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Old 04-19-2015, 11:49 AM   #1
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Bilge ventilation

Our boat has a 3.5' hull extention hung off the back of the old transom that was installed by NHD in Anacortes. It basically takes the place of the old swim platform. It has three large access hatches on the top. Inside is a large open area. The house batteries (4 GC lead acid) are installed here. I need to add ventilation to the area for battery off gassing and mildew control. The gross volume is approximately 180 cubic feet. I think I want 24/7 ventilation. Passive ventilation for the extention has some challenges in airflow restrictions. I am thinking of building the ducting out of something like 4" PVC pipe.

How much ventilation (cfm) do I need?
What kinds of small, 12v, inline fans are capable of 24/7 operation?
What other considerations are there?
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Old 04-19-2015, 01:16 PM   #2
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I would use a bilge blower as they are explosion proof ( batteries off gas hydrogen). Continuous is excessive. Can probably find a 12 volt controller that would allow you to set on and off cycle times. 5 minutes per hour would be more than enough. Maybe add a temperature switch to turn the blower on when the temperature exceeds 100 degrees or so.

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Old 04-19-2015, 01:31 PM   #3
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Pictures would help a lot.

There are a number of ways to go. Is this just a swim platform extension? Or is it more like a cockpit with coamings/bulwarks?
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Old 04-19-2015, 02:44 PM   #4
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solar powered ventilation fan?
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Old 04-19-2015, 05:38 PM   #5
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Can not figure out how to upload the pictures. The pictures are on my iPad. I click on the paper clip icon. It takes me to another screen and leads me through an upload process but they do not get transferred here?

Well here is one.......
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Old 04-19-2015, 05:43 PM   #6
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Here is another.....
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Old 04-19-2015, 07:22 PM   #7
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Are your transom bulwarks hollow? If so perhaps the vents can be mount there with ducting going down to the swim platform.

Whatever fans you use they will have to be ignition protected. And they do make continuous use rated blowers.

http://www.xylemflowcontrol.com/file...43000-0673.pdf
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Old 04-19-2015, 07:48 PM   #8
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Woodsea, I am no help but really like your hull extension. The extra storage must be huge! Here I go, dreaming again....
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Old 04-19-2015, 08:21 PM   #9
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Capt.Bill11,
The Bulwarks are kind of hollow. There is a little bit of room, maybe a 2" deep slit. I could drill holes through the old transom which would connect the hull extension to the bilge area below the cockpit. My engine start batteries, gen set start battery gen set, diesel heater are all in that bilge area.

I have a concern with connecting these two area like this. Right now my hull extension is outside the hull. If my hull extension becomes flooded from following sea or whatever, no real bid deal. It now is isolated from the rest of the hull. If I drill holes to connect the two I think I would want to try and make the PVC ducting terminate inside the hull well above the water level. Basically I have to drill through the old transom into the hull extension, turn a 90 and go up a little ways. Maybe I am making more out of this than I need to.

How many air changes per hour are required in a bilge area? Is there some sort of standard? Presently the only ventilation they get regularly is what comes in naturally through the engine air intakes.
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Old 04-19-2015, 08:24 PM   #10
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Besides ventilation for the batteries, I want to increase ventilation to decrease moisture and mildew in the space.
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Old 04-19-2015, 08:44 PM   #11
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I'm not sure how you could do it with out putting the vents up high some where. You could have the vent outlets facing in toward the cockpit.

Or run hard piping through the transom into the original bilge area and then up and out board some how. Hard piping would minimize the chances of water transferring from one area to the other.

Are you saying the area in the new swim platform gets ventilation from the original bilge or engine space now?
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Old 04-19-2015, 09:01 PM   #12
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Greetings,
Mr. w. Hmmmm....Does this give you any ideas?

http://www.bertram31.com/proj/tips/dorade.htm

http://bertram31.com/proj/bert/dorade_box.htm

I'm thinkin' on this hmmmmm......
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Old 04-19-2015, 10:33 PM   #13
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Not knowing how the extension was build and attached to the transom I can only speculate and suggest some options.

If there is an open bulkhead between hull and extension, you might only need to ensure better air circulation in the partially enclosed extension.

If the extension is totally enclosed, you might consider installing 5" diameter access ports and force the air in and out of the extension.

In both cases your bilge/ER must have sufficient ventilation and air flow to vent both, itself and the extension.

The single blower to increase air circulation (first option) or two blowers to force the air in and out of the extension (second case) must be ignition protected as mentioned already. Although nothing wrong with heavy duty unit, I would debate if you need a continuous duty model, and here is why ...

I have replaced two Jabsco continuous duty, heavy, noisy, vibrating, power suckers on my boat with well build ATTWOOD light (but not light duty) and quiet units that draw something like 25-30% of current while pushing nearly the same amount of air as compared with Jabsco. A bonus ... the ATTWOOD unit cost is a small fraction of Jabsco unit cost.

