Re: [Paddlewise] pressurize the boat

One factor involved in leakage into a bow or stern compartment is  
pressure differences. This does not arise in a skin boat or any kayak  
without bulkheads but can have a profound effect in bulkheaded  
composite boats where a small leak that in and off itself might be of  
no particular significance can admit a large amount of water.

If the leak is small (as are the small drill holes that people place  
in their bulkheads) the pressure in the compartments is rarely the  
same as the outside when you are paddling in rough water. There is a  
certain about of flex in the hull (even in heavily constructed boats)  
and as the kayak crosses the waves, the compartments alternately  
become of higher and then lower pressure than the outside. They are,  
in effect, breathing. If there were no bulkheads this would not  
happen. The tiny hole that people put in their bulkheads does not  
change this situation because the pressure can not equalize across  
such a small hole this quickly. The bellows effect is more pronounced  
when conditions get bad. People who have been puzzled with the fact  
that a small amount of water is getting into their boat while  
paddling in moderate conditions can find out at the wrong time that  
the situation is a lot worse when conditions go bad. A small leak can  
become a big problem.

This difference in pressure means that the kayak effectively sucks  
water through the small hole about half the time. The other half the  
time, it blows air back out. 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.

I have seen this phenomena be a problem even in places above the  
waterline, for example a hatch leak at the junction with the deck. A  
little puddle might collect here and be replenished frequently and  
the boat is quietly drinking from that puddle enthusiastically every  
time a relative vacuum is being created in the bulkheaded space.

One can conceive of circumstances that one could try and do something  
about this. In a boat with a hatch in the aft bulkhead, it might  
actually be worth opening that hatch so the pressure in that back  
compartment is equalized.

Understanding the disadvantages of that strategy, blah blah.

__________

Anyway, one of the advantages of building your own fiberglass boat  
(there are not many) is that as the builder you can devote attention  
to the details of inside and outside seams and hatch mountings that  
many commercial builders cannot "afford" to do.

There should be a zero-tolerance policy to leaks with the Doug Lloyd  
bubble test.

At least with the rudderless crowd. Many rudder installations  
effectively mean there is a leak into the back bulkheaded space. The  
"hole" can be tolerated if it only intermittently is flooded with  
water. If there is no "standing" water over the hole, then the volume  
of water that gets in will probably not be dangerous.

As a matter of kayak design, it is preferable if hatch-deck junctions  
and seams and rudder line entrance areas are not in puddles. Many  
designs do not meet this particular standard.


On Apr 3, 2007, at 1:24 PM, Michael Daly wrote:

> With composite kayaks, I think you have to differentiate between a  
> leak that constitutes a leak and one that constitutes a potential  
> structural failure.  The small one of the former is annoying, a  
> large one or the latter, dangerous.
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