Why does a skin tank need baffles?

[Mad Harold]

What do the baffles in a skin tank do. I am curious because although my boat is direct water cooled,I am thinking that at some time in the future, I will have a skin tank fitted.

The skin tanks I have looked at on friend's boats,are only about a couple of inches wide,so what purpose do baffles serve?

Grateful for any enlightenment.

[nicknorman]

3 minutes ago, Mad Harold said:

What do the baffles in a skin tank do. I am curious because although my boat is direct water cooled,I am thinking that at some time in the future, I will have a skin tank fitted.

The skin tanks I have looked at on friend's boats,are only about a couple of inches wide,so what purpose do baffles serve?

Grateful for any enlightenment.

The baffles are to ensure the coolant takes the “long way round” the skin tank and thus loses the maximum heat. Without baffles the flow tends to take a direct route between inlet and outlet and thus much of the surface area does nothing, and minimal cooling results.

 

Baffles are typically horizontal and arranged to force the coolant along to the far end and back a couple of times, as the coolant flows (indirectly) from inlet to outlet.

Edited December 21, 2018 by nicknorman

[TheBiscuits]

They force the hot coolant to take a longer path through the skintank, so it cools more. 

[alan_fincher]

If done well they force the water to travel around a "zig zag" path, and hence over as much of the exterior surface of the skin tank as can be managed.

Without baffles the water would probably take the shortest route from inlet to outlet, and quite possibly nowhere near most of the external surface of the tank.

It is no good a skin tank having a surface to the water of (say) 10 square feet, if (say) only 3 square feet of it are actually used for cooling.  The remaining 7 square feet would then be largely useless, and the engine overheating becomes far more likely

[NB Esk]

This should clarify what baffles do in a skin tank.  This was on a boat I built some time back and I took the pic to ensure I'd got the path of circulation correct.  In this case the tank is 1/2" wide ( thickness) and functions as expected.

Hope this helps.....

 

 

[zenataomm]

Another purpose for baffles in a tank (not necessarily in boats' keel tanks) is to avoid surging when slowing down or stopping.

Read to your heart's content

 

Although I do recall the aftermath of Ellesmere Port tug Worcester coming to an immediate halt in the 80s when the steerer fumbled reverse and she rammed the cill.  The contents of the engine bilge carried on until it met the bulkhead after which it climbed vertically to introduce itself to the roof.  Having successfully completed this manoeuvre it changed direction towards the backend until its energy was dispelled and all executive decisions were delegated to gravity.   The poor sod who climbed out resembled a Dr Who Marsh Monster suffering a distinct loss of humour.

[nicknorman]

1 minute ago, zenataomm said:

Another purpose for baffles in a tank (not necessarily in boats' keel tanks) is to avoid surging when slowing down or stopping.

Read to your heart's content

 

Although I do recall the aftermath of Ellesmere Port tug Worcester coming to an immediate halt in the 80s when the steerer fumbled reverse and she rammed the cill.  The contents of the engine bilge carried on until it met the bulkhead after which it climbed vertically to introduce itself to the roof.  Having successfully completed this manoeuvre it changed direction towards the backend until its energy was dispelled and all executive decisions were delegated to gravity.   The poor sod who climbed out resembled a Dr Who Marsh Monster suffering a distinct loss of humour.

Not really relevant. Surging only occurs when a tank is partially full, or in an open vessel. Hopefully a skin tank is full of coolant and thus no surging can occur.

[PeterF]

The width of the flow path is reduced so the velocity of the water is higher, so it is more turbulent and thus has a better capability to transfer heat from the water to the hull (heat transfer coefficient), which further increases the performance of the tank.

[Mad Harold]

Thank you all for your replies.  My question has been most satisfactorily answered.

Was nagging with a boater on the cut last summer and the subject of engine cooling came up.  He showed me the system he had made up on his boat which consisted of  a rather large vehicle radiator,complete with thermostatic electric fan,mounted over the gearbox. He said it worked fine,although I had my doubts.

Perhaps ok for an hour or two at canal speeds,but I think it would overheat if run for longer,or if a higher power setting was used.

Didn't ask him too many searching questions,although I would have liked to,it would have come across as discourteous as he had taken the trouble to show me his set-up.

[NB Esk]

1 minute ago, Mad Harold said:

Thank you all for your replies.  My question has been most satisfactorily answered.

Was nagging with a boater on the cut last summer and the subject of engine cooling came up.  He showed me the system he had made up on his boat which consisted of  a rather large vehicle radiator,complete with thermostatic electric fan,mounted over the gearbox. He said it worked fine,although I had my doubts.