I have installed them at the beginning of 2012 and they are as good as they were 3 years ago ... knock on wood ... they come with a 3 year warrant ... time will tell. Seriously, I highly recommend them even if I had to replace them every 3 years. I can have a conversation without yelling over the blowers' noise.

Here is the link to ATTWOOD Turbo 4000 page, the water resistant model for 12vdc is part number 1749 ... Turbo 4000 Series II In-Line Blowers : Attwood Marine ... there are links to specs and installation info (pdf) on that page as well.

Here is another link to a Hodges Marina page with a good selection of blower at decent prices ... Marine Bilge Blowers

Hope this helps ...
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Old 04-19-2015, 10:47 PM   #14
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Thank you all for the ideas!

The hull extension space has no ventilation presently and does not get any ventilation from any other bilges at this point. It is a great storage area except it gets mildew. The hard piping idea is how I am leaning at present. But….. how much ventilation do I need?

I like the idea of a couple of solar powered vents on the transom cap rail. But they are not ignition proof. I could run 1.5" or 2" pipes down inside the bulwark, turn a 90 and glue a short piece of pipe with 4200 in the transom penetration.

The dorade box and timer idea has me thinking….maybe I can install a bilge blower on the starboard side up high. It pulls the air out of the swim step hull extension and blows it into my existing bilge space. It does this on a adjustable timer. The air intake would be on the port side, piped down low in the bilge. It would draw air from the existing bilge.

Basically the air just gets circulated between the bilges with a fan. The fan is on an adjustable timer and comes on for x minutes per hour. I wire this back to a breaker and is one of the breakers that gets left on all the time. Its a bit of a battery drain. but lets say it is a 200 cfm ignition protected unit on for 10 minutes per hour at 5 amps. that equals 20 amp hours per day. Thats not too bad.

Any other thoughts???
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Old 04-19-2015, 11:00 PM   #15
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Richard,
Thanks! I was typing my post and did not see yours until after I had hit submit.

The hull extension is totally separate. If I make another penetration other than through the old transom, I will be outside the hull.

You raise a very good point. I had not really considered noise. I think the little 3" Attwood might be just the ticket. Can you hear it running? I wonder if I could hook it up to a rheostat type speed controller and cut the speed down and just let it run longer? It would make it even quieter. hmmmmmm
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Old 04-19-2015, 11:19 PM   #16
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The way I envisioned the extension construction, I was thinking about making 4-5" diameter ports in old transom. Just big enough to insert blower tunnels, one blowing in, another blowing out. No ducts, no pipes, and keep them on ER side for easy installation and maintenance.

They come in 3' and 4" versions, the specs are here ... http://www.attwoodmarine.com/userfil.../1238/1731.pdf ... the 3" units are most likely sufficient for your application.

I have installed two 4" units and I can barely hear them inside the boat when the engine hatch is closed. Noise comes mostly as a result of air moving thru the vent ports and outside of the hull. The old Jabsco units competed successfully with the engine noise at idle RPM.

If you can slow them down a bit, you will not hear them ... just an educated guess.
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Old 04-19-2015, 11:27 PM   #17
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Quote:
Originally Posted by Richard W View Post
The way I envisioned the extension construction, I was thinking about making 4-5" diameter ports in old transom. Just big enough to insert blower ports, one blowing in, another blowing out. No ducts, no pipes, and keep them on ER side for easy installation and maintenance.
But doing that would create passages that in the event of flooding would allow water to flow from the new extension into the original bilges.
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Old 04-19-2015, 11:42 PM   #18
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Yes, there is this danger ... the holes in a "bulkhead" which otherwise was watertight are not good.

On the other hand, the flooded extension with batteries installed there is a big problem by itself and would precede the water overflow into the ER.

Some other measures should be taken anyway to safeguard the extension. High water alarm, dedicated auto bilge pump, etc ... or drain the extension to the main bilge (which I assume is lower) via weeping holes in old transom. Judgement call ... can't say what's better/worse without knowing the boat.

EDIT: just a quick calculation ... 180 cubic feet is equal to 5 metric tons or 11,000 pounds of water. Yes, something should be done to ensure that this amount of water does not collect in the very aft part of boat unnoticed.
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Old 04-20-2015, 12:22 AM   #19
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Thank you all! I have gotten some really good ideas. I will turn some sort of a 90 so the upper part of the pipe is well above the possible flooded waterline of the hull extention. I need to get over to the boat and start laying things out. Is anyone aware of a ignition proof 1 or 2" 12v fan? Maybe 5-10 cfm. That would make locating it a lot simpler and then I could just leave it on all the time drawing maybe less than an amp.
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Old 04-20-2015, 12:23 AM   #20
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Whoops. Forgot to mention that I have a whale bilge pump located in the compartment. It is the low profile that gets all but the last 1/2 " or so.
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