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Old 06-01-2020, 02:25 PM   #1
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Keeping water out of engine room vents

My boat has the unfortunately all to common engine room vents in the hull sides. The blower discharge vents are low profile and aft facing, which has never presented a problem. The issue are the vents placed for natural draft ventilation and air intake. They're only about 3.5 feet above the waterline, and the forward set are fairly far forward and also forward facing (vents are the same as the ones in the picture).

I've ripped slats out of them a few times, although I've determined that I can avoid that by just pulling them all and upgrading the tiny tack welds that hold the slats.

The bigger issue comes when running in any kind of steep, choppy seas over about a foot where we're throwing a lot of dense spray (not the light stuff that just gets blown onto the boat, but the heavy magazine cover photo walls of spray), it's fairly common to get enough spray pushed up and along the sides of the hull to start taking some water down the forward vents. It's never been enough water to cause any real concern, but the limber holes from the outer portions of my engine room aren't quite perfectly placed, which means I have to mop up some leftover water any time it happens.

Currently the hoses come out of the bottom of the collector boxes in the hull sides. I'm not sure I can get good enough access to those areas to determine if I could flip them to come out the top and make a U bend downwards (that would likely keep most of the water out). So I'm considering building dorade boxes of sorts at the top of the engine room. Hose from the vent would come into the box with another hose coming out higher up to continue down to the lower part of the engine room where the current hoses terminate. The boxes could then be drained to a thru hull so the water just goes over the side. If I were really going crazy, I'd change it to all intake on one side of the hull, exhaust on the other rather than the "both on both sides" it has now, which is less than optimal.

Any thoughts on that method of catching the water? Would it possibly reduce airflow enough that I'd want to add another pair of vents?
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Old 06-01-2020, 08:29 PM   #2
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Here is a proper engine dorade. I built mine differently. I built the collector box in a similar shape but taller. I put the vent cover and holes lower on the side of the boat. Any water that made it into the vent would drain back out the vent. No drain tubes needed and no deflectors needed. I built them for ski boats with stern drives.
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Old 06-01-2020, 08:38 PM   #3
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Something like that would be ideal, I think. But I don't know how easily I could make that big a change to the vent layout with limited access from the inside above the salon floor level. That's what led me to the idea of an inline water separator. There's plenty of room for them under the floor, at least for the forward vents that typically take water.
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Old 06-01-2020, 09:00 PM   #4
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I had vents like those with the collector boxes with down facing holes for hoses. I was able to flip one of the collector boxes so the holes were on top to prevent water intrusion.

The area above the other vent was smaller than the first one, preventing a simple flip of the collector and hoses mounted on top. On that one, I filled in the two hose holes with circles cut out of plastic glued in with epoxy. I drilled two large holes on the back side of the collector and glued in two short plastic pipe at an upward angle with epoxy. Connected the vent hoses and created a loop to stop water coming in.

I had to cut access holes inside the boat to be able to work on the collectors.
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Old 06-01-2020, 10:46 PM   #5
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If they are angled forward can you flip them 180 so they face to the rear ??
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Old 06-02-2020, 05:09 AM   #6
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If they are angled forward can you flip them 180 so they face to the rear ??
Yes, that's definitely possible, but I've pulled a little but of water through the rear facing ones further back a couple of times as well, so with the forward ones being exposed to more spray plus having some suction on all of them, I'm not sure that would do enough. And it would definitely reduce the natural draft ventilation while the boat is sitting.

Keep in mind for the forces involved, this is typically an issue at 15+ kts.

I'll be at the boat today, so I'll have to see if I can get access to any of the collector boxes.
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Old 06-02-2020, 05:14 AM   #7
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Can you reroute air intakes to the deckhouse structure? Run pipes - install forced ventilation?
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Old 06-02-2020, 05:53 AM   #8
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Can you close them off and vent from the cockpit? You don’t really need forward facing forced air. That makes very little difference. I turned mine backwards and solved my problem.
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Old 06-02-2020, 06:04 AM   #9
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No cockpit on my boat, so no-go there. Pipes up the deckhouse structure would be ideal in my mind. I'll have to measure to see if that might be a viable off-season project (to close off the existing vents and relocate). Not sure if the cabin sidewalls are thick enough to allow it though (which might be why they chose the original vent placement).
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Old 06-02-2020, 10:36 AM   #10
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A bit of digging indicates that I can pull a panel to get to the port aft vent box and if I pull the fridge I can get to the forward one on that side. I can get somewhat poor access to the stbd aft one through the blower access panel, but I haven't found any access to the stbd forward vent.

It also looks like only the forward vents have enough height above them to bring the hoses out the top. The aft ones are too close to the deck line. I'll do some measuring and see how well I could fit a good dorade box setup on the engine room ceiling, at least for the forward vents.

As far as running the vents up the cabin sides, there's definitely not enough thickness in the sidewalls above the deck, so that option is out.
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Old 06-02-2020, 04:57 PM   #11
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What about making a kind of syphon? Meaning having the conduct going down, then doing a S so the water stays a the bottom of the S where you can install a draining hose or something like that.
Not sure I wrote that in a clear understandable way

L
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Old 06-02-2020, 04:58 PM   #12
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Originally Posted by Lou_tribal View Post
What about making a kind of syphon? Meaning having the conduct going down, then doing a S so the water stays a the bottom of the S where you can install a draining hose or something like that.
Not sure I wrote that in a clear understandable way

L

That's kinda what I was thinking with the dorade boxes, but you've got a good point that I might be able to do it more easily than building boxes. The intake air isn't moving all that fast, so it won't take a lot to drop the water out of the air stream.
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Old 06-02-2020, 06:21 PM   #13
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Agree that turning them to face aft would be a good start. Do you have gas power?
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Old 06-02-2020, 07:58 PM   #14
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Agree that turning them to face aft would be a good start. Do you have gas power?
Yeah, the engines are gas. The big vents are all non powered. The blower vents are a bit further aft and have smaller louvers, so those have never taken water.
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