Thru Hulls and Centerboard Trunks

hawkeye · Post by **hawkeye** » Tue Nov 08, 2005 1:34 pm

Instead of putting a thru hull for a bilge drain to the outside of the hull, I was wondering if it is okay to put one into the centerboard trunk. There is plenty of clearence, and and the appearence is better. It is also a shorter hose run from the bilge pump. Anybody...Anybody?

Invitation · Post by **Invitation** » Tue Nov 08, 2005 2:14 pm

We used to do this in our centerboard boat and it works fine for nusiance water but........ if the water ever reaches the top of the trunk, your boat sinks.

hawkeye · Post by **hawkeye** » Tue Nov 08, 2005 3:20 pm

I'm pretty new to the boating world, so let me try to understand this: If the water level is above a thru hull, it will no longer be able to pump out?...or does it just have to do with the water line itself. I read something that tim wrote about making sure his sink drain was above the water line or the sink wouldn't drain properly...is this the same concept as the thru hull not working. Somebody clear me up on this.

Invitation · Post by **Invitation** » Tue Nov 08, 2005 3:55 pm

If your run a line from your pump to a level at the waterline, the pump will work fine till it stops, then the water will siphon back into the boat, fill it up till the pump turns on again and around and around you go till your battery dies then the boat sinks.

If you run a line from your pump at or below the waterline you must have a section of the hose higher than the waterline with a siphon break installed at the highest point in the line so it cannot siphon back into the boat ( as long as the siphon break is working properly).

Best solution is to pump the water out as high as possible and don't let the hose get near the water so it cannot siphon at all.

Pumping it into your centerboard well will work fine until the water reaches the end of the pump hose then you start the siphon problem and down she goes.

Best solution I have for you is KISS.

Keep It Simple Stupid

Hope this helps.

catamount · Post by **catamount** » Tue Nov 08, 2005 4:31 pm

On my San Juan 21, Quasar, the cockpit drain goes to the centerboard trunk. Basically, the cockpit sole rests on top of the after portion of the trunk, and the drain is simply a hole through the sole into the top of the trunk cavity. Sometimes wave action will send geysers up the hole.

Quasar doesn't have a bilge pump (nor much of a bilge, for that matter); we just use a bailer, sponge, and bucket.

If you are dumping your bilge pump outlet into the top of your centerboard trunk, which presumably is above the waterline, then you shouldn't have to worry about a siphon break.

Regards,

hawkeye · Post by **hawkeye** » Tue Nov 08, 2005 6:24 pm

I totally understand, but shouldn't there be a check or a one way valve somewhere? The bilge pumps the water out, and the valve keeps it from coming back in right? Or when the pump has pumped 'er dry can you just shut a sea cock? Is that practical? Here is a qoute from Tim: "After winter storage the first year, the check valve I installed in the automatic pump discharge failed, so I removed it completely. Those things simply never work out, but I suppose it was worth a try."

Post by **Tim** » Tue Nov 08, 2005 6:35 pm

The problem with check valves in bilge pump setups is that they always seem to fail. There doesn't seem to be a reliable version available, but if you or anyone knows of a source for one suitable for this use, I'd love it hear about it. It doesn't seem like it should be so tough, yet not only are there few check valves in the marine catalogs, they don't last long either. In any case, I'd never rely on simply a check valve to hold seawater out in the worst case scenario.

The check valve I originally installed in Glissando's auto bilge pump discharge failed the first year, as I recall. I hoped it would prevent the backflow down the hose after the pump shut off. Now I just deal with this small amount of water.

hawkeye · Post by **hawkeye** » Wed Nov 09, 2005 1:11 am

I'm not in a salt enviornment? I know of two types of check valves: there's the rubber flap plastic, and the spring loaded brass. These are common types in the plumbing envioronment. Why can't I just use one of these?

Post by **Tim** » Wed Nov 09, 2005 7:55 am

You can certainly try. What tends to happen, though, is gunk and debris in the bilge water clogs up the springs or balls in a spring-loaded check valve, preventing proper operation. The same thing might affect a rubber flap-type (which certainly is not up to the task of outside water-withholding). Bilge water, even in a clean boat, is never very clean. It may be better in fresh water than saltwater, but it seems that the smallest particles can adversely affect the mechanisms of a check valve; they can clog in the open position, which could be terrible if the outlet were below the waterline, or can clog in the closed position (like mine did), which means the bilge pump can't effectively pump out.

