Bow Thruster Anode

[Naartjie - Duck Hatch]

I'm having my NB Blacked, including the base plate (2 pack). The surveyor suggested I put an anode in the Bow Thruster tube.  His advice was don't over anode so just one.

Any comments 

 

Edited February 26 by Naartjie - Duck Hatch

[ditchcrawler]

8 minutes ago, Naartjie - Duck Hatch said:

I'm having my NB Blacked, including the base plate (2 pack). The surveyor suggested I put an anode in the Bow Thruster tube.  His advice was don't over anode so just one.

Any comments 

 

I had one put in and at the next blacking, 4 years it was almost gone, the same 4 years latter 

[BEngo]

Bow thruster tubes are well known for corroding through.  A magnesium anode should slow this, but may not entirely prevent corrosion.

Try to get a low profile one to minimise the loss of thrust caused by an obstruction in the tube.

N

 

 

 

[blackrose]

I don't have an anode in my BT tunnel and 19 years from new it's still in perfect condition. Perhaps I've just been lucky, I don't know.

 

Are they well known for corroding thorough? There may be more, but in 18 years on this forum I think I've only heard of one case. The BT tunnel should be far enough under the waterline that it doesn't suffer unduly and I'd have thought there are plenty of other parts of a hull that are far more susceptible to corrosion.

 

I'm sure someone will start talking about cavitation around the tips the BT prop blades. Again I haven't seen that on mine, but if that happens it's a different effect and won't be ameliorated by the galvanic protection of an anode. Thinking about it, could having an anode in the tunnel cause more cavitation?

Edited February 27 by blackrose

[LadyG]

The bow thruster tube on my boat was welded up before my ownership, I assume it was corroded, there was some rumour of it sinking on its mooring, no other evidence of the incident 

[blackrose]

Ok, but how many examples of baseplates, uxter plates, hull sides, etc, corroding through can we find? I'm sure there are many more than BT tunnels even if we weighted the results to take into account the fact that perhaps only 10% of narrowboats have BTs. I obviously don't have the data but anecdotally it seems like hulls are more susceptible to corroding through in other places. 

[BEngo]

The difference between BT tubes and hull sides  etc.  is that the tube is often not as thick as the sides.  It doesn't corrode much faster but it hasn't  got the material to last as long.

 

This is compounded by a tendency not to paint them properly ( not at all on some blackings and some boats with fixed screens  cannot be got at  except by removing the BT unit) and by the stresses locked into the tube by rolling it and then resistance welding the seam.  Drawn tubes would be better (and could be thicker)  but they are much more expensive.

 

 A final factor is that many are used close to the bank and the tube gets blasted by loose gravel, damaging any paint.

 

N

 

[ditchcrawler]

I know of two. The first a boating friend on a Steve Goldsbourgh boat, I can't remember who the second was.

[blackrose]

45 minutes ago, BEngo said:

The difference between BT tubes and hull sides  etc.  is that the tube is often not as thick as the sides.  It doesn't corrode much faster but it hasn't  got the material to last as long.

 

Ok, mine is 6mm thick, the same as the sides. I didn't realise they were made thinner than that on steel boats.

25 minutes ago, ditchcrawler said:

I know of two. The first a boating friend on a Steve Goldsbourgh boat, I can't remember who the second was.

 

And how many boats do you know of which have corroded in other places?

[David Mack]

I would have thought rudder tubes to be the bigger risk. Far more boats have them than bowthrusters, and most straddle the waterline where corrosion is worst, and they almost never get blacked since you have to unship the rudder first, and even then access for derusting and painting is very limited.

[BEngo]

15 minutes ago, David Mack said:

I would have thought rudder tubes to be the bigger risk. Far more boats have them than bowthrusters, and most straddle the waterline where corrosion is worst, and they almost never get blacked since you have to unship the rudder first, and even then access for derusting and painting is very limited.

Dead right.  Not only are they neglected internally a lot are also rusting away from the accessible side as a result of a pool of water on the counter plate in the engine bay. 

 

N

 

[matty40s]

49 minutes ago, David Mack said:

I would have thought rudder tubes to be the bigger risk. Far more boats have them than bowthrusters, and most straddle the waterline where corrosion is worst, and they almost never get blacked since you have to unship the rudder first, and even then access for derusting and painting is very limited.

Rudder tubes dont get the high speed forced gravel and silt that the bow thruster tubes endure.

Coupled to the fact that many Collingwoods were launched without any blacking whatsoever in the tube... 

On 27/02/2024 at 03:11, blackrose said:

 

Are they well known for corroding thorough? There may be more, but in 18 years on this forum I think I've only heard of one case.

Yes, we see lots.

Most get the ends welded over as it is a big job to retrofit a new tube

Luckily for the owners, most are in separate lockers with bulkhead above waterline.