Keel cooling design for electric inboard

Not really, I estimate lost at 1500W.
A simple calculator with my cooler design of 40 square inches in 1/8'' stainless 316 give me it can dissipate 1750W of 50°C coolant in 35°C water.
So, that give over 5000W of transfer with 80°C coolant.
Look like I'm fine!

Edit: it's 45 000W in 1/8'' bronze and 80°C coolant

 

I think there is an error on the calculation. It can't be ~26 times the heat transfer.

 

From an engineer that has spent over 50 years as a heat transfer specialist:
For this condition, a heat exchanger has resistance to heat flow consisting of five thermal resistances acting in series. 1. The fluid outside (sea water) convection coefficient. 2. The outside surface fouling resistance. 3. The metal thermal conduction resistance. 4. the inside surface fouling resistance. 5. The inside fluid (fresh water or anti-freeze mix) convection resistance.

Most usually, the thermal conductivity of the metal surface is fairly insignificant, so it does not matter if the metal is copper, bronze, or stainless steel. The other thermal resistances, all acting in series (additive), usually dominate here. There can be exceptions.

 

Right. 45/5 is 9 time.

Playing with commercial bronze number (188W/mK) and a water temp difference of 45°C (80 coolant, 35 sea) that give 70 000W for bronze and 5250W for SS316.
Heat Transfer Conduction Calculator | Thermtest Inc. https://thermtest.com/thermal-resources/conduction-calculator

Thanks for your input.
#4 should be inexistent
#2 is a really good point. Is I have to apply fouling over SS316 cooler? Sound like it's fooling or scrub the cooler every week, right?

 

Inside system fouling can be low, provided the sealed system is protected from corrosion, and kept clean.
Sea water side fouling can be covering a wide range of values, and neglected cleaning can lead to this thermal resistance becoming dominant. I have seen outside surface fouling more than several inches thick on ships well overdue for drydocking, virtually destroying any chance of adequate cooling. Of course, your situation can be well controlled with manual cleaning.

 

Bear in mind that the formula given in the linked website is only calculating the theoretical transmission of heat at just one spot. Only at the entry point of cooling fluid (80 °C) in the cooling device its wall can be heated to 80 °C and is the sea water outside at 35 °C (if device is counter flow connected, flow of the liquids on both sides is high (turbulent) and surfaces are perfectly clean). This reflects not the real situation a boat is normally in.

 

........and that is where copper gains its points!

 

Good point.
It's why it can be important to design with a security factor of 2, even 3.

 

Why? Copper don't need anti fouling?

 

Copper is the active element in most antifouling paint. Monel would be even better; it gets no fouling.

 

Ummm...kinda-sorta. Monel is very resistant to bio-fouling (but will grow slime at flows less than ~5 ft/sec); but as a heat exchanger it can/may scale (calcify) which then will bio-foul, so you need to watch for that.

 

I will stay away from any exotic material anyway.
SS316 laser cut or managing to find rectangle bronze tube was the way to go for me.

But, I ask myself: As anything seem problematic in salt water, is simple steel cooler could do the best option?
Regular steel seem to be 4 time more heat conductive than SS316.

It's easy to find, cheap, really strong.
Downside: corrosion.
Is this downside can be compensate with proper paint, epoxy and/or anti fouling?

 

You can buy maleable copper tubing at a hardware store. It won't need to be painted. A pipe can be pressed between rollers to make oblong if that is desirable too.

 

Sure it''s easy to find and cheap, but I don't see any way to build RELIABLE cooler out of those.

It's underwater, so I want a solution to stay away from catastrophic failure or even salt water in the coolant fluid.

 

Anybody have any experience with cooper/ tin brazed instalation in ocean water ?