Adding another skin tank

[blackrose]

I'm going to get a new 4 sided skin tank fabricated in 4mm mild steel using a cardboard template to show the curve of the swim. I'm hoping this will make it cheaper as I know a welder who can weld it in for me cheaply once it's been fabricated.

 

The tank will be welded in place on the inside of the swim (it's a widebeam so there's enough room) and I'll be using the baseplate as the bottom of the tank and the swim as the outer side. Obviously a skin tank can't be internally welded onto the swim, but what about painting? Do I just leave the inside bare steel and should I scurf the paint off the part of the swim that's going to be inside the tank? I guess there's no point painting the inside of the tank because the welding will burn paint off anyway? I realise I'll lose the blacking on the outside of the boat where the skin tank is welded but I can drop the boat down on tidal waters to sort that out.

Edited September 19, 2011 by blackrose

[alan_fincher]

I don't think you need to concern yourself about unpainted steel inside a skin tank - I'd have thought most are built that way.

 

Your coolant will contain anti-freeze with required corrosion inhibitors, so the inside of a skin tank should not rust up any more than the (assuming it is iron) internals of an engine.

 

But if you just have this tank "pre-fabricated" to drop in, how will the baffles be dealt with. You can obviously not arrange to weld them to the inside of a swim, but they will presumably be pre-welded to the inner surface of the skin tank, (i.e. the bit you are having pre-fabricated).

 

Can you be sure that it it is all even enough that it will join correctly at the edges, without the baffles preventing it ll dropping neatly in to place ?

 

Didn't see it done, but on ours the tank "edges" and the baffles were welded to the hull first, I think, before the new tank face was added over the top of them.

 

(Hard to explain, but I know what I mean, even if nobody else does!).

[Timleech]

I'm going to get a new 4 sided skin tank fabricated in 4mm mild steel using a cardboard template to show the curve of the swim. I'm hoping this will make it cheaper as I know a welder who can weld it in for me cheaply once it's been fabricated.

 

The tank will be welded in place on the inside of the swim (it's a widebeam so there's enough room) and I'll be using the baseplate as the bottom of the tank and the swim as the outer side. Obviously a skin tank can't be internally welded onto the swim, but what about painting? Do I just leave the inside bare steel and should I scurf the paint off the part of the swim that's going to be inside the tank? I guess there's no point painting the inside of the tank because the welding will burn paint off anyway? I realise I'll lose the blacking on the outside of the boat where the skin tank is welded but I can drop the boat down on tidal waters to sort that out.

 

Don't paint it. Remove as much paint as you reasonably can beforehand. Corrosion inhibitors in your antifreeze should take care of the corrosion aspect. Paint will limit the heat transfer.

It'll work much better if you can get some internal baffles included, welded to the hull side (continuous welding is good because it helps heat conduction from the baffles to the hull plate), and plug weld the inner plate of the tank to the baffles to limit the water flow over the tops of the baffles (also makes it better able to withstand pressure).

 

Tim

[blackrose]

I don't think you need to concern yourself about unpainted steel inside a skin tank - I'd have thought most are built that way.

 

Your coolant will contain anti-freeze with required corrosion inhibitors, so the inside of a skin tank should not rust up any more than the (assuming it is iron) internals of an engine.

 

But if you just have this tank "pre-fabricated" to drop in, how will the baffles be dealt with. You can obviously not arrange to weld them to the inside of a swim, but they will presumably be pre-welded to the inner surface of the skin tank, (i.e. the bit you are having pre-fabricated).

 

Can you be sure that it it is all even enough that it will join correctly at the edges, without the baffles preventing it ll dropping neatly in to place ?

 

Didn't see it done, but on ours the tank "edges" and the baffles were welded to the hull first, I think, before the new tank face was added over the top of them.

 

(Hard to explain, but I know what I mean, even if nobody else does!).

