Building a heat exchanger into the hull

This is a subject dear to my heart, and I second the motion to have a look at Walters keel coolers. I've had two boats with them and I think they are the single best improvement you could make to any inboard powered boat. I've been evangelizing for years on the subject, trying to convince yachting people that their heat exchangers are completely unnecessary, inefficient and generally a pain in the butt. Do yourselves a big favor and get rid of that thing, and install a Walters keel cooler.

But I agree that if you're building a metal boat, especially aluminum, it's even more sensible to simply weld the cooling system into the hull. A good source of information on that would be Brent Swain who designs "origami" metal boats and uses the skeg as the keel cooler. You can find him at: http://groups.yahoo.com/group/origamiboats/

 

Just wondering if your boat ( with the Keel Cooler ) is a planing of displacement hull type, and if it's a planing, how much drag is there.

 

I must disagree with

"A too large heat exchanger won't hurt. The thermostat takes care of temperature control. Also, if the boat has antifouling paint, the heat transfer will be less. The paint manufacturers have tables for it. Many commercial metal boats have keel coolers"


The cooling system is most usually designed for a 15 or 20 deg drop.
Too cool return water will overcool the engine and loose efficency and service life.

FAST FRED

 

Walter Keel Coolers

The boat I have now is a homebuilt 37' heavy displacement double ender. Not fast, but very fuel efficient. I can motor at 4 knots at 1 litre per hour. I've had three boats with keel coolers, the first was homemade out of copper pipe. For my 30 hp Sabb diesel I use two lengths X 4' of Walter turbo tubing. It is adequate even when anchored in hot water, like Singapore or Panama. The drag is negligible, definitely less than the power loss of an additional pump required for a heat exchanger.

Walter has some tables to illustrate the results of tests of identical planing boats with keel cooler vs heat exchanger. Even at high planing speeds there is no measurable difference in speed. For slower displacement hulls, it's the only way to go. Consider the lower cost, simplicity and reliablility of a keel cooler, and there is no contest.

I have nothing at all to gain by promoting keel coolers. I just think they're the cats ***.

 

You need the Engine coolant flow and engine heat rejection figurers from your engine manufacturer if you want to calculate the optimal path length and channel size.

You would be best advised to obtain a copy of the
John Deere Engine applications guidelines chapter "Marine cooling systems". It is explained in detail here for the layman.
Your local John Deere rep will be able to get this for you once you tell him it exists !

 

I've been thinking about keel coolers for a long time. Specifically, why couldn't a keel cooler be built in flush with the rest of the hull? For example, an aluminum hull could just have a bunch of tubes welded on in the bilge, or cast/extruded into one of the hull's plates. The hull itself then becomes the heat exchanger. With other hull materials this plate could be bolted and gasketed in.

All the keel coolers I've seen are like radiators mounted outside the hull, requiring through hull fittings, etc. On sailboats, drag is a major concern.

90% of the mechanical breakdowns I've had in a boat have involved the raw water system. So let's eliminate it!

Why has this not been done before? Or has it? Any thoughts?

 

Welding a keel cooler into a metal hull is commonly done, but I don't know of any formula for calculating how much surface area is needed. The best approach would be to ask somebody with experience, like Brent Swain who has been doing that for years. He uses the skeg as a cooler.

The Walter keel cooler on my boat is just two tubes 5 feet long and it's oversized for a 30 hp diesel, it was adequate even while stopped and idling in hot tropical water. I doubt if the effect on sailing performance would be measurable. It probably increases my wetted surface by about .2% and slows me under sail by about.....well, a very small amount, I would be surprised if it were 1/10 of a knot, probably less.

My engine is a Sabb 2J which is 2 cylinder making 30 Hp and turns at a low speed. They still make them in Norway. They come standard with a 2 blade variable pitch which has a very rugged design without a hollow shaft. Pitch is controlled by moving the shaft fore and aft and thrust is taken on the stern bearing itself. The blades can be easily replaced if damaged.

My boat is heavy, about 15 tons and 34 feet on the waterline, and the Sabb can push her up to seven knots, burning 3 liters an hour. Slowing to four knots reduces fuel consumption to 1 liter/hour. That's about as efficient as I've ever heard of, and I think the reasons it worked so well are: the large diameter 2 blade prop with a generous aperture, minimum 6 inch tip clearance from all obstructions, adjustable pitch for fine tuning and keeping rpms low at low engine loads, the keel cooler which eliminates a lot of cooling losses, and full credit to Sabb for a very well designed marine engine. Reduction gears can be very efficient or very lossy depending on how well designed they are and obviously Sabb has done an excellent job.

If a Sabb can be fitted into your boat, I don't think you could find a better marine engine.

 

Part of the problem is the heat radiated into the hull. In warm climates it would make it unliveable.

 

That would be a problem definitely with cooling through the hull skin. More heat might go into the boat than into the water, with more surface area inside. Using the skeg seems like a good solution.

 

We are still left with the problem of a dry exhaust. Can you exit a dry exhaust through the transom or must it be a stack? And how much of the existing system would have to be changed?

 

I don't think so. Water is a much better heat conductor than air.

 

Keel cooling with Copper

:?: Does anyone have a table that indicates the length of 1.5" L grade copper pipe required to cool an engine with the following specs: Heat Rejection 4998 BTU/min...water pump 53 gal/min...water temp 78 degrees Fahrenheit (fresh water)

 

take your pick

http://www.chapters.indigo.ca/default.asp?Ntt=heat+exchangers&N=35&Ntk=SearchBooks&Section=books&Catalog=books&Lang=en&zxac=1