Venting Seat Compartments

[Rod Thompson]

If one didn't install seat hatches in any one of the four sealed seat compartments on a CS 17 would it be wise to have a vent to release air pressure and condensation? 

[John Burritt]

I don't think its necessary to relieve the pressure.  Venting would most likely increase the condensation problem.  If the compartments are sealed, there's no way for new moisture to get in.  Vented, fresh (read moist) air would be drawn in every time the compartment heated and then cooled.  I'd think about some kind of a sealable opening thru which you could check periodically that water wasn't getting in from somewhere else.

The advantage of the hatches is that they give you somewhere to store things.

[Joe Nelson CS#35]

It is a concern that if you keep your boat in the sun that the air inside the tank will find a way to get out due to pressure caused by heat (ever see a plastic gas can swell up when in the sun).  So air will eventually get in and out.  So why not install a $10 screw hatch so you can improve air circulation and still have a viable flotation chamber.  Leave the hatches open during non use.

[PAR]

Air is already in the compartment John, when it's closed up, so condensation is occurring, you just can see it. This is a great way to hide a rot problem until it pokes through into areas you can see.

All spaces on a boat need to have ventilation. In the case of sealed compartments, a deck plate is the usual answer. They don't cost much and the space can serve as storage as well as a flotation compartment.

[Tom Lathrop]

All the replies given have merit but over many years of dealing with sealed flotation chambers, one thing stands out.  A tank that is properly treated with epoxy will not have problems with rot.  Along with many others, I build water tanks with plywood and epoxy and water stands inside these tanks for years with no rot problems.

On air flotation tanks, I normally have the round access ports in each tank and these are removed when the boat is covered and idle.  Haven't put in the ports on Lapwing yet but will eventually do so.  I have never seen any expansion problem on tanks with these ports.  Probably the ports leak enough to equalize the pressure when needed.  Some FG boats like Lasers have a small 1/16" hole drilled in an inconspicuous location high in the hull to vent the air.  It does not take a lot of pressure to mess up a hull.  I saw one distorted badly just with a vacuum cleaner once.

It is not all that simple to build a perfectly sealed tank and there are often small pinholes that vent the air whether your intended that or not.  I guess it is simple to do that but it often does not happen unless you are very careful in gluing or sealing the joints.

Condensation will inevitably build up over time in any imperfectly sealed chamber when exposed to variation in temperature and humidity.  In industry, I've seen this happen in polyethlene chambers that were apparently perfectly sealed.  The porosity of the tank walls was enough to allow liquid water to accumulate after many years.

Probably the best solution is to coat the interior surfaces with 3 generous coats of epoxy, seal the tank well, install access ports and ventilate when possible.

[Rod Thompson]

Thanks for the thoughts everyone.  Hatch it is.

I noticed that when I drilled a hole in the top of the aft seat chambers air came rushing out.  I'm not real keen on seat hatches.  They all seem to leak.  In a prolonged knock down I would like to depend on the air chambers.

My cruising experience has all been in kayaks and canoes so it seems there is plenty of storage space elsewhere although I'm sure I will soon come to depend on the seat storage and enjoy the additional comforts.    Sailing does require more gear - anchors, lines, etc. which is best kept low and out from underfoot.

[Peter HK]

I'll add my 2c worth

These pry out plates http://www.beckson.com/prydp.html with thicker O rings are really water tight- I've tested them in several boats over many years. The screw-in hatches generally are not, as the threads deform and jam making them difficult to tighten enough and impossible to remove if you do.

As it says on the website if the tank is sealed and can develop positive pressure they could pop out. This means that you do need a breather hole in the compartment. I drilled a small hole from my forward seat tank to the aft one where I have the lifting seat hatch which wouldn't be airtight.