 

 

I understand exactly what you mean about the baffles, but whether they are attached to the swim or the inside of the tank seems to make no difference to me. They will never fit perfectly so some coolant will always goes under (or over) the baffles rather than around them. If the baffles of my pre-fabricated skin tank protrude a mm or two over the depth of the tank they can simply be ground off with an angle grinder until the skin tank can be tacked and then welded into place. A 4 sided tank should flex a litte - enough to tack it into place ready for welding in.

 

It'll work much better if you can get some internal baffles included, welded to the hull side (continuous welding is good because it helps heat conduction from the baffles to the hull plate), and plug weld the inner plate of the tank to the baffles to limit the water flow over the tops of the baffles (also makes it better able to withstand pressure).

 

Tim

 

I don't understand how it's possible to weld both sides of the baffles (onto the swim and onto the inside of the tank)?

Edited September 19, 2011 by blackrose

[RLWP]

<snip>

 

 

 

I don't understand how it's possible to weld both sides of the baffles (onto the swim and onto the inside of the tank)?

 

It's the plug welding that does it. One way is to drill a hole through the outside of the tank where the baffles are, then weld the baffle to the tank and seal the hole with the same weld (does that make sense???). You end up with the baffle in close contact with the wall of the tank

 

Plug welding is usually welding from the wrong side through a prepared hole

 

Richard

Edited September 19, 2011 by RLWP

[blackrose]

It's the plug welding that does it. One way is to drill a hole through the outside of the tank where the baffles are, then weld the baffle to the tank and seal the hole with the same weld (does that make sense???). You end up with the baffle in close contact with the wall of the tank

 

Plug welding is usually welding from the wrong side through a prepared hole

 

Richard

 

I see, thanks. It sounds like a lot of trouble. Surely baffles just welded to one side would suffice?

[RLWP]

I see, thanks. It sounds like a lot of trouble. Surely baffles just welded to one side would suffice?

 

I'd have thought so, but I don't have the experience that Tim has, so I'd go with what he suggests.

 

It's not too much faff, you'd drill the holes before welding the 'box' in place

 

Richard

[blackrose]

I'd have thought so, but I don't have the experience that Tim has, so I'd go with what he suggests.

 

It's not too much faff, you'd drill the holes before welding the 'box' in place

 

Richard

 

It's just a bit tricky if I'm having the skin tank prefabricated and just wanted to have someone weld it in place.

[RLWP]

It's just a bit tricky if I'm having the skin tank prefabricated and just wanted to have someone weld it in place.

 

Get the fabricator to weld it in place

 

Richard

 

Or the welder to fabricate it...

[blackrose]

Get the fabricator to weld it in place

 

Richard

 

Or the welder to fabricate it...

 

It's not quite that simple - for someone to fabricate it and weld it in place is going to be more expensive that having it prefabricated and welded in place by the welder I know. I'm working to a tight budget and he wouldn't have time to fabricate the tank.

 

Personally I'm not bothered about having the baffles welded on both sides.

[Timleech]

I see, thanks. It sounds like a lot of trouble. Surely baffles just welded to one side would suffice?

 

The issue there is - if the inner plate is not held in close contact with the baffles, water will tend to flow between that plate and the baffles rather than via the labyrinth which the baffles are supposed to create and properly across the hull plate.

 

Tim

[blackrose]

Get the fabricator to weld it in place

 

Richard

 

Or the welder to fabricate it...

 

It's not quite that simple - for someone to fabricate it and weld it in place is going to be more expensive that having it prefabricated and then welded in place by the welder I know. I'm working to a tight budget and he wouldn't have time to fabricate the tank.

 

Personally I'm not bothered about having the baffles welded on both sides.

 

The issue there is - if the inner plate is not held in close contact with the baffles, water will tend to flow between that plate and the baffles rather than via the labyrinth which the baffles are supposed to create and properly across the hull plate.

 

Tim

 

But if I'm having the tank pre-fabricated with the baffles inside, the gap will be between the baffles and the outer plate (the swim), which doesn't sound so bad in terms of heat transfer to the river water. I'd have thought most of the coolant will still flow around the baffles (the path of least resistance), and a smaller proportion will flow through the gaps.

