Keel or "Through Hull" cooling?

[PaulJS]

Does anyone know why small leisure boat engines are normally fitted with engine driven sea water cooling?
I'm actually a marine engineer, so well experienced with big sea water cooled heat exchangers, but I'm actually pretty baffled by the marine leisure engine industry's dependance on somewhat unreliable Jabsco pumps for providing the essential supply of sea water cooling to tiny, easily blocked tube type heat exchangers.
I know that one of the reasons is that the sea water cools and silences the engine exhaust after passing through the heat exchanger, but this seems to be inviting even more problems by bringing the sea water into proximity with the combustion system and the engines innards.
To my mind a more sensible, efficient, and reliable arrangement would be to have keel or "through hull" cooling and a dry exhaust system, this is what's fitted to our lifeboats which are fitted with 60hp diesels and required statutorily to be capable of running continuously for 24 hours.
The excess heat from the exhaust could be dissipated by better engine ventilation, or even have a water cooled manifold, and exhaust noise addressed by fitting silencers as on cars.
One of the well known engine manufacturers quotes a need for 7 square feet of cooling area for a 25hp engine, this sounds quite a lot until you think that a 2inch diameter pipe 11 feet long has about 7 square feet of surface area, so fitting even the simplest arrangement of keel cooling shouldn't have too much of an adverse effect on a hulls hydrodynamics, and even if this was a problem it would be relatively easy to build flush fitting skin coolers of adequate surface area.
Other advantages would be not having a sea strainer to keep clean and one less through hull cock, having a supply of hot water for boat services such as interior heating and hot water, and of course the ability to run the engine for short periods while the hull was not in the water.
The problems of fouling of the external heat exchanger could be alleviated by using a copper skin, and of course it's high skin temperature (at the inlet end) might give the barnacles reason to go elsewhere. I would always advise oversizing the cooler to provide a factor of safety against any fouling that did occur.
My best guess is that some accountant somewhere decided that it would be cheaper to fit a cheap and nasty sea water pump to push water through a relatively expensive tube type cooler, than it would be to drill two holes in the hull and bolt a copper tube onto them, and to hell with the inconvenience and need for constant vigilance and maintenance needed with a sea water cooled system...

 

[electrosys]

I did once see an article (PBO maybe ?) where a guy fitted some bronze through-hull fittings which connected to some lengths of copper pipe which ran along the side of his keel(s) - worked quite well, as I recall. But it was a small engine.


Brent Swain (http://groups.yahoo.com/group/origamiboats/) has been advocating fresh-water 'skeg-cooling' for decades, but then his boats are steel, so it's a much easier system to implement. The same circuit cools the manifold, so the exhaust gases are made cool enough to then run-out through the transom via an insulated dry exhaust, where a simple angle bend directs the gases a few inches underwater. A 100% problem-free system.

He has lots of other unorthodox ideas, which he's self-published as 'A Heretics Guide', which represents by far the best few quid I've ever spent on a book in my life - ranging from home-made blocks (stronger and much cheaper than anything obtainable commercially), to all sea-cocks being attached to standpipes (so that they can be examined at almost ANY time without prejudicing the boat), to the use of wire rope for anchoring (enabling the use of a simple DIY single or dual deck-mounted drum winch - with no dissimilar metals to corrode, and nothing to wear or go wrong). Problem-free self-steering, roller-furling - it's all in his book. Blimey - I should be on commission ...

Of course, his ideas will be of no interest whatsoever to those who believe that frequent chandlery pilgrimages are essential prerequisites to boating activities.

 

[KellysEye]

>To my mind a more sensible, efficient, and reliable arrangement would be to have keel or "through hull" cooling and a dry exhaust system

Our boat has internal saddle tanks on the hull for fresh water cooling. It also had a dry exhaust. Firstly dry exhausts are extremely noisy, second it caused a fire on the boat. The previous owner fitted a Jabsco pump to pump salt water through the exhaust, other than the odd impellor replacement it works fine. I don't see any problem with cleaning the salt water strainer occasionally, we usually go years without needing to clean it but it does depend where you sail.

The reason yacht engines are made as they are is they work, last a long time if they are maintained properly, and Jabsco pumps are fine, as said above.

While you have eperience of large engines I'd suggest you need to learn more about yacht engines, why they are designed as they are, as they have been for decades.

