might have water up against it. The key is  
that that hole is not exposed to water all day long.

If you have the round rubber hatches that I prefer which are air- 
tight, you can just plug up any bulkhead hole vents and take the boat  
from the hot air into cold water or from a cold spot to a warm one  
and look at the hatch. It should bulge out or suck in. If it does  
not, you have a leak.

Not as useful as the Lloyd test because you do not know the location  
but it has alerted me to the problem when I would notice my front  
hatch bulging out but not the back hatch or on one boat but not the  
other. My recent experience had to do with deck fittings that I  
THOUGHT I had installed watertight. If they are in recessed  
depressions, they are perfect places for the bellows effect --  
patiently pulling in water.

I am compulsive about this stuff because I like my boats dry for  
camping. I do not store everything in water-tight bags. Water  
resistant bags are easier for many things. Once of those safety vs  
convenience tradeoffs. But I have been on trips where someone in a  
commercial boat had a gallon of water accumulate in a day of vaguely  
rough water paddling without a clearly visible leak even looking at  
the situation on dry land. That is up there in the "effects handling"  
category.

When I read these stories of experienced people getting into trouble  
with leaks that they did not "know" about it, it is troubling. They  
are sinking and they do not know why! There is no "obvious" hole. I  
believe this occurs largely in the context of bulkheads. The same  
holes in a boat without bulkheads would admit substantially less  
water. -- or they would learn about it sooner :)

On my earlier boats I would make hatches that were essentially just  
two cockpit rims covered with two spray covers, one slightly smaller  
inside the other. Actually it worked pretty well in an era that good  
watertight hatches were hard to find ( or some sand would get in the  
threads or whatever). If the boat was upright it worked perfectly. If  
you rolled a lot or spent time with the boat upside down then some  
water would get in so it is was not as good as the commercial rubber  
hatches available today. I built a small, very "rigid" kayak for  
surfing and camping and I would watch endlessly -- what else is there  
to do :) -- the front hatch puffing up and sucking back as I  
travelled through the waves. I would have never guessed how much  
compression and relaxation of the hull was taking place without  
having the benefit of watching this little rhythm. Suddenly it made  
sense that "leaks" that would normally not admit any water of  
consequence could pull in a lot of water. In calm water paddling, you  
did not see it.


> While a boat
> without bulkheads won't have this problem, isn't
> it likely true that much more water can enter the
> boat and fill spaces around float bags and other
> buoyancy in the event of a capsize and flooded
> boat, than with a bulkheaded boat?

ABSOLUTELY. I believe in bulkheaded boats with the cockpit volume  
minimized. That means no space behind the seat, that back bulkhead  
wrapping around the hips and the feet up against the front bulkhead.  
I think that a boat should paddle reasonably well with the cockpit  
area completely flooded and a dysfunctional spray skirt. It is  
possible to design boats this way. Few are.

I am just saying that people with bulkheaded boats should be a little  
more compulsive about looking for leaks. For me that means no leaks  
-- air tight (except for the bulkhead vent hole)

I am no fan of the float bag approach. But for the open canoe or the  
skin boat that is the way to go, obviously. Just another trade-off.
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