Cabin Ventilation

I have a 40ft spotted gum cruiser where the cabins are, for all intentional purposes, below the water line. This is great for keeping a nice temperature in the warm of Australia however I would like to improve the ventilation without leaving hatches open. The boat appeared to have had a ventilation pipe from the top deck to the lower cabin previously however it was removed for some reason. I am interested in putting the pipe back in and am interested in getting some opinions on the functionality. The pipe from the top deck to the lower deck would be approximately 2200mm long and be either circular, with a diameter of 100mm, or rectangular, 120mm by 90mm. On the top deck there would be a vent of some type to capture air and channel in down the pipe and at the cabin end there would be a valve a some type to shut of the air if required.

I am interested to hear whether there will be airflow down a pipe of this size and whether it's worth the effort to refit?

 

It's possible you had a cowl vent (traditional, mushroom or clam shell vent), mounted on the top of that pipe, but what seems more likely, is it was a heater or stove exhaust stack. Most 40'ers would have more then one cowl vent, at least two and likely four.

At rest the flow on most boats is from aft to bow, through the below deck spaces, so the forward cowl vents would be facing aft and the aft vents facing forward. Underway these vent may be moved to pick up air flow over the forward moving vessel, but many leave them so boarding water doesn't enter the forward vents in particular.

Speaking of boarding water, it would be wise to use a Dorade box under each vent to help ship water away from the inlet. A Nicro Water Trap (or other check valve) can also be installed to do the same thing.

Ventilation is often a misunderstood subject. Good ventilation is a must, not only for comforts sake, but maintenance and upkeep too. You can use two styles of ventilation systems, forced (active) and natural (passive). Forced is as the name would suggest, it forces air flow with electric fans, passive uses convection, ship movement or high and low pressure differential to move air. You can have a completely solar system, self contained battery driven, ship power or combination with the forced air systems. Several different types and styles are available. On the passive side, you have vent style choices and that's about it.

 

I don't think they were for stove exhaust as they are not installed near the galley. There are two, one starboard and the other port side. I have a solar powered ventilator, solar cell run fan, in the bridge ceiling and it works a treat and is extremly quite.

I had already considered a dorade, even though the vent will be about 3500-4000mm from the water surface, to prevent rain from entering the pipe. Given that even with the slighest breeze the vessel sits bow into the wind I expect forward facing vents would capture air. I just don't know if the cost, both finacial and time, would result in an appreciative flow of air. I don't want to be blown away but a hole through the deck is a hole through the deck and I don't really want to do it if there is no value to it.

The patch that was fitted when the pipes were removed leaks something horrid during the rain so I need to fix the deck anyway. My partner wants the vents back in, which I am happy to do, but not if ther really don't make a lot of difference.

 

Passive systems will channel moving air through the boat, but if it's a still air day, you get nothing, which will have your mate all over you like an off the rack suit. This means you'll want an active system and it sounds like you have the beginnings of one started. Install some Nicro vents in the two pipes with a Dorade box or water trap, to keep the rare boarding sea (or splashes) from getting below, because you know she'll be sitting under one when it does happen. If you choose to go with the solar/battery version, then you'll be off the house banks and not have a draw to worry about. Playing around with the different fan blades, to get a cross flow will work wonders on a hot afternoon. These type of stand alone ventilators can be placed anywhere, like a hatch cover or companionway door. Just keep some new ni cad batteries handy. If you use ship power then you can really move some air with 12 VDC fans.