My overheating problems.

[bizzard]

And the welder needs to go all over his welds afterwards with a centre punch and hammer to check for any porosity.

[blackrose]

I've seen welders approach this in different ways and don't think its a real problem. True the steel you're trying to weld gets hotter than the steel you're welding to, but thats in the technique. When I've seen them do it well, instead of trying to run the seam after its been tacked, they do 6" or so and move to another bit, let it cool and come back. It means you have to check carefully so it doesn't leak and with it in the water there is always the risk of a blow through!

 

A blow through? Is that what I think it means?

 

Could a welder burn through the 6mm steel of my swim accidently?

Edited June 8, 2012 by blackrose

[Barry]

A blow through? Is that what I think it means?

 

Could a welder burn through the 6mm steel of my swim accidently?

 

Unlikely where the hull is below the waterline but below the waterline the welder might overheat the new bit of steel being welded on and blow through that

[RobinJ]

A blow through? Is that what I think it means?

 

Could a welder burn through the 6mm steel of my swim accidently?

The thicker the steel the less likely, as it tends to happen where there is already a weld (which may not be too good) and I think its caused by gases trapped in the old weld, but if the weld cracks can lead to seepage!

[blackrose]

Here's where the new skin tank will be welded in:

 

 

And a drawing of the proposed prefabricated tank:

 

 

I got a very reasonable quote for pre-fabricating this skin tank today but the guy says he'd do it in 3mm plate because his rollers will have a job to roll 4mm steel. I went up to Uxbridge boat centre to buy the fitings. Alan the owner asked me what the fitings were for so I explained and showed him the diagram.

 

"It isn't going to work" he said. "As the weld cools it's going to pull away from the keel."

 

I asked him for clarification but he got p*ssed off with me. He seems to view questions as undermining his expertise rather than a genuine attempt to understand the issues and resolve the problem.

 

"Listen, it's going to pull away! I could spend half an hour talking to you about temperature differentials, memory effects and the granular structure of steel, but that's not going to make it work!"

 

He said the tank can't be pre-fabricated and has to be fabricated in-situ. I wondered what stops his skin tanks from pulling away and he said the keel holds it in place. That made no sense to me. Surely the keel will hold a pre-fabricated tank in place too if it's properly tacked in first?

 

Can anyone shed some more light on this please?

Edited June 13, 2012 by blackrose

[RLWP]

Yes. He's saying 'I've done this before, got it wrong, this is what happened. I'm giving you the benefit of my experience'

 

What he is is a welder/fabricator. What he isn't is a communicator or teacher. You are upsetting him by questioning his experience

 

It's up to you, if it were me, I'd go with the welding expert, even if he can't explain why

 

Richard

[blackrose]

I did (and do) accept what he says. I'm just having trouble understanding how I'd get the job done even if it was fabricated in-situ? Since the main curved plate has to be pre-rolled I don't understand how that could be welded onto anything without distortion as it cools?

 

Edit: All I'm doing is folding one end of the curved plate and pre-fabricating a top and a baffle onto it. If anything I'd have thought that would strengthen the overall fabrication.

Edited June 13, 2012 by blackrose

[RLWP]

I did (and do) accept what he says. I'm just having trouble understanding how I'd get the job done even if it was fabricated in-situ? Since the main curved plate has to be pre-rolled I don't understand how that could be welded onto anything without distortion as it cools?

 

My guess (!WARNING!WARNING!WARNING!) is that it's hard to make the weld along the bottom with a welded up tank, so he would rather do that with a curved plate first. That's all speculation (!WARNING!WARNING!WARNING!)

 

Richard

[blackrose]

My guess (!WARNING!WARNING!WARNING!) is that it's hard to make the weld along the bottom with a welded up tank, so he would rather do that with a curved plate first. That's all speculation (!WARNING!WARNING!WARNING!)

 

Richard

 

It's not a welded up tank. It's a curved plate with a top, a side and a baffle. Have a look at the diagram - there's no back, no bottom and only one side. Perhaps he didn't understand that either?

[Paul C]

I got a very reasonable quote for pre-fabricating this skin tank today but the guy says he'd do it in 3mm plate because his rollers will have a job to roll 4mm steel. I went up to Uxbridge boat centre to buy the fitings. Alan the owner asked me what the fitings were for so I explained and showed him the diagram.

 

"It isn't going to work" he said. "As the weld cools it's going to pull away from the keel."

