Engine Cooling

So, my question is a pretty simple one but it should be fun to kick it around a bit. The boat I'm looking to build is a 39' molded ply sloop (a Dix) and I would prefer to use a non-marinized industrial/tractor diesel (eg Kubota or maybe automotive). To do this, I'm thinking about a keel cooler but, being a sailboat it's important to keep the flow clean over the hull.

I have thought about casting cooling lines into the (lead) ballast shoe, using a plate molded to the shape of the hull as a heat sink, or using a seachest as a cooling chamber. My principal goal is to eliminate the raw water through hull and the secondary aim is parts cost and global availability. Using antifreeze will also keep the corrosion down and this layout will make it far easier to plumb heat exchangers.

The next question then is how to address exhaust cooling and discharge as given that I'd rather not bring raw water into the boat.

I'm pretty good with boatbuilding practices but powerplants aren't one of my strengths so I'm looking for thoughts, ideas, etc.

Thx all

 

You could use a dry exhaust, but heat and getting the stack high enough is always a problem. A wet exhaust is a much better solution, though you will need a raw water pump and manifolds.

I'm not sure about the heat sink idea with the ballast, but it seems like there's a whole new world of issues it could raise, especially expansion and contraction or heat/cold cyclic difficulties.

Raw water inlets are quite safe, if installed properly and kept in good repair. Parts for sea cocks, hoses and other fittings are generally available world wide. A home brewed cooling system may require custom made parts, which will not be as readily available in out of the way locations.

Some fear thru hull fittings, possibly after having one leak and sink a boat. If you trace back the cause, invariably it leads to neglect of the fitting, hoses or other related equipment or installation. The thru hull can hardly be blamed for this.

 

Casting cooling lines in the ballast shoe is at least an original solution. That will certainly work, lead can absorb and conduct thermal energy very well. And there will be plenty of surface to transfer the heat to the water.

You might also consider constructing an exhaust heat exchanger which transports at least a significant part of the exhaust heat towards the keel. Probably not enough to allow the use of rubber exhaust hoses though.

I've seen very efficient heat exchangers from a Swedish company, Alfa Laval. A stack of thin stainless steel plates, cleverly arranged, so that two circuits are formed, one for the gases or liquids to be cooled and one for gases or liquids that transport the heat.

 

Having dealt with this question many times, the problem is not the engine cooling but the exhaust. It all depends on the HP of engine. A small engine can be cool with any keel cooler and use a small dry exhaust. But if you have 200hp the cooling that much is much harder and the dry stack can be huge heat producer.

 

Skin cooling

So, my question is a pretty simple one ...................

Nahhh... in this field nothing is simple.
As PAR mentioned is a dry stack on a sailing vessel a problem, and IMHO NEVER a good idea. Exhaust running up the Mast? Counterpressure? Soot?
On a 39´ boat I expect the (motorbox) engine room already cluttered and you need some sufficient space for insulation.
A 100hp will need about 8m² or 90 ft² surface if made in steel. I do´nt know about the heat transmission of lead, should be much better, but is´nt the much higher thickness a problem?
Anyway a proper designed and serviced heat ex. system is by far the smaller problem.
my 0,02 €
Regards
Richard

 

Thanks for the replies...some food for thought for sure.

About the only way I could figure to use a dry exhaust was to 'disguise' it as a radar mast on the stern rail. I'm not sure how to wrap a cooling jacket around it without it looking like a dog's breakfast tho...maybe a transom vent with a dorade box on the inside ?

Paul, my concern about the through hull comes not from a fear of fittings but simply from a desire to avoid holes in the hull where possible. All of the other through hull locations will be open for visual inspection, not hidden inside cabinetry and under floors. The beauty of building this one for myself is that, unlike so many clients, I'm not trying to conceal the fact that it's a boat first and foremost, lol. And as you adroitly pointed out, there's no substitute for vigilance and sound maintenance.

Considering that, I intend to seal and insulate the engine compartment to deaden the noise and heat transfer. Good practice dictates keeping the raw water lines as short as possible. Logically then, that puts the seacock inside the engine compartment. Forget the 'ready access' (ABYC) issue for a minute, it's conceivable that a soundproofed compartment could take on a hell of a lot of water through a 2" hole before one was even aware of a problem, fire and bilge alarms aside.
My concern is not so much for the seacock itself because they do tend to be pretty robust but for the rest of the system's plumbing...I've seen what a broken belt can do to a plastic raw water basket.


