Keel cooling with 2 separate circuits

[hjc457820]

Hello,

First, thanks for all the constructive informations in this forum, that's very nice.

I am designing my motorsailer, 50ftLOA, she gonna be made out of steel. Twin keel, twin engines, twin rudders design.

I bought the engines few months ago, two Volvo TMD40A. Just one is enough to move the boat forward even in windy conditions, the whole project is designed around complete redundancy.

After... a year?! of thinkings about this, let's go with dry exhaust system and keel cooling. But i'm not sure how to do it.

Ma favorite way, now, is to have +- 150L of coolant fluid in both keels, and use the original impeller pump and Bowman heat exchanger (integrated thermostat) to cool the internal circuit of the engine. The only maintenance will be the impellers, that's quite inexpansive and fast to do, so no problem.

The idea behind that is that is very simple to make, and if something fail i can run it with raw water with just two hose to plug. The large volume of liquid in the keels makes it possible to run the engines on the yard for few minutes if needed, and to run as long as i want the 6kVA genset.

I did some calculations, and it will work. But what do you think about that?

Thank you
Benjamin

 

[comfisherman]

So basically your still using the raw water pump and exchanger but having a captured reservoir attached?

Back in the late '80s early '90s a lot of the high horsepower Detroit's only came raw water cooled for after Cooling. Many used built in keel cooler much like you describe. My first boat was configured this way on the aftercooler side. It' corroded around 7k hrs and began to leak. We cut them out at tremendous effor and cost and put new ones in. Those did not last as long and had corrosion pin holes around 6k hrs. Begrudgingly we ended up going raw water with a large tube type cooler and that is how it's still configured a decade later. This happened to many boats up here. Belief is the high flow rates causes higher electrolysis than the water jacket side with a lower flow from the main coolant pump.

Conversely we had to patch our old steel boats keel cooler around the 40 year mark from internal corrosion from no more than block water pump flow rates. Honestly f your 50 Or older it's probably never going to be your problem ever again....

 

[hjc457820]

Yes, basically it's that.

Thanks for youre reply.

How Can it corrode if we use coolant, with regulary renew it? I think that solution is also fine to fill empty part (keels) of the Hull to prevent rust.

 

[baeckmo]

I’m not enthusiastic about the suggested ”series cooling” system. Comfisherman is referring to turbocharged engines with an aftercooler. For those, you have to split the system in two separate; one for the engine and one for the aftercooler. Normal practice here is to use the standard raw water pump for the aftercooler circuit, since it operates at lower temperatures (better for the rubber impeller).

The TMD40 in your case has no aftercooler (that comes with the TAMD40). The engine, exhaust manifold and turbine housing are fresh water cooled, and the engine and transmission oil coolers are raw water cooled. At 3500 rpm (max power comes at 3800) the FW flow is close to 11 m3/h, and the RW flow is almost exactly half of that.

IIRC, the RW design temperature is 25 degrees C, anything above that requires a resetting of fuel injection. Now with your idea, there is an additional heat exchange “barrier” between the sea-water and the new “primary system”, which causes an elevated temperature level in the primary system. To compensate for the reduced temperature difference available over the coolers and the engine heat exchanger, the primary flow has to be almost doubled.

This is why you should use the FW cooling all the way including the keel cooler; it is better for the engine, and it is better for redundancy.

The suggested “pool” of low velocity water will have low heat transfer efficiency, unless you can arrange the flow along the hull skin with a velocity about 2 to 3 m/s. The pool will require a massive volume of the expansion tank. Note that all cooling systems will have hot spots, often in combination with low pressures. In these locations there is a de-gassing and a “stationary” cavitation, trigged by vibrations.

Many of the keel cooling systems I have surveyed, have been “afterthoughts”; there has been no pressure control devices, just an expansion vessel ventilated directly to atmosphere. The system pressure has to be high enough to suppress cavitation in the critical spots, and the system has to be “closed enough” to avoid vaporization (ie loss of fluid), because lost water is compensated for by the addition of new, oxygen rich fluid. If no corrosion inhibitor is added each time, then the corrosion is accelerated.

So, I suggest that you rethink the arrangement, use a full FW keel cooling system with correct fluid velocities. If you need operating capacity while drydocked, add an air cooler.

 

[philSweet]

Redundancy is a terrible thing. It breeds complacency and 99% of the time is doing nothing except for doubling the rate of failures and cost of maintenance and repairs. It's a sailboat. Zero engines is a reasonable option. One engine is often desirable. Putting a pair of 40hp in a 50'er is ludicrous. If you want to make the boat more reliable, hire the best system installers you can find to design the installation and install one engine properly. It will cost less than two crate engines and will be much safer having only half the number of ways to sink the boat or catch fire to the boat or fill it with potentially deadly fumes.