 

I asked him for clarification but he got p*ssed off with me. He seems to view questions as undermining his expertise rather than a genuine attempt to understand the issues and resolve the problem.

 

"Listen, it's going to pull away! I could spend half an hour talking to you about temperature differentials, memory effects and the granular structure of steel, but that's not going to make it work!"

 

He said the tank can't be pre-fabricated and has to be fabricated in-situ. I wondered what stops his skin tanks from pulling away and he said the keel holds it in place. That made no sense to me. Surely the keel will hold a pre-fabricated tank in place too if it's properly tacked in first?

 

Can anyone shed some more light on this please?

 

He's more or less right, but being a bit over-cautious. If it were fabricated and then welded in, assuming the welds are good enough quality, it wouldn't actually physically pull away from the hull, instead internal stresses would be set up within the steel. If he fabricated it in situ, he can ensure those internal stresses are minimised and that the welds have a much better chance of holding, long term.

 

 

 

Here' an analogy: if I posted a spring to you in a box, you'd be annoyed when you opened the package, if I had compressed it and fitted it into a smaller box. It would be a surprise! If I chose a bigger box and sent it stuffed with bubble wrap, you'd be happier.

[blackrose]

Here' an analogy: if I posted a spring to you in a box, you'd be annoyed when you opened the package, if I had compressed it and fitted it into a smaller box. It would be a surprise! If I chose a bigger box and sent it stuffed with bubble wrap, you'd be happier.

 

As long as the smaller box isn't opened and the spring stays inside I'll be happy!

 

But seriously, there must be internal stresses all over our hulls, especially in the bows and stern end?

Edited June 13, 2012 by blackrose

[Timleech]

 

There are no advantages, (and considerable disadvantages), in making a "thick" skin tank. Strange Barrus should recommend 2" to 3" "thick", as it goes against normally given advice. More like 1" is the usual recommendation.

 

It presents the same cooling surface, gives the hot water less chance of bypassing it, and limits the amount of expansion the system needs to be capable of dealing with. It will also reduce your anti-freeze requirements by up to 67%.......

 

I know they supply the engines, but I think that bit of advice is highly suspect, and best not followed!

 

Having a thin tank also gives a more turbulent flow, which helps with heat transfer (or so I'm told).

 

Tim

[blackrose]

I'm beginning to wonder whether I should just get a full tank fabricated in 3mm plate (with a top, back, bottom and sides), and then just stick it to the swim with one of these epoxy resisns?

 

http://www.epoxies.com/therm.htm

 

It would save a lot of hassle but I wonder if it would still lose the required heat through the extra 3mm steel and the epoxy?

[Timleech]

I got a very reasonable quote for pre-fabricating this skin tank today but the guy says he'd do it in 3mm plate because his rollers will have a job to roll 4mm steel. I went up to Uxbridge boat centre to buy the fitings. Alan the owner asked me what the fitings were for so I explained and showed him the diagram.

 

"It isn't going to work" he said. "As the weld cools it's going to pull away from the keel."

 

I asked him for clarification but he got p*ssed off with me. He seems to view questions as undermining his expertise rather than a genuine attempt to understand the issues and resolve the problem.

 

"Listen, it's going to pull away! I could spend half an hour talking to you about temperature differentials, memory effects and the granular structure of steel, but that's not going to make it work!"

 

He said the tank can't be pre-fabricated and has to be fabricated in-situ. I wondered what stops his skin tanks from pulling away and he said the keel holds it in place. That made no sense to me. Surely the keel will hold a pre-fabricated tank in place too if it's properly tacked in first?

 

Can anyone shed some more light on this please?

 

Not sure what the 'keel' is that it's going to be pulling away from? Certainly if that baffle is welded to the new inner skin before assembly, it will be in little or no contact with the outer hull plating by the time it's finished.

If I were doing it I would weld the baffle to the hull first, as a separate piece (with a decent thickness of flat bar, say 6mm). Then the inner plate needs a series of holes or slots such that it can be pushed against the baffle & plug welded through. This could be tricky with only 3mm plate if the holes don't line up perfectly with the baffle, much easier with at least 5mm plate.

Bear in mind that the bottom corners, against hull/bottom and bulkhead/bottom, can be absolute bu**ers to get a good watertight weld in if you don't get it right first time, especially so if access is poor.

 

Tim

[blackrose]

Not sure what the 'keel' is that it's going to be pulling away from? Certainly if that baffle is welded to the new inner skin before assembly, it will be in little or no contact with the outer hull plating by the time it's finished.

