Bow thruster tubes

[Bunny]

We just had a very helpful Manchester Ship Canal surveyor on board . Got talking .... as you do . He happened to mention that he is seeing quiet a few boats with leaking bow thruster tubes . Some leaks even sink the boats in question. He's doing at least 3 or 4 re sleeving a month . Has anyone heard about this problem . I have heard it mentioned but not to this degree . Cheers . Bunny.

[Greylady2]

I've heard this too, don't take advice from me because I am new to boating.

 

We avoided buying a boat with a bow thruster for this reason and I've read a lot about people just plating over the bow thruster holes/inlets/outlets.

 

 

We have a 57 ft boat and we never need a bow thruster even single handed.

Edited June 25, 2016 by Greylady2

[magnetman]

talking about bowthrusters on here is almost as bad as religion .

 

Anyway. Bowthrusters are ----------------

 

Going back to the op I wonder if these leaking tubes were just random or if there was a bias towards any particular boat builder or equipment manufacturer?

 

Age of boats? I suppose if the tube is thinner stuff than the hull plating then it will go through quicker - how long have bowthrusters been around for on canal boats? Maybe it's the first generation beginning to show their age.

 

 

Typo

Edited June 25, 2016 by magnetman

[gazza]

I witnessed a guy with an outboard powered seamaster 28 move off from Downham Market mooring in the lee of the wind with the aid of a bow thruster. I was stunned! I mean, they are girly buttons on tin tubs but a 28' outboard powered grp is taking the piss!

It really did highlight how piss poor his boat handling was!

[Alan de Enfield]

There is not a 'problem' per se with 'Sissy Buttons' when used correctly and when necessary, however the skipper who has never learned to control his boat without one WILL end up in the Do-Do WHEN it fails.

 

Maybe you should have to take a 'driving test' before you are allowed to buy a boat with a 'Sissy-Button'

[magnetman]

The topic is about the tubes rusting through

(But I do agree !! )

[Dalslandia]

If they are not painted on the inside and outside/inside the boat, they will rust, and galvanic protection is a must too.

[Murflynn]

if it isn't used, fill it with PU foam.

[gazza]

The topic is about the tubes rusting through

(But I do agree !! )

True, not a worry on the seamaster

[Claude]

If they are not painted on the inside and outside/inside the boat, they will rust, and galvanic protection is a must too.

Why would they rust any more than a baseplate will rust? My bow thruster tunnel is unpainted and 11 years old but showed no sign of rust when the boat was out the water last summer.

 

Also, the tunnel needs to be insulated on the inside of the boat otherwise it will condensate. That's much more important than painting it.

[Claude]

if it isn't used, fill it with PU foam.

To a certain extent PU foam will soak up and trap water, and in conjunction with gases/air in the foam cells it really will create a corrosion issue.

[Claude]

We just had a very helpful Manchester Ship Canal surveyor on board . Got talking .... as you do . He happened to mention that he is seeing quiet a few boats with leaking bow thruster tubes . Some leaks even sink the boats in question. He's doing at least 3 or 4 re sleeving a month . Has anyone heard about this problem . I have heard it mentioned but not to this degree . Cheers . Bunny.

I don't think one person's experience necessarily constitutes a phenomenon. I've spoken to plenty of surveyors and marine engineers and none have ever raised it as an issue.

[Bunny]

Yes we have a bow thruster . No I don't use it very ofter ,yes I can can drive perfectly well without it. Yes may even have it covered next time she's twin epoxyed . I just wondered if anyone here had experienced a bow thruster sinking or heard of one . Apparently the problem is with oxygen eating into the tube metal during cavitation whilst the bow thruster propeller is in use , right by the blade . Nothing to do with thin steel or apparently the age of boat or builder the chap said . Anyone heard of this ?. Bunny

[gazza]

Yes we have a bow thruster . No I don't use it very ofter ,yes I can can drive perfectly well without it. Yes may even have it covered next time she's twin epoxyed . I just wondered if anyone here had experienced a bow thruster sinking or heard of one . Apparently the problem is with oxygen eating into the tube metal during cavitation whilst the bow thruster propeller is in use , right by the blade . Nothing to do with thin steel or apparently the age of boat or builder the chap said . Anyone heard of this ?. Bunny

Whoever told you that is not familiar with how corrosion occurs in steel.

 

Have a think about a weedhatch and the amount of time a prop is turning compared to a bow thruster

[Woodsy]

I have been involved in the offshore oil & gas industry for well over 30 years tunnel thrusters are used far more aggressively and for longer continuous periods than on narrowboats, yet have not heard or seen this occurring, I would imagine that there are rare instances of it, but certainly not to the extent that the industry or classification societies are unduly concerned about it.

