The Proper Fixture for Water-Cooled Stern Tube

[RigelKent]

After days of looking I found the leak that has been threatening to flood my new (to me) Beneteau 311. The cause was I imagine to be very substandard plumbing of the stern tube cooling fixture. I am wondering if someone could tell me the right (and safe) way for this to be done.

I had originally thought it was the rubber boot/shaft seal. This is a shaft-drive Volvo Penta MD2020. There is a through-hull behind the engine with a hose that connects to the stern tube behind the shaft seal to cool the shaft.

I pumped grease under the shaft seal but that didn't solve the problem. Every time I ran on the engine (20 minutes or so on the river here) the leak got worse. Today I arrived to find the bilges flooded and my floor boards floating. Engine fortunately still high enough not to be flooded.

Where the hose connects to the stern tube it looked like someone had sealed the connection with red "wax. "As it turned out the "wax" is some kind of plastic that had been melted on in an attempt to seal a corroded nipple (clamped in the hose) to the stern tube. That plastic weld was all that held the hose in the stern tube. Running on the engine caused the stern tube to warm up and the red plastic (with nipple and hose) came away, causing a fountain of water about the width of a pencil to spout, flooding the bilges.

Temporary solution - I have a threaded bolt pressing down on the spouting hole, with the weight of some spare conduit pressing down on it to supply pressure. Surprisingly this has completely stopped the leak for now - Joy!

What I am wondering is, how should this fixture have been done in the first place? Where am I likely to get a replacement nipple? Is the nipple supposed to thread into the hole in the stern tube?s there any bonding material that's used? Are the any specific types/materials I should look for/look out for?

Thanks for any replies or thoughts on the proper way this should be done so I can keep the boat dry again.

The previous owner put a considerable time and resources into having engine work done and had the the through hull that feeds sea water to the stern tube replace (it had been badly corroded. However I'm very surprised that the actual connection to the stern tube was left this way. If I had put to sea, instead of sailing in the river, this could have turned very serious...

[Vasco]

This is a common problem with Beneteaus. They used brass nipples and they turn to dust fairly quickly. The nipple threads into the hole in the stern tube. Replace with bronze nipple. I think it's 1/4". You may have to tap the hole for the threads again.

[RigelKent]

Thanks very much Vasco for your knowledgeable reply. Off to find a brass replacement.

[RigelKent]

For benefit of anyone having a similar problem with the Volvo Penta stern drive

Beneteau has a repair kit for this problem that includes a tapered threaded hose-end fitting (original) and some space-age epoxy:
Part : 460646 STERN TUE FITTING REPAIR KIT, $34.93

I am still trying to source a compatible hose ending, threaded and tapered, in bronze.

[Wotname]

Originally Posted by RigelKent

.....I am still trying to source a compatible hose ending, threaded and tapered, in bronze.

If you can't source a bronze one with the correct thread and hose barb AND you can identify the thread (or have a brass one to copy), I might be able to knock one up for you from bronze as I have a access to a lathe if necessary. Come to think of it, I could probably do a stainless one as we would have more chance of having suitable 316 stock than suitable bronze stock.

Does anyone know what material the stern tube on a Beneteau 311 is?

[RigelKent]

The local Beneteau dealer is helping source stock parts, so I should have a model to copy from. Haven't had any luck identifying the stern tube material as yet, but I've asked the dealer if he knows.


I have a picture of the tapered, threaded spar/nipple and hose attached (first picture). The red blotch is piece of the red glue that was used to keep the hose end attached to the stern tube. There were no other parts or components (other than hose clamps, not shown(



The second picture (diagram) is what the dealer thinks could be replacement parts. If the diagram represents an improved design, and is compatible with the Oceanis 311, I am wondering if it will fit into the existing threaded hole in the stern tube, or would it require being welded in place? The diagram shows a sea cock with handle. It's hard imagine how this will connect with a 1/4 inch hose!

[RigelKent]

Parts shown in the diagram need to connect to the stern tube at point 20 in the diagram.

[Vasco]

A buddy got a nipple from a hardware store. I bought a couple of spares but have not needed them yet. If I remember right the shaft log is some sort of composite material. Just get another nipple and tap some threads in the shaft log if the nipple is larger than the original.

[RigelKent]

Been looking for hardware stores too - Couplers in Malaga may have something roughly compatible. Thanks for the suggestions and kind offers.

[Wotname]

Thinking more about it and especially if the stern tube is composite, maybe the best solution is to get a bronze hose tail where the diameter of threaded end is slightly larger then the existing flogged out hole in the stern tube and then just drill and tap the stern tube to suit the thread on the hose tail.

If done while still in the water, the incoming water stream will even carry away the swarf from the tapping and so keep it out of the Volo shaft seal.

[RigelKent]

The tube aft of the rubber seal is metal for a hundred cm or so (where the hole is). Aft of that the tube runs into a composite skeg integral with the hull.

For the moment, have a ratchet strap holding in a wooden plug down into the leaking hole, under some tension. Was a trick to get the strap under the stern tube - can't slip it around as the hull is in the way. Used a strip of styrene to slide under the tube from one side to the other. Used the strip to feed a length of string around. Used the string to feed the ratchet strap. Then ratcheted in a wooden dowel with end shaped roughly like the original hose tail/barb.

Have a repair kit on order, but will need to be out of the water to apply he space-age epoxy. If the barb can't thread I'll have to re-thread for a larger hose end. Couplers in Malaga has some bronze and SS parts that should do.

Can a hole be re-threaded in the water? A pencil thin fountain comes in through the hole. Would be hours before a serious amount of flooding occurs.

[Wotname]

Originally Posted by RigelKent

.....
Have a repair kit on order, but will need to be out of the water to apply he space-age epoxy. If the barb can't thread I'll have to re-thread for a larger hose end. Couplers in Malaga has some bronze and SS parts that should do....

OK, I wonder if the original part screwed in or was glued in. If it was screwed in then why the space-age epoxy - just for sealing???

Originally Posted by RigelKent

..... Can a hole be re-threaded in the water? A pencil thin fountain comes in through the hole. Would be hours before a serious amount of flooding occurs.

If the tube is a hard metal (SS or Silicon Bronze or similar) then probably not because one would likely need to use a taper tap before the intermediate or bottoming tap and the shaft would get in the way.

If the tube was soft(ish) like composite, it may be possible to tap the hole starting with a bottoming tap. This way the shaft would not be an issue. It would take a bit finese but perhaps doable.

It looks like getting the original parts and glue might be the way to go after all.

[RigelKent]

From what I read there are only a couple of turns on the thread. Don't know if the part was originally glued in or not, but the repair kit includes the special epoxy (perhaps in hindsight after recognition of problems with the design).

Anyway, with the hole plugged its relatively secure and sail-able close to home.

[RigelKent]

Just an update for the sake of posterity: The kit(s) arrived at the dealers. The spur is brass and isn't threaded at all. The 'space age' epoxy was missing from the kit - apparently it is a hazardous substance that can't be shipped via air. Don't know if Beneteau just didn't ship it or if it was removed by customs? The kit also contained an instruction sheet and a very nice piece of gritty sand paper. The dealer is trying to source the special epoxy locally.
Moving on...

[Wotname]

Let's look at the good news here. The brass spur can be tossed aside as no one wants brass below the water line BUT before we put it in the bin, we can use it as a sample from which to turn up a bronze or SS one . I suspect the special epoxy is no better than some epiglue or just run of the mill boat epoxy.

I am away this coming week but maybe we can catch up next weekend and work out if we can make a better spur and admire the very nice piece of gritty sand paper.