If I were doing it I would weld the baffle to the hull first, as a separate piece (with a decent thickness of flat bar, say 6mm). Then the inner plate needs a series of holes or slots such that it can be pushed against the baffle & plug welded through. This could be tricky with only 3mm plate if the holes don't line up perfectly with the baffle, much easier with at least 5mm plate.

Bear in mind that the bottom corners, against hull/bottom and bulkhead/bottom, can be absolute bu**ers to get a good watertight weld in if you don't get it right first time, especially so if access is poor.

 

Tim

 

I meant the swim - keel cooling.

 

I wonder how many builders make holes or slots and baffle tanks as you describe? Doing it as I planned I imagine the baffle would only be a mm or 2 away from the swim so not all the coolant would pass under it and whatever coolant did would have turbulant flow. Perhaps I could weld an identical baffle on the swim just above or below the one on the tank? That seems like a simpler idea.

Edited June 13, 2012 by blackrose

[RobinJ]

It's not a welded up tank. It's a curved plate with a top, a side and a baffle. Have a look at the diagram - there's no back, no bottom and only one side. Perhaps he didn't understand that either?

I think I understand what he's saying, and it will depend on the welding sequence I suspect! If the welding is started at the bottom and given suffucient time to cool, any expansion in the vertical direction will be reduced.

 

When I cut out one of my drainage channels, I stuck a block of wood in to ensure the hull sides didn't push in, to my horror, when cutting through, they sprang out!

 

Are you planning to weld along the top of the baffle too?

[blackrose]

I think I understand what he's saying, and it will depend on the welding sequence I suspect! If the welding is started at the bottom and given suffucient time to cool, any expansion in the vertical direction will be reduced.

 

When I cut out one of my drainage channels, I stuck a block of wood in to ensure the hull sides didn't push in, to my horror, when cutting through, they sprang out!

 

Are you planning to weld along the top of the baffle too?

 

Once the tank is taked into place, surely any decent welder would know that they should weld an inch or two here, an inch or two there, continually changing positions in order to allow the heat to dissipate, rather than trying to weld a complete seam in one go. Eventually it's just a case of filling in the gaps.

 

The baffle will be welded to the inside of the tank before the tank is welded to the swim, but following on from what Tim said I think an identical baffle will be welded onto the swim just above or below the baffle on the tank to prevent water going underneath any gap.

[bizzard]

Although i've never welded in a skin tank i've done plenty of other welding mainly motor trade,truck chassis and MOT repairs ect Gas,stick and Mig.

 

A good welder shouldn't have any trouble doing the job,good welding is all about getting plenty of practice at it.If only an occasional welder beware!.And beware of people who often leap up and say ''I can weld'' 99% of them can't.

I think i would shore the tank up with timber and wedges as i went along too,from say engine mounts to tank, to keep it close up to the swim.

And i would certainly go over all the welds when cool with a fine centre punch and hammer especially in corners and awkward bits,checking for dirty or porous welds BEFORE your welder departs,though he might be offended by this,but he should have done it himself anyway on anything that has to contain liquids or pressure.

[blackrose]

Although i've never welded in a skin tank i've done plenty of other welding mainly motor trade,truck chassis and MOT repairs ect Gas,stick and Mig.

 

A good welder shouldn't have any trouble doing the job,good welding is all about getting plenty of practice at it.If only an occasional welder beware!.And beware of people who often leap up and say ''I can weld'' 99% of them can't.

I think i would shore the tank up with timber and wedges as i went along too,from say engine mounts to tank, to keep it close up to the swim.

And i would certainly go over all the welds when cool with a fine centre punch and hammer especially in corners and awkward bits,checking for dirty or porous welds BEFORE your welder departs,though he might be offended by this,but he should have done it himself anyway on anything that has to contain liquids or pressure.

 

Good tip, thanks.

[blackrose]

I've been to the fabricator and it should be ready in about 2 weeks (he's busy). Now I'm worrying that it isn't going to be big enough! I couldn't really make it any bigger because the fuel agglomerator and fuel pipes were in the way.

 

The new tank will be 5.63ft sq or approx 60% of the area of the existing tank (9.5ft sq). That's a total of 15ft sq, which should be plenty except that because the existing tank isn't baffled I've no idea what its effective cooling area really is? All I know is that the whole thing gets very hot - even at the corners away from the feed/outlet pipes.