Edited September 19, 2011 by blackrose

[Timleech]

 

But if I'm having the tank pre-fabricated with the baffles inside, the gap will be between the baffles and the outer plate (the swim), which doesn't sound so bad in terms of heat transfer to the river water. I'd have thought most of the coolant will still flow around the baffles (the path of least resistance), and a smaller proportion will flow through the gaps.

 

If you can keep that gap tight, any flow through it will be 'disturbed' which is just what you want. How 'thick' is the tank going to be?

 

Tim

[Bob18]

One solution is shown at the bottom of this page

http://nbsiskin.com/?cat=3&paged=14

 

(Rest of this blog is a superb read on how a replica was/should be built)

[Guest]

It's not quite that simple - for someone to fabricate it and weld it in place is going to be more expensive that having it prefabricated and then welded in place by the welder I know. I'm working to a tight budget and he wouldn't have time to fabricate the tank.

 

Personally I'm not bothered about having the baffles welded on both sides.

 

 

 

But if I'm having the tank pre-fabricated with the baffles inside, the gap will be between the baffles and the outer plate (the swim), which doesn't sound so bad in terms of heat transfer to the river water. I'd have thought most of the coolant will still flow around the baffles (the path of least resistance), and a smaller proportion will flow through the gaps.

I once read (may have been on here?) about somebody whose engine was overheating, and he found if he put a scissor jack between the engine mount and the skin tank and pressed the side of the skin tank towards the hull, that the overheating stopped. It seemed that the side of the skin tank was moving a little causing the baffles to part company with the side a few mm and letting coolant bypass the baffles. From this persons experience I can see why the plug welding idea might be good. Surely not a massive task to measure, drill a few holes in then get the welder to plug them up?

[RLWP]

I once read (may have been on here?) about somebody whose engine was overheating, and he found if he put a scissor jack between the engine mount and the skin tank and pressed the side of the skin tank towards the hull, that the overheating stopped. It seemed that the side of the skin tank was moving a little causing the baffles to part company with the side a few mm and letting coolant bypass the baffles. From this persons experience I can see why the plug welding idea might be good. Surely not a massive task to measure, drill a few holes in then get the welder to plug them up?

 

Except I guess Mike will get the tank prefabricated with the baffles welded to the inside of the tank, not to the swim. So the holes would have to be drilled through the hull. Perfectly feasible in a dry dock, but tricky in the water

 

Richard

[tosher]

Blackrose -I had an additional skin tank fitted about 2 years ago in exactly the same way as you suggest in your OP. I sanded down to bare metal all the parts of the swim that formed part of the new tank and it was welded into position with the boat in the water. It works a treat and have had no temperature problems at all since it was fitted (did prior to fitting)and can now run at full power indefinitely with engine temp normal. When lifting out about 1 year later for re-blacking we could see where the original blacking was burnt off along the outside of the welds during fitting. tosher

[Guest]

Except I guess Mike will get the tank prefabricated with the baffles welded to the inside of the tank, not to the swim. So the holes would have to be drilled through the hull. Perfectly feasible in a dry dock, but tricky in the water

 

Richard

Point taken, I wouldn't want to drill the hull anyway, but if the baffles are welded to the hull first I cant see this being a huge task "on site." Drilling a few holes in the tank side and plug welding can't be *that* much more work? Surely worth a few more quid for the welders time to get it right first go? Presumably the original tank isn't up to the job, and needs to be right this time?

[blackrose]

If you can keep that gap tight, any flow through it will be 'disturbed' which is just what you want. How 'thick' is the tank going to be?

 

Tim

 

I guess about an inch and a half external thickness - is that about right?

