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Nestor
Nestor

Posted: Sat Oct 19, 2013 4:11am

Post Subject: Vaccum port in mudbox?

Hello, I have a 30' cruiser with a BMC engine and a bit of a problem with the raw water cooling system. The mudbox is a 6" long plastic tube, 3" diameter, threaded on both ends, with plastic caps. The line to the gearbox cooler comes out of one cap, the other one is removable to access the strainer. The cylinder is horizontal, it is bolted at the middle to the hole in the hull, and right opposite that, i.e, on the top of the cylinder, there is a plastic hose connection with a valve. I attached a photo. When I bought the boat, there was suction on that hose connection at the top of the mudbox, and it worked as a bilge pump. Very handy, because the channel for the prop shaft is narrow, with less than an inch of clearance either side of the shaft, so no bilge pumps fit in there. Today the suction stopped and I noticed that there was some rubbish in the hose leading to the top of the box. Then removed the whole hose connection and could see the strainer being very dirty. I then opened the mud box and was very surprised by the amount of water that came out, I barely managed to screw the cap back in. Now that the strainer is clean, there is no way I could remove the top and look inside, water gushes in. And when I run the engine there is no suction from that hose connection, on the contrary, water comes out of it at all times. So my questions are: -Is it normal to have a vacuum port out of the mud box? -Now I am thinking that maybe the inlet to the mud box was dirty and that created the suction, could that be the case? -Maybe there is some part that came out when I removed the strainer and I lost it? If the mudbox does not have a vacuum port, would it be a good idea to connect a hose to the suction side of the raw water pump (Jabsco) to work as a bilge pump? I get a lot of water in when the prop is turning, 20-30 litres per hour. The electric bilge pump can cope with that, but the level doesn't go below the shaft and thus the pulleys are submerged and the belts slip. Thanks in advance Nestor

Vaccum port in mudbox?
Tony-B
Tony-B

Posted: Sat Oct 19, 2013 8:41am

Post Subject: Vaccum port in mudbox?

Please email the photo, making sure it shows the hull connection, to Tony@TB-Training.co.uk. If you would rather email it to the magazine so they can email it to me but be prepared for a delay in answering. I will comment at present that as the inlet looks (as far as I can see from that small photo)as if it is well below the waterline so of course water flooded in. I want to see if there is a valve between the "mud box" and hull. I may have more to say about reliability etc.but need to see a proper photo first. Tony Brooks

Tony-B
Tony-B

Posted: Sat Oct 19, 2013 8:46am

Post Subject: Vaccum port in mudbox?

PS, please also include a photo of where the shaft enters the stern tube, also the details of the engine drip tray. If you have just bought the boat I would expect it to fail the next Boat safety examination if what you say about water on the belts is true (unless it is not on UK inland waterways). Also a photo/description of the position of the bilge pump if possible. Tony Brooks

Tony-B
Tony-B

Posted: Sat Oct 19, 2013 1:42pm

Post Subject: Vaccum port in mudbox?

4 huge photo files later, her are my two replies. First of all I have in 50 years of working with boats never seen anything like that. Normally I would expect the small horizontal cylindrical section below and set at right angles to the main body to be a valve with a lever on one end. You would use this to isolate the seainlet when cleaning the strainer. However the second photo clearly shows it has a nut on a thread rather than a lever. I can make no sense of this unless the nut and whatever it is fitted to turn as one. If it does then I expect it will be a 1/4 turn valve. I will come back to the assembly in a short while. I had hoped to get a photo of your stern tube and gland because if as much water as you suggest leaks in when running something is seriously wrong. It could be something as simple as needing to adjust or repack the gland, but it could be something worse. Although I can not see the gland I am struck by the lack of grease on the hull so wonder if you are using the stern gland greaser enough. I can see what appears to be the lower portion of a greaser so suspect you have a conventional packed gland. The Boat Safety Scheme requires an oil tight drip tray under the engine and you do not seem to have one. On a metal boat this will often be metal plates welded across the boat between the engine beds. You are also not allowed a bilge pump in the engine tray because it can then pump oil into whatever you are floating on. I further note that your bilge pump is lying on its side so it is not surprising it is not draining the bilge sufficiently. Back to the mudbox. The whole of the raw water system between the sea inlet and raw water pump is subject to the suction created by the rubber impellor in the pump. As air is far less viscose than water even the slightest airleak can compromise the raw water flow through the heat exchanger. I would NEVER have anything that risks allowing air into the system in that part. You seem to be totally relying upon that valve in the top of the assembly. to stop the raw water pump sucking air rather than water. A blocked strainer could (depending upon design) allow the raw water pump to create sufficient suction to act as a bilge pump but I do not feel this is a good technology. If you look at that nut on the side of the small, lower cylinder you will see the rod it is fixed to appears to have a screwdriver slot in it. This may be an adjustment to restrict the water flow into the assembly and thus build up some suction. If it follows convention then the slot (if that is what it is) indicates the valve is fully open. With the slot horizontal it would be fully closed. This would also give you the means of turning the raw water flow off. I doubt anything has gone missing. I would strongly advise you not to try adjusting the nut and shaft to get the "bilge pumping" working. That way lies potential overheating and expensive engine damage. Make sure the stern gland is sorted out so it does not leak. I fear that you will have to address the drip tray issue and move the bilge pump. If you send a photo of the sterngland I will see if I can give you more help. *** second reply****** Thanks, that is a conventional gland. First of all clean and dry the area around the gland, including the bit where the whole assembly bolts onto the hull. Ten run the engine in gear to see if you can see where the water is coming in. I hope that I am wrong but I gear it could be between the whole assembly and the hull. You just might also have a bent shaft. If the water comes in between the gland and shaft then try adjusting it first. You have a thin and a very thick "nut" around the shaft. Loosen the thin locknut by turning it clockwise to wind it away from the thick gland nut. The gradually tighten the gland nut while you try turning the shaft by hand. When you feel the merest resistance to turning lock the locknut against the gland nut and run in gear. The gland may get a little warm but NEVER hot. If it does then you have either overtightened the gland, it needs repacking, or the shaft is out of alignment. There is a whole section of glands in the maintenance notes on my website. Depending upon design it is also possible, or even probable that the water is entering via the rudder tube assembly. There may be a gland like the stern gland on top or it may have O rings in it, if O rings they are probably worn out. Tony Brooks

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