vents for flotation compartments?

[Steve Miller, Jr]

In the aft section of my 16' plywood skiff that I am close to completing, there are two epoxied and taped flotation compartments that I built. They are about 24" wide and I filled them with the blue foam insullation board from Lowes. I recently took the boat to the shop to have the steering and motor installed. They screwed a clamp to the .25" plywood cover on the flotation compartment to attach the throttle cables, penetrating the cover. I of course didn't like the location of the clamp, so I removed the screw and clamp. As I removed the screw, I heard "Pssssss", air either entering or escaping through the screw hole. Air is obviously trapped inside the compartment and expanding and contracting with the weather/temp. Everything is epoxied and taped, so it is air tight. Should there be some type of ventilation for the compartment? Or is it better sealed up. I could drill a hole where it would not be exposed to rain water, but then water could enter if I swamped, also, would ambient moisture be more likely to accumulate in the compartment. What I don't want is moisture to accumulate in the bottom of the compartment, leading to problems down the road. What do you all think, seal up or vent?

[Gary E]

Your asking what others would do... I think that is a waste of time because it shows either a lack of confidence in the floatation actually working, or maybe a lack of your construction quality.

Now before you get all huffy and say I dont know what I am talking about, your right, I do not know you or your construction quality or the percieved floatation and wether it will do any good if the boat is actually swamped. Have you done the calculations to see what this floatation chamber actually gains you even if it does help? are you really going to run in water so lumpy where it will likley swamp?

Therefore only you can look at your situation and determine what you will do. If it was me, and I did have an all welded 16 ft alum boat with ZERO floatation, and 90 HP motor to push it. That boat was taken places where there was no help available, but QUALITY CONSTRUCTION, able seamanship, confidence in what we were doing was all we needed. If I had your water tight box, it would end up as a ice cooler for the fish.

Good luck

[AngWood]

Gee, that was helpful.

[Gary E]

Originally posted by AngWood:
Gee, that was helpful.Whutz ur problem?? Where is your sugestion???
I told him what I would do...
"If I had your water tight box, it would end up as a ice cooler for the fish."

[ 08-22-2005, 01:59 PM: Message edited by: Gary E ]

[Ian McColgin]

Interesting that you'd fill a sealed box with foam. At any rate, I've come to believe that whether filled with air or filled with foam, tha ability to vent every compartment is of real value.

Epoxy is water"resistant" but not vapor proof. You can get structural damage if either heating or cooling occurs more rapidly than vapor can pass through the material. Witness some time how a kayaker puts the fore and aft rubber hatches on while ashore and then launches the boat. The hatches immediatly bow in due to the cooling.

But it's more likely that you'll just get noxious condensation inside the compartment. If it pools just right and if the epoxy cracks - both quite probable, you'll have rot in a while. Whether a year or two or a decade or two is harder to predict. Depending on the quality of the foam, you may also get some deterioration there.

I favor foam place such that the boat can float upright. Small watertight access hatches are readily available and should be placed as low as possible. Whenever possible, they can be left open. If the boat is beached or otherwise upright, any water in the float compartments will drain out. Store the boat upside down on a nice hot day to steam vapor out.

G'luck

[Ian McColgin]

Oh yeah. The other reason for putting the vent-access hatches low is that way they will not be much stressed when the boat's swamped. In some installations, I've been happy with just a 3/4" hole right at the bottom of the bulkhead. The air will trap just fine.

[JimD]

From my kayak progess thread. It's a $5 drain plug, one inch diameter. There is a 3/4" plywood block on the the other side of the bulkhead. The hole goes all the way through, of course.

http://www.imagestation.com/picture/sraid183/peed21bf16ccd6576f58fc155a6828b6c/f2b94fba.jpg

[Ian McColgin]

In a kayak I agree that the best location might not be at the bottom. In this case I'd have worked the hole up into the apex under the deck to facilitate draining any water. But, different boats, different longsplices. She'll be a grand boat when done.

[Todd Bradshaw]

It's common practice to drill a tiny hole in the bulkheads of sealed chambers to prevent the air inside from heating up, expanding and blowing out or fracturing the seams. All that has to pass through the hole is a little bit of air, so they're often very small (like 1/32") holes. As long as you don't have unsealed raw wood inside the bulkhead, you'll likely be dead from old age before the potential for moisture entry ever begins to cause any kind of problem worth worrying about.

[Stephen Hutchins]

-Better safe than sorry. Since you've filled them with foam, I'd use some sort of plug.

[JimD]

Originally posted by Ian McColgin:
In a kayak I agree that the best location might not be at the bottom. In this case I'd have worked the hole up into the apex under the deck to facilitate draining any water. But, different boats, different longsplices. She'll be a grand boat when done.I thought about placing it on the deck but decided I didn't want to look at a plug on the deck. The only way water can get in is either through the plug hole itself (unlikely) or if the boat is punctured, in which case I'll have a hole to let the water out whether I want it or not :D

[Steve Miller, Jr]

Gary E, You're part right, I have little confidence in the flotation actually working, and I will most likely never take the boat in to "Lumpy" water, (but after watching the lobster skiffs this weekend, I know I could.) This is my first boat building project, and I purchased the plans. There is one on the port and starboard sides, they are 24" wide and taper from 6" deep at the sheer rail to 0" at the floor. The shape will not work as a fish box. As for the construction quality, I feel it is as high a quality construction as could be achieved by a first time builder. Okoume coated multiple times, taped, coated etc.. I have confidence in the construction quality. The question was Should I vent or not? How about those 4" water tight hatches you see?