 

[testmonkey]

you will typically find larger no-thruhulls keel-cooling motorboats up north in Gulf of Bothnia, typically used by locals who live on small islands along the coastline and by authorities like coast guard, police, postal service... This design has specific benefits in colder climates in year-round use - no worries when leaving the boat in solid freezing conditions and when there is a need to operate in a couple of centrimeters of solid ice. Key is to really have zero thruhulls - this then also applies to heads etc.

Other than that especially the air cooled exhaust is not so practical - takes up a lot of space and increases noise.

 

[paul.norton]

I did once see an article (PBO maybe ?) where a guy fitted some bronze through-hull fittings which connected to some lengths of copper pipe which ran along the side of his keel(s) - worked quite well, as I recall. But it was a small engine.


Brent Swain (http://groups.yahoo.com/group/origamiboats/) has been advocating fresh-water 'skeg-cooling' for decades, but then his boats are steel, so it's a much easier system to implement. The same circuit cools the manifold, so the exhaust gases are made cool enough to then run-out through the transom via an insulated dry exhaust, where a simple angle bend directs the gases a few inches underwater. A 100% problem-free system.

He has lots of other unorthodox ideas, which he's self-published as 'A Heretics Guide', which represents by far the best few quid I've ever spent on a book in my life - ranging from home-made blocks (stronger and much cheaper than anything obtainable commercially), to all sea-cocks being attached to standpipes (so that they can be examined at almost ANY time without prejudicing the boat), to the use of wire rope for anchoring (enabling the use of a simple DIY single or dual deck-mounted drum winch - with no dissimilar metals to corrode, and nothing to wear or go wrong). Problem-free self-steering, roller-furling - it's all in his book. Blimey - I should be on commission ...

Of course, his ideas will be of no interest whatsoever to those who believe that frequent chandlery pilgrimages are essential prerequisites to boating activities.

Searched the web for 'A heretics Guide' by Brent Swain but without luck - where can I buy a copy?

 

[vyv_cox]

I used to help to look after a dry-exhaust rescue boat for a sailing club. The biggest problem was the flexible joint between engine and exhaust pipe, which corroded and went solid on an annual basis. I was welding a new one together every season. As said above, it was very noisy and people regularly burned themselves on the pipe.

I recently changed the exhaust hose on my boat, almost certainly original, 27 years without a problem.

 

[PaulJS]

>To my mind a more sensible, efficient, and reliable arrangement would be to have keel or "through hull" cooling and a dry exhaust system

Our boat has internal saddle tanks on the hull for fresh water cooling. It also had a dry exhaust. Firstly dry exhausts are extremely noisy, second it caused a fire on the boat. The previous owner fitted a Jabsco pump to pump salt water through the exhaust, other than the odd impellor replacement it works fine. I don't see any problem with cleaning the salt water strainer occasionally, we usually go years without needing to clean it but it does depend where you sail.

The reason yacht engines are made as they are is they work, last a long time if they are maintained properly, and Jabsco pumps are fine, as said above.

While you have eperience of large engines I'd suggest you need to learn more about yacht engines, why they are designed as they are, as they have been for decades.

Hi KellysEye,

I appreciate that leisure boat engines have used this sort of system for a while, but I am also aware that they have also used keel cooling for a long time, and of course the abomination that is direct sea water cooling... Now there was a really bad idea!
However, the very nature of an indirectly cooled sea water system sucking sea water via a design of pump which can not tolerate any sand or other solid bodies or being run dry for any appreciable length of time through a strainer which is usually going to block when you need it most strikes me as being well along the path toward breakdown before even starting the engine.
Sorry to hear that your dry exhaust had caused a fire, but surely there must have been a reason such as poor installation design or something having been left or splashed on the hot exhaust. As to the issue of noise, I think that could easily be addressed by appropriate design, after all my 140bhp car diesel is pretty quiet.
As to your last two paragraphs, I think that we'll have to disagree about the fitness for purpose of a pump that requires it's impeller to be changed annually and has other problems as detailed above, but I'll agree that the engine should last as long it's well maintained.
Your final paragraph seemed a little insulting, I'm currently sitting on a barge in Saudi where the generators use fairly large Jabscos for sea water cooling via tube type heat exchangers because keel cooling, as fitted to the propulsion engines, doesn't work very well when the keel is 22 feet above the water! Also, you'd probably be surprised at how many small diesels and outboard motors there are, even on the biggest ships, and I haven't only worked on big ships.

 

[electrosys]

Searched the web for 'A heretics Guide' by Brent Swain but without luck - where can I buy a copy?