For this reason, I would never rely on either of these to keep out a backsiphon or other inflow from the surrounding water, as with a discharge through hull located near or below the waterline. For preventing nuisance backflow from the hose length, either would be OK...until they weren't. A brass shoreside plumbing check valve might be an acceptable choice in a freshwater environment, and would probably be of better quality than the plastic alternatives that one could consider with saltwater. I don't know of a bronze check valve that's available, at least not in applicable sizes.

Bilge pump outlets belong well above the waterline, in my opinion. Anything else is a setup for potentially serious trouble. How you envision using and storing the boat factors in to your choices as well, of course. A trailer-sailed boat might be able to get by with an installation that would never pass muster (to me) in a constantly floating boat.

Too many boats with bilge pump installations that took the "easy and convenient" route have ended up submerged at the dock. Is this a risk worth taking? Even at best, every hose and through hull fitting, regardless of location, is tempting fate on a boat. So one ought to try to do what one can to minimize potential pitfalls.

For a deep bilge or long, uphill hose run, the best sort of pump isn't one of the cheap centrifugal ones that we normally use, but instead an impeller or diaphragm pump, remotely located, with an intake hose located in the bilge. These pumps don't allow backflow through the pump, and are longer-lasting and better-pumping in the first place. Of course they cost several times as much as the standard pumps. The benefits, though, are many--not the least of which is the fact that the pump body and wiring can be located well above the damp, nasty environment of the bilge.

bcooke · Post by **bcooke** » Wed Nov 09, 2005 9:01 am

Bilge pump outlets belong well above the waterline

Does the counter on a Triton constitute "well above the waterline"?

-Britton

Post by **dasein668** » Wed Nov 09, 2005 9:04 am

bcooke wrote: Does the counter on a Triton constitute "well above the waterline"?

No!

Post by **Tim** » Wed Nov 09, 2005 9:27 am

bcooke wrote:Does the counter on a Triton constitute "well above the waterline"?

No...that'd be the center of the transom!

Admittedly, the counter is a compromise solution, not ideal. Typically, it doesn't cause a lot of problems...but we know of the exceptions!

bcooke · Post by **bcooke** » Wed Nov 09, 2005 10:38 am

I can't tell you how much I hate the placement of my bilge pump outlet. It is SO unladylike to have a stream of water exit the middle of the transom. Effective maybe, but I shudder when I think of it.. or when someone brings it to my attention!

-Britton

Post by **dasein668** » Wed Nov 09, 2005 11:17 am

Tim wrote:Admittedly, the counter is a compromise solution, not ideal. Typically, it doesn't cause a lot of problems...but we know of the exceptions!

Yes, and those exceptions are not much fun!

Allen · Post by **Allen** » Wed Nov 09, 2005 11:44 am

Slightly of topic, but related. :)

In This Old Boat by Don Casey there is a section dealing with bilge pumps and nuisance water. His solution was to have two pumps, a small one which used a reducer to take the out put hose diameter down to 1/4 inch (if memory serves). This small capacity pump was located at the lowest point in the bilge and removed nuisance water. The small diameter hose allowed only a small amount of water to back-flow into the bilge. Mounted slightly above the nuisance pump was a high capacity pump with a normal size discharge hose which was designed to deal with serious bilge water. The small pump was on a float switch and the high capacity pump was on a manual switch. This always seemed like the best way to deal with nuisance water to me.

Regarding discharge hose exit from the hull, I prefer on the transom's center-line. This generally keeps the exit point out of the water regardless of which tack you are on.

hawkeye · Post by **hawkeye** » Wed Nov 09, 2005 11:58 am

So my plan of attack is to install a rule 1500 with a bronze check valve into the aft portion of a centerboard trunk...as high as I can put it. Now that I have patched that transducer hole, my water intake should me minimal at the dock. Thanks guys!

hawkeye · Post by **hawkeye** » Wed Nov 09, 2005 4:11 pm

Funny thing...the city water guy just came into my studio to do an inspection (which I passed), and I asked him about all the different check valves out there that he had seen. There is a valve that has a cone shaped pressure fitting that is adjustable...it is 3/4 size and comes in plastic and bronze. He said the cone is less prone to failure. Anybody...Anybody? I'm goggling like a mad man. hehe