[Tom Lathrop]

Well, if you don't trust your air tank to be sealed, it would be best to use the foam without the tank. Been using air tanks on racing sailboats for decades and the most accepted technique is to have drains at the bottom of the tank (fittings are readily available), inspection ports for airing out when in storage and inspection of course. Then drill a tiny 1/16 inch hole near the top of the tank for pressure equalization (you already know why that is needed.

Of course a lot of air tanks don't work because someone mounts unsealed fittings on them or routes control lines through them (yes, they do that too). If that is going to happen, then maybe the blue foam is a good idea but the tank should still have the other three options.

[Stephen Hutchins]

Mike O'Brien mentioned in a kayak review once that if you insist on puting bulkheads in a kayak, they should have drain holes near the bottom. Reason being, many sea kayakers damage the stem from pulling up on shore and eventually wind up taking water into the foward compartment. The drain plug allows the kayaker to pump the water out of the cockpit area while out at sea. It is a most informative article on kayak design. I think a copy of it is in "100 Designs Reviewed"

[Gary E]

There is one on the port and starboard sides, they are 24" wide and taper from 6" deep at the sheer rail to 0" at the floor. I may not have the correct picture of this, but is sounds like you have the 2 chambers and each is ...
24 x 6 x SOME unspecified length

Let me assume the unspecified lengh is also 24 inches...

Vol of each chamber is (24 x 6 x24 ) divided by 2 = 1728 cubic inches or 1 cubic foot...

If that was filled with foam and not sealed so that air could circulate and water will drain, you will have at the most 2 cubic ft of floatation in this example... Measure the unspecified dimention and figgerit out...
Maybe it's worthwhile..

[John Turpin]

I've got four sealed flotation chambers in my TwoPaw 8 filled with ping pong balls and whatnot. This discussion has been very helpful and surfaces an issue that I'd never thought of. I think I'll go home tonight and drill a few tiny holes in those compartments.

[George Roberts]

John Turpin ---

Drill very tiny holes in each ping pong ball also.

[Gary E]

very small (like 1/32") holes ummm... how come you wouldn't do that to the boats bottom? or maybe the deck above your bunk?

Me thinks that all that little hole is gona do even if it is through a epoxy plug or a brass screw or something that wont let the moisture into the wood, is allow a LEAK... or it'l get clogged up... it will not allow air circulation...

i know, you couldnt care less what I think..

[Todd Bradshaw]

that's right....

[Tom Lathrop]

Gary,

Volume = pressure X temperature. If the temperature goes up and you try to keep the volume constant, just what do you think is going to happen? Look at any Laser for instance. You will find a tiny hole near the daggerboard to keep the hull from deforming in changing temperatures.

[ssor]

If you are concerned about condensate in the floatation chambers then venting them through a silica-gel trap will take care of that and if the vent hole is less than 1/16 inch flooding the floatation chamber won't be a problem.
Remember water vaper is lighter than air so placing the vent at the top is correct but the drain hole must be at the bottom.

[jzeigler]

You can probably address the air pressure issue, the moisture issue, and water access issue with a small clear 1/16" ID tubing with a loop or two at the outer end. The tube will be at the top of any airtight bulkhead. You'd have to roll the boat a couple of times to get water in, yet it will breathe.

This prevents some of my plastic racing boats from blowing out their seams. I plan to use this method in the strip planked Bangor Packet I'm starting to build.

[Gary E]

Originally posted by Tom Lathrop:
Gary,

Volume = pressure X temperature. If the temperature goes up and you try to keep the volume constant, just what do you think is going to happen? Look at any Laser for instance. You will find a tiny hole near the daggerboard to keep the hull from deforming in changing temperatures.Might want to check that formula...by checking Charles Law, but I know what you are meaning. If the "box" is closed and sealed tight, it will condense moisture inside dependent on tempeature, unless he intends to evacuate it an charge it with nitrogen, and I doubt that. So to make a practical "box" and have it usefull why not eliminate all the complications and condensation and use the foam with a big enough drain at the low point not a 1/32 Dia pinhole.

[David123]

I'm going to use watertight 4" inspection ports on my bulkheads....will serve two purposes. Access/inspection port that can be aired out when not in use and also watertight storage for trips.

Was thinking of foam, but thought better of it. Foam will be placed in other areas should i decide to use it.

Just an added note. On the aluminium boats i've had, the foam is placed in areas that are not sealed. i suspect that this is for two reasons. #1-It's cheaper and less labor intensive.

#2-Breathes better that way.

[Tom Lathrop]

Yeah, There's Charles, Boyle and Avagadro to give all the relationships of a gaseous state. I should have used a proportional symbol instead of = but don't have one.