Directly from Brent - address/ details etc can be found at the Yahoo group site I mentioned - in the /Files section.

BTW - for the naysayers, I did a quick search on OrigamiBoats for 'dry exhaust', and Brent recently posted:
"I insulated my dry exhaust with fibreglass house insulation, covered with cloth like 3 inch wide muffler tape. This, I covered with common caulking gun silicone. After days of motoring in the tropics , it is cool to the touch. The tape alone is not enough, the fibreglass makes all the difference. On 1 1/2 inch sch 40 pipe the end result is roughly 4 inches in diameter."

Yes - I agree with others that wet exhausts are common on yachts, and have been 'for decades'. But so has seawater finding it's way into the cylinders - it shouldn't happen - but it does, with uncanny regularity.

 

[PaulJS]

I used to help to look after a dry-exhaust rescue boat for a sailing club. The biggest problem was the flexible joint between engine and exhaust pipe, which corroded and went solid on an annual basis. I was welding a new one together every season. As said above, it was very noisy and people regularly burned themselves on the pipe.

I recently changed the exhaust hose on my boat, almost certainly original, 27 years without a problem.

Hi Vyv, been there and done that! I agree that the flexi joint between engine and exhaust is a seriously weak point, but if car manufacturers can manage to quietly get the exhaust gases from a flexibly mounted, average 100bhp engine to exit the engine via the tailpipe for, let's guess at an average of 20,000 miles, or approximately 500 hours while the car is bouncing over British roads, why do boat owners not have a similar level of reliability?
Another point about sea water cooled wet exhausts that keeps raising it's head on these fora is the build up of scale...

 

[PaulJS]

Directly from Brent - address/ details etc can be found at the Yahoo group site I mentioned - in the /Files section.

BTW - for the naysayers, I did a quick search on OrigamiBoats for 'dry exhaust', and Brent recently posted:
"I insulated my dry exhaust with fibreglass house insulation, covered with cloth like 3 inch wide muffler tape. This, I covered with common caulking gun silicone. After days of motoring in the tropics , it is cool to the touch. The tape alone is not enough, the fibreglass makes all the difference. On 1 1/2 inch sch 40 pipe the end result is roughly 4 inches in diameter."

Yes - I agree with others that wet exhausts are common on yachts, and have been 'for decades'. But so has seawater finding it's way into the cylinders - it shouldn't happen - but it does, with uncanny regularity.

Oh! Thanks Electrosys, I'd completely forgotten to rant about anti-syphon arrangements!

 

[wotayottie]

I'm actually a marine engineer, so well experienced with big sea water cooled heat exchangers, but I'm actually pretty baffled by the marine leisure engine industry's dependance on somewhat unreliable Jabsco pumps for providing the essential supply of sea water cooling to tiny, easily blocked tube type heat exchangers.
.

As a marine engineer you should realise that Jabsco pumps are not unreliable ( I have only had one fail on me in 25 years cruising and that was a drive belt failure) and nor are heat exchangers easily blocked. You are trying to solve a problem that doesnt exist.

Sure I suppose in a boat with a proper lead keel you could cast a passageway into that keel and route engine cooling water through that. Whether the engine pump would be up to it I do not know. Certainly having hot exhausts would add cost and complication through the need to fabricate difficult pipe layouts and then insulate them sufficiently well in a small boat.

 

[PCUK]

There are myriad reasons why you are wrong about so much, but I really can't be bothered to put you right.

 

[DanBurrill]

Most narrowboats use keel cooling with internal skin tanks, as there's rather a lot of stuff in the cut that can clog up a strainer. A lot easier to do with a steel boat (although I'd always worry about corrosion on the inside of such tanks, as it's virtually impossible to coat them with anything).

I suspect one of the main reasons why keel coolers aren't very popular is that when they're mounted externally on a GRP or wooden boat, they're very prone to damage from grounding, collision, or when the boat is being lifted in or out of the water. Add that to the extra drag they cause, and perhaps replacing an impeller every year (if you're conscientious) and keeping a strainer clear doesn't seem to be that much of a hassle.

 

[prv]

of course the abomination that is direct sea water cooling... Now there was a really bad idea!

Oi! I *like* my raw-water-cooled 2GM20 Simple, less to go wrong, haven't noticed any downsides so far.

I'm inclined to agree with you about keel-cooling / dry exhaust though. Definitely feels tougher and more reliable, but would only work well in the right kind of boat.