 

Blackrose -I had an additional skin tank fitted about 2 years ago in exactly the same way as you suggest in your OP. I sanded down to bare metal all the parts of the swim that formed part of the new tank and it was welded into position with the boat in the water. It works a treat and have had no temperature problems at all since it was fitted (did prior to fitting)and can now run at full power indefinitely with engine temp normal. When lifting out about 1 year later for re-blacking we could see where the original blacking was burnt off along the outside of the welds during fitting. tosher

 

Cheers, that's good to know.

 

Point taken, I wouldn't want to drill the hull anyway, but if the baffles are welded to the hull first I cant see this being a huge task "on site." Drilling a few holes in the tank side and plug welding can't be *that* much more work? Surely worth a few more quid for the welders time to get it right first go? Presumably the original tank isn't up to the job, and needs to be right this time?

 

The original tank is fine, it's just too small for the HP of the engine. The boat should have had 2 tanks to start with.

 

The problem with plug welding is that not only does the welder have to fit the skin tank, he also has to weld the baffles onto the swim and cut the holes in the tank before plug welding. If he doesn't do all of this he won't be able to make sure that the baffles fit the holes. So the holes can't really be pre-cut during tank fabrication.

[Guest]

I guess about an inch and a half external thickness - is that about right?

 

 

 

Cheers, that's good to know.

 

 

 

The original tank is fine, it's just too small for the HP of the engine. The boat should have had 2 tanks to start with.

 

The problem with plug welding is that not only does the welder have to fit the skin tank, he also has to weld the baffles onto the swim and cut the holes in the tank before plug welding. If he doesn't do all of this he won't be able to make sure that the baffles fit the holes. So the holes can't really be pre-cut during tank fabrication.

I see. Mine too is a bit marginal on the skin tank, but not too bad.

 

I am not a welder/fabricator of any sort,but trying to think logically, I would think if the baffles are made of substantial thickness that they would hold the shape better, and perhaps achieve the effect without the plug welding?

[RLWP]

<snip>

 

The problem with plug welding is that not only does the welder have to fit the skin tank, he also has to weld the baffles onto the swim and cut the holes in the tank before plug welding. If he doesn't do all of this he won't be able to make sure that the baffles fit the holes. So the holes can't really be pre-cut during tank fabrication.

 

Use the holes in the tank to mark out where the baffles go!

 

Richard

[blackrose]

Use the holes in the tank to mark out where the baffles go!

 

Richard

 

Yes and that would all have to be done by the welder. As I keep saying, I am working to a tight budget (not out of choice, but because I don't have the money). To be honest, he doesn't really want the job but if it's simple then he'll do it as a favour and I might get away with bunging him fifty quid or so. I'm hoping to get the tank made for about £100 - 150, so it doesn't end up being an expensive job.

[RLWP]

Yes and that would all have to be done by the welder. As I keep saying, I am working to a tight budget (not out of choice, but because I don't have the money). To be honest, he doesn't really want the job but if it's simple then he'll do it as a favour and I might get away with bunging him fifty quid or so. I'm hoping to get the tank made for about £100 - 150, so it doesn't end up being an expensive job.

 

Mike, I do get it, and I'm pretty sure that as it is an additional tank, it should be fine not plug welded

 

Out of interest, how are you going to match the curve of the swim?

 

Richard

[blackrose]

Mike, I do get it, and I'm pretty sure that as it is an additional tank, it should be fine not plug welded

 

Out of interest, how are you going to match the curve of the swim?

 

Richard

 

I'm going to make a cardboard template of the curve of the swim.

Edited September 19, 2011 by blackrose

[alan_fincher]

I believe the advice that a skin tank should be made thin is the best.

 

Most people seem to say 1" "thick" internally max, so your 1.5" (external) would be better if made less, I believe.

 

Don't know how "thick" current tank is, but whatever scope you have for expansion of the coolant will need to be able to deal with combined size of 2 skin tanks, plus calorifier, plus engine when you are done, and not overflow.

 

On many engines, unless there is a big external expansion bottle, that might be more than just a built on "header" tank can cope with.

 

Also remember the bigger you make the total capacity, the more anti-freeze you will need to buy when replacing coolant.