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Old 12-09-2018, 03:16   #1
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Engine room ventilation outlet

Hi all,

I recently rebuilt the aft deck on my boat, removing the two very tired old dorade boxes and cowl vents that served as engine room blower exit points.

I was about to replace them with two new deck mounted vents with built in dorade boxes when I stopped myself and asked if I really wanted to cut two holes in my beautiful new deck.

So I wondered about vents that exited the boat through the topsides.

I have 1.4 meters (4 feet 7 inches) of freeboard at the stern, and the boat is a canoe stern design.

What’s the feeling about such a solution?

Face my fears and cut holes in the new deck or are topsides exits a better option? (Topsides are 27 mm (1 inch) thick in that area so strength is not a big issue)

Photo of the stern from a few years back, you can just make out the old dorade cowls peeking up over the stern.

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Old 12-09-2018, 03:59   #2
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Re: Engine room ventilation outlet

I would stick to the deck, well inboard. With dorades boxes over them and some means of sealing them from outside if need be. I guess closable vents right aft and high with good water baffling could work and be safe enough, but making them would be a lot more work than the dorades.

Plenty of good ways to keep water out of the plywood that are much easy enough.
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Old 12-09-2018, 04:01   #3
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Re: Engine room ventilation outlet

Good point. Hadn’t thought about the work involved with the topsides vents.

But cutting holes in my nice new deck!!!
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Old 12-09-2018, 04:42   #4
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Engine room ventilation outlet

Duplicate post.
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Old 12-09-2018, 05:26   #5
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Re: Engine room ventilation outlet

There are plenty of venturi collector box designs which are cheap and easy to buy online. They are designed to keep water out.

If you already have the holes in your topsides, these would be my choice. Think of that pretty deck that you spent so much time on!

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Old 12-09-2018, 07:41   #6
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Re: Engine room ventilation outlet

Haven’t seen those before. Thank you for pointing them out.

Don’t have holes in the topsides. (Yet). I’ll have a look into that concept and see what is involved.
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Old 12-09-2018, 08:31   #7
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Re: Engine room ventilation outlet

Stick with deck or cockpit.
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Old 12-09-2018, 15:00   #8
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Re: Engine room ventilation outlet

If you are using forced inlet air, the inlet to the outlet pipe should be near the top of the engine room. Where it goes from there--since it will not be convection that is the main cause of air movement--is up to you. Mine exited via vents behind the coach house under the coach house roof overlap. Because the exhaust air is usually not very hot, one could get away with twin 150mm PVC pipes each side of the hull with an angle fitting on the end to prevent rain entering. Not having the outlets above the roof of the coach house means they do not foul ropes or the boom. Had I built the coach house myself, I could have used these pipes as supports by wrapping them in a couple of layers of biaxial cloth, instead of just fitting them into the corners.

One could possibly also use hollow dinghy davits as an outlet, even a mizzen mast if it was hollow.
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Old 12-09-2018, 17:21   #9
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Re: Engine room ventilation outlet

Quote:
Originally Posted by Mike Banks View Post
If you are using forced inlet air, the inlet to the outlet pipe should be near the top of the engine room. Where it goes from there--since it will not be convection that is the main cause of air movement--is up to you. Mine exited via vents behind the coach house under the coach house roof overlap. Because the exhaust air is usually not very hot, one could get away with twin 150mm PVC pipes each side of the hull with an angle fitting on the end to prevent rain entering. Not having the outlets above the roof of the coach house means they do not foul ropes or the boom. Had I built the coach house myself, I could have used these pipes as supports by wrapping them in a couple of layers of biaxial cloth, instead of just fitting them into the corners.

One could possibly also use hollow dinghy davits as an outlet, even a mizzen mast if it was hollow.

I love the davits idea!

Mine is the extractor fan style of ventilation, twin blowers that suck air from the top of the engine box and blow it out the back of the boat. The hard work of plumbing the air pipes from the engine to the stern has already been done, so the advantage of pumping air out the stern and away from the midships cockpit is hard to ignore. Anywhere else would be a step backwards in terms of crew comfort.

On balance, looking at the size of holes I'd have to cut for those venturi boxes, I think I'll just get over my protective feelings about the new deck and cut the holes. Like Snowpetrel says, there are ways of making the plywood stay dry and I hadn't really thought about how much work would be involved in setting up the drains for topsides vents.

There's me overthinking things again.
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Old 12-09-2018, 19:55   #10
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Re: Engine room ventilation outlet

The dry engine room heat can be very useful. If you could push it through a deck box it would make a great dryer, quickly drying ropes and fenders or wet shoes and dive gear. They do get a bit smelly from typical engine smells. But still handy. And if you run metal pipe inside the boat it can warm up a compartment to some extent as well.
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Old 12-09-2018, 21:49   #11
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Re: Engine room ventilation outlet

The reason I use forced air is simplicity. A small cowled fan can be used to push air into a duct almost anywhere on the vessel--thence to the engine room. Mine mounted on the engine room bulkhead itself and was the fan from a car radiator. I think it cost me fifty dollars. It pushes a LOT of air. Once it is in there, the higher pressure allows air to escape via any number of simple pipes. A more elaborate one uses a blind to close off the hole in the bulkhead when the fan is not running, so no diesel small comes back into the saloon or galley.
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