Perhaps ok for an hour or two at canal speeds,but I think it would overheat if run for longer,or if a higher power setting was used.

Didn't ask him too many searching questions,although I would have liked to,it would have come across as discourteous as he had taken the trouble to show me his set-up.

Might have given trouble on a really hot day, unless there was really good ventilation through the engine room/compartment.

 

 

[nicknorman]

14 minutes ago, Mad Harold said:

Thank you all for your replies.  My question has been most satisfactorily answered.

Was nagging with a boater on the cut last summer and the subject of engine cooling came up.  He showed me the system he had made up on his boat which consisted of  a rather large vehicle radiator,complete with thermostatic electric fan,mounted over the gearbox. He said it worked fine,although I had my doubts.

Perhaps ok for an hour or two at canal speeds,but I think it would overheat if run for longer,or if a higher power setting was used.

Didn't ask him too many searching questions,although I would have liked to,it would have come across as discourteous as he had taken the trouble to show me his set-up.

It would probably work OK provided the heat can escape the engine area (hatch etc) but eg if in a cruiser stern covered in boards, where will the heat go?

[ditchcrawler]

17 minutes ago, Mad Harold said:

Thank you all for your replies.  My question has been most satisfactorily answered.

Was nagging with a boater on the cut last summer and the subject of engine cooling came up.  He showed me the system he had made up on his boat which consisted of  a rather large vehicle radiator,complete with thermostatic electric fan,mounted over the gearbox. He said it worked fine,although I had my doubts.

Perhaps ok for an hour or two at canal speeds,but I think it would overheat if run for longer,or if a higher power setting was used.

Didn't ask him too many searching questions,although I would have liked to,it would have come across as discourteous as he had taken the trouble to show me his set-up.

There is a boat set up like this on the T&M

 

1 minute ago, nicknorman said:

It would probably work OK provided the heat can escape the engine area (hatch etc) but eg if in a cruiser stern covered in boards, where will the heat go?

What about all the air cooled engines

[nicknorman]

5 minutes ago, ditchcrawler said:

There is a boat set up like this on the T&M

 

What about all the air cooled engines

Air cooled engines have forced cooling “over the sides” ie typically an intake grill at one side and a shrouding over the engine with a fan that blows air directly over the engine fins and via ducting, out of the exit grill on the other side. Which is one of the reasons why they are so noisy - the engine is acoustically connected to the outside world via the cooling duct and grill.

[bizzard]

Very common practice in the 1950-60's in old cruiser and lifeboat conversions, when most folk had little money to spare. Using the engine and gearbox from a scrapped car. Even the gearboxes could be converted to give almost equal fore and astern ratios, usually by swapping cogs and welding in chain sprockets and chan and by using a plummer thrust block to take the thrust. A good fan and ducting and they were pretty reliable. Ford 100E and E93A were favourites because the 3 speed boxes was easy to adapt with a bike brake cable and lever fiixed to the gearlever to work the clutch.  A complete transverse Mini engine and transmission was once  used to make a paddle boat which was successful. Most folk were much more practical and inventive in those days.

[nicknorman]

7 minutes ago, bizzard said:

Most folk were much more practical and inventive in those days.

Well that plus they didn’t need to reprogramme the ECU and CANBUS interface to get stuff to work!

[Kingdom Isambard Brunel]

That is an exceptionally well baffled skin tank, most are just 3 or 4 "passes". 

I have seen several boats with pipes outside under the counter, they seem to work well.

The hulls with the tank flat on the base plate under the engine are no so good.

[Tony Brooks]

11 hours ago, Mad Harold said:

What do the baffles in a skin tank do. I am curious because although my boat is direct water cooled,I am thinking that at some time in the future, I will have a skin tank fitted.

The skin tanks I have looked at on friend's boats,are only about a couple of inches wide,so what purpose do baffles serve?

Grateful for any enlightenment.

2" is too wide, ideally the water passages should be a skinny as possible so water can not "get away" from the cols hull side and, as pointed out earlier, create turbulent flow. The wider the tank  the more antifreeze it needs and the larger the space for coolant expansion has to be provided.

[Kingdom Isambard Brunel]

28 minutes ago, Tony Brooks said:

2" is too wide, ideally the water passages should be a skinny as possible so water can not "get away" from the cols hull side and, as pointed out earlier, create turbulent flow. The wider the tank  the more antifreeze it needs and the larger the space for coolant expansion has to be provided.