In particular, you really need a metal hull, so that the cooling tank can be integral - running pipes along the outside of a wooden or GRP hull, though I've seen it done, always looks vulnerable to damage and fouling, and adds drag.

You'd also need to design the layout with the exhaust stack in mind - retro-fitting it to a typical boat would involve a lot of unsightly boxing-in of hot components, and you probably couldn't follow the route that the original hose did.

Pete

 

[PaulJS]

Some helpful replies, some less so (!).
I appreciate the point about keel coolers possibly being more vulnerable to damage, but in the event of a total failure the engine would then be directly sea water cooled until repairs were made, and if you've hit anything hard enough to damage it you might have more urgent problems anyway...
Regarding the issue of internal corrosion, this wouldn't be an issue if the keel cooler/skin tank wasn't steel.

Prv, sorry wasn't meaning to disparage direct cooling... it's a perfectly good idea if your engine isn't steel or cast iron and can tolerate chronic scaling of coolant passages.

 

[wotayottie]

But what about the pipe run for the dry exhaust. Have a look at the run of the rubber pipe in your boat and think what would be involved in duplicating that in steel and then with sufficient insulation to avoid heating up the inside of the boat, getting burnt flesh etc.

I saw it done on the British Steel Challenge boats but they were 67 ft and built of steel with no concession to flashy type interior lay outs as most boaters require.

 

[bikedaft]

interesting ideas

have wondered about the pump myself - we also have an old raw water cooled sabb - the pump is plastic mushroom type with springs (no idea what its called really)

we took it apart once (to check) in 40 years and ended up putting the old plastic bits back in - no problems whatsoever.

seems positively odd to have to carry a spare impellor and tools at all times for what can be a safety critical part?

 

[rob2]

I think others have covered the issues with keel cooling , particularly on a GRP sailing yacht that would like to achieve maximum speed. Must admit, though, that now that car manufacturers are making radiators out of plastics it should be possible to manufacture - but at what cost? It would be a great idea to incorporate it into the design of a new yacht, but probably no way it could be retro-fitted.

The dry exhaust system has similar problems. Cars exhausts are suspended on flexible mountings and generally rely on having sufficient length to absorb the vibration, they are after all only fixed at one end, albeit to a moving engine! I wonder if most boat installations would have enough space amongst all the other gubbins twixt engine and transom to design a system to absorb movement without those infernal flexible sections - yes, I too have replaced them as they corrode and seize regularly.

In summary, I agree there are many advantages to be gained but only if the systems are designed at the outset so as to incorporate the components needed, e.g. a Steven Jones style bustle for the keel cooler, perhaps.

Rob.

 

[nimbusgb]

A cheap pump through an expensive heat exchanger

....against a heat exchanger ( radiator ), isolated exhaust mounts, silencer, heat insulation, added fire protection ( I had a Discovery once that lost a section of heatshield above the exhaust during a trip to Spain. It melted 2 square feet of the carpet in the back! ) engineering challenges it making it all fit flush, regular silencer and exhaust section replacement (?) keeping it sealed and to gain what? A little less vigilance? Hmm I'd rather people checked their boats more often not less.

Do engines really flood that often. Not in my experience!

The great thing about a leaking wet exhaust is that the water becomes obvious quickly in a boat. A leaking dry exhaust may just kill people before its noticed!

After 26 years my old Perkins still ticks along at between 80 and 90 deg C and if the heat exchanger died tomorrow a grand and a couple of hours would fix it for the next 20 years!

KISS!

 

[Brent Swain]

On Metalboatsociety.org a diesel mechanic was quoted as saying;
"If everyone went for dry exhaust and keel cooling, us mechanics would all be out of work!"
I have been using my skegs for cooling for decades, with no problems . 1 1/2 sq ft of surface per HP works well.
Water soluable machine oil in the coolant eliminates corrosion.
I insulate my dry exhausts with fibreglass house insulation. This over a 1 1/2 inch sch40 stainless exhaust pipe, gives me an OD of about 4 inches ( 100mm).
This I wrap with 3 inch wide muffler tape, and silicone over that. After days of motoring in the tropics, it remains cool to the touch on the outside .
Where it exits the transom, thru a flush welded in stainless ring, about 6 inches above the waterline, an elbow directs it downward , to about 3 inches below the waterline,
where another elbow directs it aft, with a rubber flap to keep a surge out . A 1/4 inch hole in the first elbow makes a siphon break . This eliminates the need for a muffler.