Keel or "Through Hull" cooling?

[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.
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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.