That's my problem, too big a volume. Took 24 liters of antifreeze and 80 liters of water. I have difficulty accommodating the expansion. Cools well though!

[blackrose]

5 hours ago, Boater Sam said:

That is an exceptionally well baffled skin tank, most are just 3 or 4 "passes". 

 

 

Possibly too well-baffled for some engines. One must keep in mind that the water pump on most engines isn't really a pump it's more of a stirrer. The more complex the baffling, the more difficult it becomes to circulate the coolant, so there is a happy medium between a tortuous path for the coolant and adequate heat transfer. Beta recommend a more simple baffle arrangement which is what I followed when I installed a second skin tank.

 

https://betamarine.co.uk/keel-cooling-calculations/

 

I've taken IR thermometer readings all over the tank and once it gets hot there isn't much temperature difference in different parts of the tank.

Edited December 22, 2018 by blackrose

[WotEver]

2 minutes ago, blackrose said:

I've taken IR thermometer readings all over the tank and once the tank gets hot there isn't much difference in different parts of the tank.

I’m sure you don’t mean this as it reads, what you’ve written suggests that the tank isn’t cooling the water at all. 

[alan_fincher]

7 minutes ago, blackrose said:

 

I've taken IR thermometer readings all over the tank and once the tank gets hot there isn't much difference in different parts of the tank.

 

Presumably you don't mean exactly this, or it wouldn't actually be working?

 

Clearly there should be temperature gradient between where the hot water is coming in at the top, and the cooled water is  going out at the bottom...

 

EDIT: Cross posted with Wotever

Edited December 22, 2018 by alan_fincher

[blackrose]

6 minutes ago, WotEver said:

I’m sure you don’t mean this as it reads, what you’ve written suggests that the tank isn’t cooling the water at all. 

I meant there isn't much difference in terms of temperature between the centre and corners of the tank.

[PaulJ]

41 minutes ago, blackrose said:

 

Possibly too well-baffled for some engines. One must keep in mind that the water pump on most engines isn't really a pump it's more of a stirrer. The more complex the baffling, the more difficult it becomes to circulate the coolant, so there is a happy medium between a tortuous path for the coolant and adequate heat transfer. 

Although a valid point -all the skin tanks Ive ever seen are lower than the engine. So the hot water being pumped out of the engine only has to fall as it cools , which its going to do without a pump anyhow. I think the test is-if you pour it in the top and it comes straight out the bottom its going to work!

[cuthound]

4 hours ago, blackrose said:

 

Possibly too well-baffled for some engines. One must keep in mind that the water pump on most engines isn't really a pump it's more of a stirrer. The more complex the baffling, the more difficult it becomes to circulate the coolant, so there is a happy medium between a tortuous path for the coolant and adequate heat transfer. Beta recommend a more simple baffle arrangement which is what I followed when I installed a second skin tank.

 

https://betamarine.co.uk/keel-cooling-calculations/

 

I've taken IR thermometer readings all over the tank and once it gets hot there isn't much temperature difference in different parts of the tank.

 

It surprised me how powerful the water pump was on my Beta 43 engine, although I appreciate this may not be the case for other engine types.

 

When I bought my boat the calorifier had just been replaced, and the numpty who fitted it had connected the two coils in series, so the engine was heating the domestic hot water, the Webasto boiler and the central heating radiators. The total circuit must have been over 120 feet in length, so quite a resistance for the engine water pump to pump against.

[Tony Brooks]

4 hours ago, blackrose said:

 

Possibly too well-baffled for some engines. One must keep in mind that the water pump on most engines isn't really a pump it's more of a stirrer. The more complex the baffling, the more difficult it becomes to circulate the coolant, so there is a happy medium between a tortuous path for the coolant and adequate heat transfer. Beta recommend a more simple baffle arrangement which is what I followed when I installed a second skin tank.

 

https://betamarine.co.uk/keel-cooling-calculations/

 

I've taken IR thermometer readings all over the tank and once it gets hot there isn't much temperature difference in different parts of the tank.

 

Although not a positive displacement pump it is still a centrifugal pump, be it with larger impeller to case clearances. yes,you can stop the flow by putting your thumb over the pipe at idle but it becomes progressively more difficulty as the speed rises. As long as the water galleries in the skin tank are large enough, and I have yet to see any that aren't, then the pump will be fine.

 

I think this notion comes from the days when we had very tall, slim radiators in cars when the water pump was not doing much more than assisting thermo-circulation. Today with low profile horizontal radiators the pumps seem much better.