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Bow thruster tubes


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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?

 

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

 

The tube wall should be the same thickness as the hull, the weld should not give up,

erosion and corrosion can be from sand and gravel passing the propeller at the bottom, I think, but also just good old bad luck with galvanic should have a couple of small anodes on the inside of the tube

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In terms of welds "giving up", I don't see what possible difference there is between a weld around a bow thruster tube and a weld anywhere else on a steel boat. Either it's a proper weld with good integrity or it isn't.

 

As for prop cavitation causing bow thruster tubes to corrode and leak it sounds like more "canal purist folk myth" to me.

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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?

I guess so if the welds were no good in the first place, but the same would go for any other weld at the bow area.

 

 

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

No. The prop blade tips on a bow thruster are supposed to be within a few mm of the tunnel wall.

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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?

I guess so if the welds were no good in the first place, but the same would go for any other weld at the bow area.

 

 

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

No. The prop blade tips on a bow thruster are supposed to be within a few mm of the tunnel wall.

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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 -

20160622_133359_zpst59m9njs.jpg

 

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.

Looks like that one year old boat was never (or very badly) blacked.

 

I wonder why these small stones and silt have never worn or affected my bow thruster tunnel or any others I've seen?

 

In my personal opinion the best advice is to stop listening to unsubstantiated heresay and nonsense and just carry on enjoying your boating with whatever equipment you want on your boat, whether that includes a bow thruster or not.

  • Greenie 1
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I didn't mentioned cavitation.

 

exactly.

 

he did, and you ignored it.

 

cavitation is effectively the creation of bubbles of water vapour in a near vacuum, which is probably never experienced with the propeller but will very likely happen with an impeller set within a tube of tight tolerances.

 

do you know that cavitation conditions are not the cause of corrosion or 'corrosive erosion'? cavitation certainly results in the erosion of blade tips of a fast moving propeller in fast vessels.

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exactly.

 

he did, and you ignored it.

 

cavitation is effectively the creation of bubbles of water vapour in a near vacuum, which is probably never experienced with the propeller but will very likely happen with an impeller set within a tube of tight tolerances.

 

do you know that cavitation conditions are not the cause of corrosion or 'corrosive erosion'? cavitation certainly results in the erosion of blade tips of a fast moving propeller in fast vessels.

Correct.

 

But the chances of cavitation being the cause of tube failure in a bow thruster tube are vanishingly small.

 

I think the problem is the confusion between cavitation and ventilation hence why I used that as an example.

 

Long and the short if it is it will be down to a combination of corrosion and wear. Nothing more, nothing less.

Couple it with the possibility of undersized tube and it's all likely to fall.

 

Another poster mentioned it should be in a watertight bulkhead - of course they should, but as ever, ditch crawler engineering and marine engineering are often light-years apart.

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Correct.

 

But the chances of cavitation being the cause of tube failure in a bow thruster tube are vanishingly small.

 

I think the problem is the confusion between cavitation and ventilation hence why I used that as an example.

 

Long and the short if it is it will be down to a combination of corrosion and wear. Nothing more, nothing less.

Couple it with the possibility of undersized tube and it's all likely to fall.

 

Another poster mentioned it should be in a watertight bulkhead - of course they should, but as ever, ditch crawler engineering and marine engineering are often light-years apart.

The tube and thrusters on our boat are in a watertight compartment and fitted with its own bilge pump so all are not the same

 

Ray

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Looks like that one year old boat was never (or very badly) blacked.

I wonder why these small stones and silt have never worn or affected my bow thruster tunnel or any others I've seen?

In my personal opinion the best advice is to stop listening to unsubstantiated heresay and nonsense and just carry on enjoying your boating with whatever equipment you want on your boat, whether that includes a bow thruster or not.

Very dangerous attitude.

 

Nothing I refer to regarding boat maintenance or painting is unsubstantiated, all of it has been experienced on a vast range of canalcraft, I share it to help people be aware of issues they may otherwise not know about.

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As for prop cavitation causing bow thruster tubes to corrode and leak it sounds like more "canal purist folk myth" to me.

Proper ship bow thrusters have a steel "wear ring" to protect against "cavitation attack", e.g.

 

http://www.hercules.org.uk/bow-thrusters/kt120.html

 

http://www.gallois.be/ggmagazine_2014/gg_02_03_2014_54.pdf (p. 3 has a picture)

 

http://www.thmarco.com/en/productos/eck-Equipment__7/item/ide-thrusters__16.html

 

http://brent-occ.nl/en/offshore/straalbuizen-boegschroeven-2

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If it is a problem that several NB's and or WB's have sunken because of this, it might be a small problem, at least for those owners.

 

question is how thick was those tubes from start.

 

was they painted in and outside of tube

 

offer anodes inside the tube?

 

it must be a problem for insurance companys

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I had a BT tube fitted when my boat was built in 2007. My thinking was that whilst I didn't need a BT to put my boat where I wanted, in the future I might have more need of it. I later realised that if I couldn't handly my 57' boat I should get a smaller one.

 

The tube gave rise to a lot of condensation in the compartment under the well deck and made it difficult to access the water tank taps behind it. So two years ago I had it taken out. It was made of solid 8mm drawn tube and showed next to no corrosion, but then it hadn't been used. The engineer doing the work to remove it said that a lot of BT tubes, particularly older ones, were made from thinner formed plate welded along the seam. These were the ones he had seen that leaked.

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The tube and thrusters on our boat are in a watertight compartment and fitted with its own bilge pump so all are not the same

 

Ray

Every Tyler/Wilson shell with a BT tube has a bulkhead aft of it to prevent water entering the cabin bilge in the event of tube failure. IIRC, there's a mesh screen bolted over the tube ends to allow entry for blacking or just inspection.

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I was more looking at the failure to black the tube at all.....

It's not what you'd call a quality job is it?! I'd bet it's quality money though!

 

Absolutely highlights all I've said in the past about the varying standard of fabrication, while there are no doubt some highly talented and skilled fabricators building canal boats, there are still plenty that have graduated from the skip repair business!

The vertical weld looks to have been done downhill too, naughty naughty ;)

 

ETA, what thickness is it? The weld profile is very small.

Edited by gazza
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A decent welder by the look of it smile.png

The cable tied on mesh sums up the gulf between some of the builders out there.

 

Sam springer wouldn't have let that go smile.png

 

OTOH, the cable ties mean that can be removed easily for blacking clapping.gif

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OTOH, the cable ties mean that can be removed easily for blacking :clapping:

Except for the fact that the manufacturer didn't bother...

This yet again puts Claudes go and boat and stop worrying about non facts comment into clarity.

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if the boat ovner, buyer didn't demand the builder to paint the damn tube, the poor worker at the builder will nor bother

On the other hand, the fabrication work is of poor quality and the fact they couldn't be arsed to paint the tube sums up to me the rest of the standard the boat is built to.

 

If the profit or loss for the builder hinged on not painting the tube they are in dire straits.

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