[Iain_S]

There was one sunk a few years ago on the Forth & Clyde by failure of the bow thruster tube when the canal froze.

Thinking about it, the tube could be a weak point due to difficulty in blacking the interior, presence of more noble metals than steel, aeration of water while in use, etc.

[gazza]

Corrosion is a many faceted subject, but in the main this is the primary cause of corrosion on a tin tub.

 

http://metalcorrosion.blogspot.co.uk/2010/11/differential-aeration-corrosion.html?m=1

 

I thank the original poster of that blogspot for saving me some flaky typing on my phone.

[mross]

I spoke to an old hand who knows of four bow thruster leaks and one that sunk the boat. The tube was, in each case, made of welded tube and the weld failed. If seamless tube was used failure would take much longer and be slower. On passenger ships, where you can stand up in the 'tunnel', it is normal to weld in a liner. The liner is of stainless steel and can be renewed. It's also protected with sacrificial anodes. If I built a boat with a thruster I would want a watertight bulkhead aft of the thruster and a designated bilge pump and alarm. The motor could be off to one side and fitted on a sleeve that came above the waterline so that the seal could be serviced afloat.

 

Does anyone have any old photos of narrowboat sterns craned out of the water under a bridge - used to be quite common practice, presumably they chose a bridge with no traffic!

[Dalslandia]

Yes we have a bow thruster . No I don't use it very ofter ,yes I can can drive perfectly well without it. Yes may even have it covered next time she's twin epoxyed . I just wondered if anyone here had experienced a bow thruster sinking or heard of one . Apparently the problem is with oxygen eating into the tube metal during cavitation whilst the bow thruster propeller is in use , right by the blade . Nothing to do with thin steel or apparently the age of boat or builder the chap said . Anyone heard of this ?. Bunny

Yes we had that problem, right under the prop, but was cured, last 10-15 years it have not come back

[magnetman]

I spoke to an old hand who knows of four bow thruster leaks and one that sunk the boat. The tube was, in each case, made of welded tube and the weld failed. If seamless tube was used failure would take much longer and be slower. On passenger ships, where you can stand up in the 'tunnel', it is normal to weld in a liner. The liner is of stainless steel and can be renewed. It's also protected with sacrificial anodes. If I built a boat with a thruster I would want a watertight bulkhead aft of the thruster and a designated bilge pump and alarm. The motor could be off to one side and fitted on a sleeve that came above the waterline so that the seal could be serviced afloat.

 

Does anyone have any old photos of narrowboat sterns craned out of the water under a bridge - used to be quite common practice, presumably they chose a bridge with no traffic!

I wonder if impact of the front of the boat with lock entrance walls could actually cause a weld failure due to flexing of the hull?

Yes we had that problem, right under the prop, but was cured, last 10-15 years it have not come back

Is it to do with the propeller being too close to the tube wall?

[matty40s]

I have seen a few of these, one on a boat less than 10 years old, which would have sunk it within a few months.

 

Some boats have a couple of bolted bars you can remove to black the inside of the tube -

 

This is a one year old boat!!!

 

some cheaper boats have a steel mesh over, welded to the hull - so blacking cannot take place.

 

The damage is usually caused along a weld - or in a circular arc where the prop cavitation and blasting silt and small stones around the tube leads to wear.

 

Best solution is to weld two plates over the end and sell the device on ebay.

[magnetman]

Yes and maybe the tube itself could be used as a beer cooler?

[bizzard]

I call em ''bank eroders and vole evicters''. Because of the mess they've made to some of the natural banks on the river Stort where people use them to shove off with, big gouges torn out and has made many bank living creatures homeless.

[Murflynn]

Whoever told you that is not familiar with how corrosion occurs in steel.

 

Have a think about a weedhatch and the amount of time a prop is turning compared to a bow thruster

cavitation does not occur in a weedhatch, even if the 'plug' is absent.

[matty40s]

I have been involved in the offshore oil & gas industry for well over 30 years tunnel thrusters are used far more aggressively and for longer continuous periods than on narrowboats, yet have not heard or seen this occurring, I would imagine that there are rare instances of it, but certainly not to the extent that the industry or classification societies are unduly concerned about it.

 

The boats in these scenarios are not generally moored in shallow, silty ditches where mud and small stones are pulled into the tube when mooring up, and then act like gritblasting material rotated at high speed when used to shove off the same bank next mooring.