If air pressure can ‘blow out’ a bulkhead, how can the same bulkhead, assuming footpegs are removed, take the pressure of paddlers’ feet/body weight while paddling/surfing.

Question came to mind not just from the bike pump mentioned in the leak thread, but also from various threads on venting bulkheads.

Thanks,

Lyn

I’m no physicist
but I’ll hazard a guess:

maybe that’s because there can be MORE pressure with air pressure than there is with the paddlers’ body weight/G’s. Air pressure holds up your car, a 747, etc., and those weigh how much?

Well
The positive pressure would act over the entire surface of the bulkhead and the force would be equal and over the entire cross-section. Your feet would apply more force but only over a small area.

Work with vacuum and pressure a bit at work. Have learned the amount of force a small vacuum has on a large surface. We have imploded more than one container over the years with a small vacuum.

Mark

Taper
Kayaks are tapered. Bulkheads (the foam type) get wedged in, so when you are pushing with your feet from the cockpit side much of the force you are applying is resisted by the the wedging action. It would stay in place without any glue at all actually.

If you push towards the cockpit, the wedge isn’t working for you any more.

To put some rough numbers on things. I can leg press around 300 pounds. If I’m really paying attention to form, and leg drive I don’t put anywhere near that amount of force on the bulkhead. But lets just say a world-class paddler might. So that would be 150 pounds force.

Front bulkhead is roughly 10 inches high and 14 wide = 140 square inches. So 1 psi air pressure would put 140 pounds of force.

Some numbers.
Good question. Agree with everything said. Here’s some numbers to bring it home.

If you pump in 10 psi, and assuming a 22 wide by 12 deep boat, and using a 0.8 factor to approximately convert from rectangular area to oval area, then the total force on the bulkhead would be:

10x22x12x0.8 = 2112 lbs.

Feet might exert 300 lbs if you really press, depending on how much you can leg press. As already pointed out, some of that’s absorbed into the deck and hull by the tapered press fit of the foam. (I don’t think you want to get that press fit overly tight either because of stress on the deck/hull, sort of a balance.)

I had a shop guy advise me to be careful on rough landings not to press to hard on the foam foot pads. Deceleration is a whole nother ball game in forces. F=MA. Staying with the car analogy, think of the force exerted on a crash dummy by it’s own weight at say 10 mph, if it stops abruptly.

Paul S.

Question:
Does air pressure actually cause some bulkheads to “blow out”?? Foam ones? FG ones? ABS ones?

I can see how a cockpit full of water might cause a lot of pressure on a bulkhead, but I would guess that air pressure from the sun would cause a rubber hatch seal to pop off or a bulb-sealed FG hatch to leak long before a bulkhead would blow.

Or am I just clueless?

Jim

yep
what they said…

it does not take many pumps on the pump to push the rubber hatch way up and begin to hear creaks and groans from the boat. I’d guess it would take a bunch of pressure to pop a hatch but I have heard of b/h seams failing with air pressure. tho I’ve never seen it.

steve

Who has a flying car?
“Air pressure holds up your car”

Who has a flying car?

60lbs per sq in.
of air pressure produces enough force to throw a 16lb bowling ball over a half mile. This is a fact that has been demonstrated many times.

I have been in the Automotive/Truck repair industry for +25yrs as both a technician, College Instructor, & buisness owner.

Most people take the simple physics that exist around them for granted.

10lbs per sq in. of air pressure in a confined space such as a tire is enough to lift a 2 ton vehicle.

If you don’t believe it, just let the air out of your own tires, add 10lbs & watch the vehicle rise.

Wouldn’t the hatch covers blow out first
1.

Wouldn’t the hatch covers pop-out first, and let the air pressure squeeze out? If the bulkhead pops out before the hatch cover, the bulkhead may be damaged already.

2.

Air is expandable and pressurable, in opposite to water, thus can absorb a good amount of expansion or retraction in volume caused by temperature changes. Think about pressure in tires, who seldom explode or crack. If your bulkheads blow out due to a change in air temperature, they might have been damaged, or installed inproperly in the first place?

3.

Why worry about a problem which is highly, highly uncommon, and usually does not derail (never ever derailed?) expeditions, even the ones done in most extreme climates?

Temperature change and pressure
Temperature changes that you are likely to encounter in real-world boating conditions are not going to generate that much pressure.

Increasing a fixed volume of air from 60 degrees F to 90 degrees F will raise the pressure by 1 psi.

Wouldn’t the hatch covers blow out first
Air is a fluid (defined as - any substance that conforms to the shape of its container) just like water, & as such will usually seek the path of least resistance.

If a hatch is more secure than a bulkhead (i.e.-due to area coverage), the bulkhead will fail first.

Most good ruber hatches that I have encountered get somewhat more secure with pressure (helping prevent implosion).

Round rubber hatches like Valley’s or Kayjaksports tend to grip the lip they are secured to more tightly when being pushed outward also, like a Chinese finger cuff.

Hope this helps…

a more realistic change would be
70+ degree day, kayak in sun, kayak launched in ~40 degree water (a fair description of last Saturday).

Lyn

You have heard of tires?
Haven’t you?

…and be done with it. There is no good reason not to do so and it eliminates thermal expansion issues completely.

1/16" hole
poked in the center of yer b/h.

or get a …naw, let’s not go there.

steve

Vacuum
Ok Lyn, that would produce about half a psi vacuum.

I think venting bulkheads is a fine idea. My hatch covers leak like sieves, so I haven’t done it yet. If I ever get them reasonably watertight, I probably will.

that is at pressurs higher than 1
atmosphere.

where’s the sense
in THAT. work hard at getting them air tight so you can then ‘vent’ 'em.

=:-0)

with most of my leakers I just have good drybags and not worry. that’s why it gives me a warm/ fuzzy feeling to see that little bulge of pressure. IMO for it to actually do damage is minimal.

sorry I can’t help to poke fun at leaky/ dry hatches. it’s only taken us how long???

steve

gimme dry hatches
that’s my bottom line. I have more dry bags than I’ll ever need, but I love to end a long day on the water and open totally dry hatches. We all have our different needs and loves in our boats, but this is major for me.

Lyn