I worked on nuclear weapon systems in a previous life. They had some redundancy built in. That meant basically that the redundant systems had to individually be more reliable than what you would ordinarily spec for a single system, which drove the cost up a lot and still about doubled the manpower needed to maintain the stuff. My skill set is engineering economy and maintenance engineering. If you are concerned about reliability, put one engine in a space big enough for two - then you can actually see what you are doing and access all the parts of the engine and fuel and exhaust systems. You can expect today's engines to run trouble-free for thousands of hours and many many years (excluding the tier III and tier IV exhaust systems, which remain a nightmare) That's much longer than what a home grown cooling or exhaust system will last, and those will sink the boat or catch it on fire. Two of them will just double the risk. A twin 40hp installation that is equal in reliability to a single 60-75hp will cost at least 3 times as much money to buy and install. It will cost double in maintenance. It will take twice the time to maintain, will require considerably more on-hand spare parts. And you are wasting a lot of valuable floorspace.

The single best way to improve reliability, maintainability, and availability in a fixed space is to simplify things as much as possible. If it isn't there - it can't fail. And you have more space to access what is there. The best motor-sailor arrangement I've seen was an old Colin Archer. When you went down the companionway into the salon, the diesel was sitting there right in the middle about 3 feet in front of the steps, raised up about a foot above the floor so you can change the oil. That thing will run forever because you can see it and get at it.

 

[comfisherman]

I didn't realize the 40a had an integrated aftercooler circuit.

So this is single circuit like a c 18, with one pump servicing cylinder water jacket as well as aftercooling. As opposed to a di scania that has divorced pumps one for water jacket and oil and another for aftercooler and water jacket manifold.

I read double circuit like the di 13. Guess in this vernacular he's referring to coolant circuit and a keel circuit.

I would concur with baeckmo that it's not optimal to have those kinds of flow rates and a captured pool of coolant.

 

[baeckmo]

Phil, these engines are nominally 130 hp for light duty; the number 40 is referring to engine swept volume. And I completely agree with you about redundancy.

Comfish: the TMD engines are all turbo without aftercoolers (no integrated either), and yes the Scania parallell cooling is typical for the needs where you have an aftercooler. In our climates (Alaska? and Scandinavia) the cooling areas for both circuits are about the same, but as soon as the raw water goes up just about five degrees, there must be more cooling surface for the aftercooler.

 

[philSweet]

Phil, these engines are nominally 130 hp for light duty; the number 40 is referring to engine swept volume.

Duh. I normally go back and read the OP after writing an answer, but I lost internet right as I posted, and haven't had a chance to get back in since.

 

[Milehog]

While I don't have cooling system expertise, I have operated diesel equipment, from 13 hp to 4,400 hp, for 45 years.
When I was young, I had similar thoughts on ultimate fantasy yacht propulsion systems. Now I understand the significant disadvantages of crowded equipment and how, even with good practices, you just will not as reliably catch developing issues. I suggest you sell one of the engines and do a first-class, professionally designed installation of the other one. Also, go with one very robust rudder. The sails provide redundancy.
You will have more reliability and easier maintenance at no more expense.

 

[BlueBell]

I couldn't agree more, redundancy has its' place, when it's prudent.

Otherwise, do it once and do it right.

 

[Milehog]

Hello???

 

[IronPrice]

I'm responsible for a small fleet of commercial vessels. We use keel cooling in 15 of them. 2 - 3 circuits per vessel. One per engine.

It works really well. Ours are on the outside of the hulls. That ensures sufficient conduction of heat into the water.

You would want to be sure you will dump enough heat through the keel, which will presumably be painted.

We header tanks in our cooling systems, with low level alarms.

 

[comfisherman]

I didn't read it as value of keel coolers, thousands of commercial craft all around the globe have proven that.

Think he was curios to the use of a large keel cooler more inline with a keel reservoir and still utilizing the engines raw water system. Like a capture pool for the raw system to utilize.

 

[IronPrice]

I'm now no longer sure I understand what OP has planned .

If it's using the keel cavity as sort of open-topped reservoir to circulate cooling from ... I wouldn't. I'd go with an enclosed system. That might still use the keel cavity, but it would need a sealed top, and a plumbed inlet and outlet. I'd also consider baffles to stop water sloshing around. I addition I'd have the inlet and outlet pipes terminating at opposite ends, to avoid thermal stratification forming. The inlet should discharge into the aft end of the keel cavity and the outlet should pick up at the forward end.

And I'd still have a header tank, with a low-level alarm. The trouble with keel cooling is if that if it leaks it's at the lowest point and you can lose all your coolant.