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Old 08-09-2016, 09:12 PM   #1
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Thru-hull and bilge pump issue

On a long, multi destination cruise on my 2003 Mainship 30 Pilot the this past week I became nervously aware that the thru-hulls for the forward and midship bilge pumps sit about 3 inches above the waterline. What is concerning is both pumps sit below the waterline by several inches and the hoses have no loops. In fact the hoses have no incline at all and pretty much run directly down into the bilge below the waterline. This is obviously an original design flaw by Mainship. I probably should have caught it when I was purchasing it last November but I'm even more surprised my surveyor didn't flag this. Another surprising thing was the previous owners must have recognized the water accumulation but never addressed it for 13 years. The way it came to light to me was no matter how much I dried the bilge there always seemed to be more water in the bilge each time I arrived at a destination. On one part of the cruise I leaned over the gunnel to check the thru-hulls relationship to the waterline and realized the thru-hulls were often submerged. The water seems to back flow slowly if it's calm and faster if its rough. Since the bow on my Mainship rides fairly high in the water when cruising, the water doesn't pool and initiate the forward or midship pumps, it collects at the transom where it's kind of hidden unless the rear deck hatch is opened. At one point the high water pump at the stern initiated and the alarm sounded for a few seconds. It's a high water pump but with the bow up it really doesn't take a lot of water to raise the float switch as the water pools at the transom. When the boat slows, the water runs forward and most of it gets pumped out by the midship pump at the rear of the engine. I am planning to remedy the issue with a vent loop for each that rises well over the waterline. I am hoping for some advice from someone who has dealt with this type of issue on their boat even if it is not a Mainship. I have included a couple of images below for reference. Thank you.





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Old 08-09-2016, 09:36 PM   #2
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I'd also want seacocks on those.
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Old 08-09-2016, 09:38 PM   #3
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Wondering if there is a spec for height of the loop to avoid to avoid the vent. Looks like the loop could go all the way up to the wash board (maybe a couple of feet) before turning down. Would rather not have a vented loop if it wasn't needed.

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Old 08-09-2016, 09:39 PM   #4
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Amazing! You'd swear some builders are not only building their first boat but the first bost ever build. Over and over again.

There are basically two ways to correct this. One is to add a vented loop to the pumps hose. Or add a check valve.

With a check valve you will end up with a drier bilge. But you run the risk of the valve sticking shut at the wrong time and they restrict the flow. Which is already pretty poor in all but the very largest pumps.
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Old 08-09-2016, 11:19 PM   #5
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My preference would be for the vented loop. If you want a dryer bilge, add a small pump with a small discharge hose that will be holding less water when the pump shuts off.
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Old 08-09-2016, 11:20 PM   #6
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A check valve will stick open eventually with the normal bilge debris. Even sooner on a shower sump. If they stick open when the outlet is under water, water runs into the bilge. If the bilge pump switch fails, the boat sinks.
A vented loop breaks suction. It causes the water to exit the hose, either over the side or to the bilge pump. If you want a safe dry bilge, use a vented loop, fix any leaks, and mop up the water.
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Old 08-09-2016, 11:20 PM   #7
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Add a raised loop up in the space outboard of the water pump, up where the purple hose is. No need for a vented loop. If the boat sinks 'that far' a vented loop is the least of your needs. Just raise the hose up so the top is a foot above the thru hull.

like this:
One of the problems with these bilge pumps installed like this is the hose drains back into the boat after it shuts off. Nothing like wanting a dry bilge then having the hose contents siphon back into the boat. The solution is to make a raised loop as close to the pump, so the amount of water that flows back into bilge is minimized. In your case I would consider installing the hose straight up from the pump on the FWD bulkhead, up to a loop and running it down and over to the thru hull to make as short a run to the loop, and have longest run after loop to thru hull. Make sure you have a constant downward slope in the side from the loop to the thru hull that way it drains completely and doesn't let algae grow in the hose to clog it up.

I wouldn't depend on a thru hull mounted checkvalve to stop water ingress. Checkvalves get gunked up with bilge scum. (although your bilges look pristine) Freely moving parts and seawater don't last long.
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Old 08-09-2016, 11:31 PM   #8
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Cappy is right, nice bilge. I'll never publish a photo of my bilge in trawler forum. I'd never hear the end of it and that's after I just did an engine out cleaning!
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Old 08-09-2016, 11:38 PM   #9
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Man that is just wrong. Check valves will keep the bilge drier, loops are cheaper but increase head pressure on the pump decreasing efficiency. It's hard to believe the builder installed the hoses that way, harder to believe the surveyor didn't catch it... A few years ago there was a Bayliner that sank at the dock when waves started forcing water through a thru the hull, a check valve installed would have prevented this....
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Old 08-10-2016, 06:35 AM   #10
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I agree with Cappy, and also feel check valves are a really bad idea. Another foot or so of head is not a big deal. And no seacocks either; you want as clean a run with as few connections and potential obstructions as possible. If you like, you can add clamshells to the outlets to divert water from them while under way. Personally I'd add a bilge pump to the rear in addition to the high water pump.

It may not be a Mainship caused issue, those hoses look newer and perhaps not OEM; higher quality than most OEM BP hoses.
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Old 08-10-2016, 06:58 AM   #11
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Quote:
Originally Posted by HopCar View Post
Cappy is right, nice bilge. I'll never publish a photo of my bilge in trawler forum. I'd never hear the end of it and that's after I just did an engine out cleaning!
Me too...
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Old 08-10-2016, 07:17 AM   #12
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Use a diaphragm style pump to get the last water out. I did that on my old Mainship 34 and it worked great.
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Old 08-21-2016, 08:32 AM   #13
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I wonder if the original pumps had check valves. But in any case once you fix the pump hoses, secure your wires.
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Old 08-22-2016, 07:18 AM   #14
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"This is obviously an original design flaw by Mainship. I probably should have caught it when I was purchasing it last November but I'm even more surprised my surveyor didn't flag this."

It should have been caught at the first purchasers pre-delivery survey.
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