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?

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.

Ted

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?

solar powered ventilation fan?

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.......

Here is another.....

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/files/357X0_43000-0673.pdf

Woodsea, I am no help but really like your hull extension. The extra storage must be huge! Here I go, dreaming again....

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.

Besides ventilation for the batteries, I want to increase ventilation to decrease moisture and mildew in the space.

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?

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......

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 ...

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???

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

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/userfiles/store/product/files/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.

Richard W said:

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.

Click to expand...

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.

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.

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.

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.

woodsea said:

Whoops. Forgot to mention that I have a whale bilge pump located in the compartment.

Click to expand...

Whew ... problem solved ...

woodsea said:

Is anyone aware of a ignition proof 1 or 2" 12v fan? Maybe 5-10 cfm.

Click to expand...

Check them out, if anybody knows where to find the fans it would be Fisheries Supply ... Fisheries Supply - Marine Supplies Since 1928 Page ... I often go there for the ideas proven on the working boats. They also carry hard to find replacement/repair parts/kits.

Another idea ... you probably can find a cooling fan used in some electric/electronic equipment indented for installation in ER. The marine battery charger comes to mind ... one on my boat has two small 1 or 2 inch square fans installed. They come in various sizes, from 1 to 6 inches, and with various air flow capacity ... you just need to track the one that works for you.

You don't need a fan to clear out the hydrogen.

It's lighter than air.

Any hole in the top area of the compartment will bleed off accumulated hydrogen.

It won't solve the damp or the mildew though

Before I compromised my transom I'd get rid of the hydrogen and spray a dilute bleach on the walls occasionally to kill the mold.

The easiest answer might be to switch to AGM batteries. And as mentioned add a high water alarm and a high capacity bilge pump as an emergency back up.

Capt.Bill11 said:

The easiest answer might be to switch to AGM batteries. And as mentioned add a high water alarm and a high capacity bilge pump as an emergency back up.

Click to expand...

Yes to all.

How hot does it get in this enclosed area with diesel heater running?

Ah, I forgot about the heater.

Where does the heater get its air now? Through some kind of in and out ducting?

If you make any penetrations through the old transom SEAL those holes well. I don't know if the transom is cored and if so with what but do not allow any moiture to get into the core.

The huuricane hydronic heater has been inoperative since I got the boat last fall so I do not know how warm it gets. There is all kinds of stuff in that compartment and the items immediately adjacent to the heater don't show any effects so I think I am ok. The heater exhausts out the port side. Not sure where the combustion air comes from. I will check that out. The engine and gen set lead acid start batteries are all in the same space. Hmmmmm. Looked at lots of 47's when we went through the buying process. A lot of boats had similar heaters and batteries all installed in this space.

I will definetly seal any penetrations with epoxy.

I have considered the agm solution. Still a possibility.

I have cleaned and wiped down the hull extension. Things stored in there are just continually damp at present. I would just like to solve the problem if I can do it without compromising the boats safety.

Our Hurricane heater is in the lazarette with exhaust about 18" above water line and inside loop closer to 30 ". Hard to see how your heater would work with so little head height available. The intake air comes in on the outside of the exhaust to cool it, a pipe in a pipe arrangement.

Enclosed as yours is the heat build up would be considerable and if not flooded to extinction may well cook the batteries when running. But, if as you say many Bayliners of similar size are set up this way I have just one more thing to ruminate on.

sunchaser said:

Our Hurricane heater is in the lazarette with exhaust about 18" above water line and inside loop closer to 30 ". Hard to see how your heater would work with so little head height available. The intake air comes in on the outside of the exhaust to cool it, a pipe in a pipe arrangement.

Enclosed as yours is the heat build up would be considerable and if not flooded to extinction may well cook the batteries when running. But, if as you say many Bayliners of similar size are set up this way I have just one more thing to ruminate on.

Click to expand...

I've been meaning to ask some of those same questions, so I'll do it now...

Form the OP's description the house batteries and the furnace are both in the hull extension.

My first question is WHY???

The 4788 has a huge lazarette. The house batteries along with allot of other stuff fits there just fine. The hatches in the lazarette are gigantic as well. That means that you can stand or even kneel in the lazarette and work on things.

Why would anyone go through the trouble to relocate the house batteries? Why would anyone go through the trouble of installing a furnace all the way back there?

The logical place for a hydronic furnace on the 4788 is either in the lazarette or in the forward/outboard area of the port engine.

The hull extension would be the last place on a 4788 that I'd mount a furnace. The problem is that if the hull extension is at the same level as the transom door its only a foot or so above the water line. I cannot imagine getting the exhaust situated so that water does not ingress to the furnace.

Thinking about woodseas post that allot of Bayliners have this setup makes me think. The hull extension while sorta popular is not all that popular. A small percentage at best have this modification. North Harbor Diesel did a large percentage of these modifications. Perhaps they were behind the battery relocation and furnace installation as a sales tool to free up space in the lazarette? I don't know but it doesn't make all that much sense to me. I don't know what I'd put there but it certantly would not be the furnace, and probably not the batteries either unless I had some reason to free up space in the lazarette.