 

In theory a 55 hp engine needs 13.75ft sq of skin tank - but that's a properly baffled tank. However, my engine only revs to 2000rpm (producing about 42hp) because it's over-propped.

Edited June 15, 2012 by blackrose

[bowten]

I've been to the fabricator and it should be ready in about 2 weeks (he's busy). Now I'm worrying that it isn't going to be big enough! I couldn't really make it any bigger because the fuel agglomerator and fuel pipes were in the way.

 

The new tank will be 5.63ft sq or approx 60% of the area of the existing tank (9.5ft sq). That's a total of 15ft sq, which should be plenty except that because the existing tank isn't baffled I've no idea what its effective cooling area really is? All I know is that the whole thing gets very hot - even at the corners away from the feed/outlet pipes.

 

In theory a 55 hp engine needs 13.75ft sq of skin tank - but that's a properly baffled tank. However, my engine only revs to 2000rpm (producing about 42hp) because it's over-propped.

I have just posted on the other thread about overheating,have a look.

[George94]

I've been to the fabricator and it should be ready in about 2 weeks (he's busy). Now I'm worrying that it isn't going to be big enough! I couldn't really make it any bigger because the fuel agglomerator and fuel pipes were in the way.

 

The new tank will be 5.63ft sq or approx 60% of the area of the existing tank (9.5ft sq). That's a total of 15ft sq, which should be plenty except that because the existing tank isn't baffled I've no idea what its effective cooling area really is? All I know is that the whole thing gets very hot - even at the corners away from the feed/outlet pipes.

 

In theory a 55 hp engine needs 13.75ft sq of skin tank - but that's a properly baffled tank. However, my engine only revs to 2000rpm (producing about 42hp) because it's over-propped.

 

I should have thought that if your engine can't reach maximum rpm because you are, in effect, over-loading it (by using an over-sized prop), it is going to get hotter than it would if it had a lighter load and could reach full rpm (and faster water circulation).

 

I wonder if this is part of your problem.

[blackrose]

I should have thought that if your engine can't reach maximum rpm because you are, in effect, over-loading it (by using an over-sized prop), it is going to get hotter than it would if it had a lighter load and could reach full rpm (and faster water circulation).

 

I wonder if this is part of your problem.

 

Could well be. It had crossed my mind and I'd made plans to get 1" taken off the pitch after speaking to Crowthers and Norris - another job that I still haven't got around to.

 

With my 19 x 13" (3 blade RH prop) it revs to 3000 rpm out of gear but just under 2000rpm in gear on the river. Crowthers said I could only expect to get another 250rpm by reducing the pitch by an inch, so it will never actually achieve full revs.

Edited June 17, 2012 by blackrose

[George94]

Could well be. It had crossed my mind and I'd made plans to get 1" taken off the pitch after speaking to Crowthers and Norris - another job that I still haven't got around to.

 

With my 19 x 13" (3 blade RH prop) it revs to 3000 rpm out of gear but just under 2000rpm in gear on the river. Crowthers said I could only expect to get another 250rpm by reducing the pitch by an inch, so it will never actually achieve full revs.

 

That extra 250 rpm should (all else being equal) give you 12.5% more cooling (because the pump will be going 12.5% faster). Not a huge difference, but perhaps enough. Might be worth cutting down the prop before doing anything more drastic.

[blackrose]

That extra 250 rpm should (all else being equal) give you 12.5% more cooling (because the pump will be going 12.5% faster). Not a huge difference, but perhaps enough. Might be worth cutting down the prop before doing anything more drastic.

 

Two points:

 

If you look back over the thread you'll see that I contacted some people with a bigger boat from the same builder and an identical setup in terms of engine, geabox, prop and skin tank. Their boat also only revs to 2000rpm but never goes above 80C. As they have the same prop it's probably unlikely to be over-propping that's causing the overheating problem. The only thing I don't know about their boat is whether their skin tank is baffled.

 

The other thing is that various knowledgeable contributors to this thread have suggested that faster circulation of the coolant will not in fact cool the engine more efficiently, but could in fact do the opposite. While this advice may seem counter-intuitive, if you think about it, moving the coolant faster across a heat exchanger (in this case the skin tank), will only reduce its opportunity to diffuse that heat to the river water on the other side of the swim. This theory only works up to a point of course - the coolant still has to move through the skin tank, but I've already established that it heats up the tank from top to bottom and that the return pipe to the engine gets hot.

Edited June 18, 2012 by blackrose