CDK, thanks for the Alfa Laval tip...their Canadian distributor is now a bookmark...
Every time I worked on a sailboat, my thoughts kept going to the massive potential heat sink capacity hanging off the bottom of the boat that was just going to waste. Now that it's my own project, it's time to try and use it.
To put it another way, I'm trying to figure out a good way to take the heat to the heatsink rather than bringing the sink to the heat.

Regarding power...hull speed comes from a 20-30 hp based on the specs, but I'll probably stay to the high side of that because if it's feasible I'd like to spin a second alternator, a fridge compressor, and an onboard air compressor as well. If I do go this route with closed-loop cooling, I'll maintain all the 'accessory' heat loads (bus heaters, hot water tank, maybe radiant or finned-tube) as a completely separate system which will also require its own circ pump as well. I could probably isolate all of the secondary loads from the driveline requirements through a PTO, but I want the option of running everything at once without overworking the engine.

That was one of the reasons for considering a newer automotive diesel (blasphemy I know...I spat when I said it too ;^). The increased power to weight ratio of a newer small turbodiesel will give me more power for accessories and reserve without requiring the trim adjustments that a larger marine powerplant would require. I realize that the automotive market doesn't require anything near the durability of the marine environment, but virtually any powerplant available from that market would provide sufficient power that the engine would never be loaded heavily in the first place. Make sense ?

I figure now is the time to be playing with these ideas: before starting the build! While I'm not normally one to buck the tried-and-true methods of doing things in a production environment, I now have the luxury of spending my own time on R&D. That's why I'm glad I (finally!) get to play with these ideas.

I'm grateful to everyone for sharing their hard-won experience and knowledge with me. Any and all thoughts are appreciated !

 

There's a lot about the subject in Dave Gerr's new book Boat Mechanical Systems Handbook. Just got my copy fresh from the print...

 

Some Suggestions;
If you are going to enclosed engine then raw water cooling engine and exhaust is best.

Put seacock on raw water entry and at least one filter. Put Anode in filter.
On water entry, consider looping pipe above waterline. Use a heat exchanger close loop system, don't use raw seawater in engine.

Remember air intake for engine, place that as high as possible and make it big enough.

On water exhaust make sure it loops high above waterline.

Make separate box for batteries, outside of bilge, with independent venting

 

There's an exception to this assumption and it could be an issue in BC. definitely it's an issue here (70N). It's freezing temps!

 

There is no problem with leading the exhaust pipe inside the mast , as long as the exhaust pipe ends above where someone might touch it.

A small sailboat will probably burn under 1 gph while motoring . If there is about 160,000 BTU in a gallon of diesel, no more than a third will be exhaust heat.

So it wont melt the mast or even warm it much.

If the masthead is left as open as possible , there will be almost no exhaust noise ever heard on deck.

Sounds like a winner.

FF

 

If that works it's definitely a winner. However I have a doupt about the excess heat involved. Allthough the amount of burnt fuel doesn't sound much, on third of it still is about three times more than a tow burner kerosene stow can generate. Cooling the motor department and the dry stack might however work efficiently if the mast profile is large (by lateral area inside) enough to allow high volume of air to be vented. Chinney effect of a tall mast provides a lot of boost for the air anyways. What's trickier to do, is how to lead the exhaust from the engine to the mast if they not happen to be close to each other. Either case inside the hull dry exhaust got to be insulated bcs of the surface temps up to 400C/900F..

 

Exactly that will cause a problem: counterpressure........
If you run the pipe several meters above the engine you produce what truckers know as a "motorbrake" counterpressure. To solve that prob you increase the diameter of the organ pipe sufficiently. And have the next issue......
How does that look to have such a barrel on a boat of this size?

And guys do not underestimate the insulation issue. There is not enough space in his motorbox to do it proper, I am sure!
Just to prevent the manifold from burning the "engine apartment" is a real headache for pro´s.

It is just not a good idea to have a dry stack on a sailboat. period

Regards
Richard