boat hull

[MtB]

Thanks for the replies, as someone who has a degree in chemistry I feel somewhat silly I didn't think about the lack of oxygen the further down you go and therefore the less corrosion! Goes to show just how useful this forum can be! Thanks!

 

This is of course complete bollocks, or bottom-feeding fish would not survive in the canal. I suspect there is plenty of oxygen in the water at all depths.

 

How could one test this hypothesis?

 

[Neil2]

 

This is of course complete bollocks, or bottom-feeding fish would not survive in the canal. I suspect there is plenty of oxygen in the water at all depths.

 

How could one test this hypothesis?

 

I agree - people use this reasoning all the time but what evidence is it based on? Most narrowboats draw about two foot - so there's not enough oxygen to cause rust at that level?

 

And if bottom plates rust at a much slower rate, why are they commonly much thicker than side plates these days? How many times have we seen comments on here to be wary/avoid older narrowboats with "only" 6mm thick baseplates?

 

I don't think anyone has conclusive proof, but I still reckon there are far more boats around with overplated bottoms than hull sides and the reason is it's so much easier to routinely black the sides.

[Paul's Nulife4-2]

Hi ya Gang,

Just a thought on this, Could it be more a COMBINATION of things like Oxygen, Sunlight, Water Disturbances ect, that excel the Rust / Corrosion problems.

And as there might be less of all 3 at lower levels, !!!!.

Edited May 3, 2014 by Paul's Nulife4-2

[blackrose]

I agree - people use this reasoning all the time but what evidence is it based on? Most narrowboats draw about two foot - so there's not enough oxygen to cause rust at that level?

 

And if bottom plates rust at a much slower rate, why are they commonly much thicker than side plates these days? How many times have we seen comments on here to be wary/avoid older narrowboats with "only" 6mm thick baseplates?

 

I don't think anyone has conclusive proof, but I still reckon there are far more boats around with overplated bottoms than hull sides and the reason is it's so much easier to routinely black the sides.

 

I don't know how you've drawn that conclusion, but I can only go on my own experience having personally seen far more boats with replated/overplated sides than bottoms.

 

I'm sure there's plenty of scientific evidence to support reduced levels of dissolved oxygen as a function of depth in a water column, but as you say, whether that has any effect over a 2ft depth isn't clear. Again, I can only go on my own personal experience. 4 years after blacking my boat the rust patches on the sides looked more aggressive than the light surface rust on my unpainted baseplate which I could almost brush off with my hand.

Hi ya Gang,

Just a thought on this, Could it be more a COMBINATION of things like Oxygen, Sunlight, Water Disturbances ect, that excel the Rust / Corrosion problems.

And as there might be less of all 3 at lower levels, !!!!.

 

Yes at the surface of the water that's right - and most canal boats tend to intersect the water surface along their sides!

Edited May 3, 2014 by blackrose

[John Williamson 1955]

 

I don't know how you've drawn that conclusion, but I can only go on my own experience having personally seen far more boats with replated/overplated sides than bottoms.

 

I'm sure there's plenty of scientific evidence to support reduced levels of dissolved oxygen as a function of depth in a water column, but as you say, whether that has any effect over a 2ft depth isn't clear. Again, I can only go on my own personal experience. 4 years after blacking my boat the rust patches on the sides looked more aggressive than the light surface rust on my unpainted baseplate which I could almost brush off with my hand.

 

Yes at the surface of the water that's right - and most canal boats tend to intersect the water surface along their sides!

I think the difference in corrosion rates between sides and bottom is caused not so much by the dissolved oxygen near the surface, but the alternating wetness and exposure to atmospheric oxygen along the waterline due to wave action. I noticed on my boatwhile it was on the side before I bought it that there is a fairly well defined strip of about 6 inches in width which is noticeably pitted, with the rest of the hull being fairly smooth, both above and below that strip.

[blackrose]

I think the difference in corrosion rates between sides and bottom is caused not so much by the dissolved oxygen near the surface, but the alternating wetness and exposure to atmospheric oxygen along the waterline due to wave action. I noticed on my boatwhile it was on the side before I bought it that there is a fairly well defined strip of about 6 inches in width which is noticeably pitted, with the rest of the hull being fairly smooth, both above and below that strip.

 

Yes, that's right. I didn't mean to imply it was dissolved oxygen that was chiefly responsible for corrosion along the waterline.

[Ex- Member]

It's not the case there's no oxygen in deeper water, it just there's less oxygen than at the surface as you go deeper it decreases.

 

This basically pertains to lakes but close to canals as water flow is minimal and doesn't create much oxygen except around locks or where water enters the canal system at a high rate.

 

 

 

"Oxygen levels depend on whether water is flowing or not, whether there are rocks or other obstacles for water to flow over, how many plants are growing in the water, and the temperature of the water. There is more oxygen in cold, flowing water with many obstacles and a moderate amount of plants. Plants take up carbon dioxide and release oxygen, but if there are too many plants all of the oxygen will be used up when bacteria decompose them after they die. Oxygen levels are higher in very cold water compared to very warm water. This might make us think that water in the winter has lots of oxygen but this is actually not true. During the winter, ice covers lakes and rivers and very little oxygen enters the water from the atmosphere– the lake is effectively sealed up.

 

Oxygen in lakes changes with depth. In deep lakes that do not get very much wind, oxygen levels go down as we get deeper. In all lakes, oxygen is generally low right at the bottom where water meets the lake sediment or mud. This is because there are many bacteria and animals that live and breathe in the sediment. These bacteria and animals decompose dead material that sinks to the bottom and use up oxygen. In some lakes and ponds that have very low oxygen, we install aerators to keep oxygen levels high. This is quite common in lakes that are stocked with fish and in lakes that receive sewage inputs. Oxygen levels depend on whether water is flowing or not, whether there are rocks or other obstacles for water to flow over, how many plants are growing in the water, and the temperature of the water. There is more oxygen in cold, flowing water with many obstacles and a moderate amount of plants. Plants take up carbon dioxide and release oxygen, but if there are too many plants all of the oxygen will be used up when bacteria decompose them after they die. Oxygen levels are higher in very cold water compared to very warm water. This might make us think that water in the winter has lots of oxygen but this is actually not true. During the winter, ice covers lakes and rivers and very little oxygen enters the water from the atmosphere– the lake is effectively sealed up.

 

Oxygen in lakes changes with depth. In deep lakes that do not get very much wind, oxygen levels go down as we get deeper. In all lakes, oxygen is generally low right at the bottom where water meets the lake sediment or mud. This is because there are many bacteria and animals that live and breathe in the sediment. These bacteria and animals decompose dead material that sinks to the bottom and use up oxygen. In some lakes and ponds that have very low oxygen, we install aerators to keep oxygen levels high. This is quite common in lakes that are stocked with fish and in lakes that receive sewage"inputs.

 

www.enr.gov.nt.ca/sites/default/files/dissolved_oxygen.doc‎

[Neil2]

 

I don't know how you've drawn that conclusion, but I can only go on my own experience having personally seen far more boats with replated/overplated sides than bottoms.

 

 

Did I say it was a conclusion? I was expressing an opinion based on my own experience, just as you are. neither of us can claim to know for certain.

 

 

There are some indisputable facts though. The narrowboat industry started moving towards 10mm and thicker baseplates in the 1980's, and there must have been good reasons for it.