Keel cooling

About how many feet of 1 1/2" aluminum tubing with a 1/4" wall thickness, would it take to keel cool a Kubota, model 1505, 4 cylinder diesel engine. Estimated HP is 25. We are installing this in a 34' pontoon houseboat.

 

hi forkliftking....firstly i would not be using alu tubing ,many issues there .
however for copper ,which is the usual material for the job ,the following applies.for a diesel engine you need 12 sq inches of exposed area per hp the equivalent length of 3/4 copper tube is 0.43ft per hp ,also you need to increase these figures by 25 per cent if the exhaust manifold is water cooled
all this is only a guide ,the best way is to fit as much pipe as practical ,and to fit a thermostat to keep the water outlet temp to around 175 degrees, your engine already has a thermostat i suppose. also you have to make sure your pump has the capacity to pump this water around .hope this helps
cheers hartley

 

Thanks for the reply. The reason for the aluminum tubing is that we were going to weld it along the pontoon. It will be in fresh water and using anti-freeze year round.

 

Aluminum is used all of the time. I know a 1963 Marco gillnetter with original aluminum keel cooling. Skin cooling is even better. Find out from the manufacturer what the cooling requirements are. If no satisfaction, ask "Ski" on boatdiesel.com. Don't get copper near that thing.

 

Hows your physics?

looking up the engine specs I find 1650 BTU per minute as the heat rejection running at full power. Then you need to know the warmest the water gets in the lake and the flow rate of the water pump/labyrinth combo.

There is a good publication put out by John Deere on marine cooling "Engine application guidlines" Publication AG-24. You can get this from A John Deere agent. It will lead you through the calculation.

 

One thing you might want to locate is an external thermostat .

On a vehicle the cooling system is designed to lower the coolant temp about 20deg with each pass thru the radiator.

The vessel cooling system will be designed for a "worst case" 105F water and full throttle for hours and growth on the cooler surface ..

This will surely result in over cooling during the usual operation, which can be very hard on the engine.

180F out 160F return ,just right,

180F out 100F return will shock the engine and cause inefficiency .

A by pass thermostat will allow the coolant to leave the engine , and the return water to be at the proper temp.

FF

 

Thanks Fred. That sounds like a good idea.

 

We have built some alu-workboats with this type of cooling. Generally, with Scandinavian conditions; sea water at max abt 20 centigrades and vessel speeds 10 to 20 knots, the low-speed surface (mostly inside) is taken as 1 m2 per 100 hp. For a closer calc you need to know the performance curve of the circulation pump, but if you increase surface by 20 % you should be quite safe, provided you keep painting layers at a minimum. Overdue length will ultimately increase pipe resistance so that throughflow is reduced, so there is a balance to be found.

Then check the internal thermostat, in modern engines you have a 3-way shunting thermostat, dividing the flow from shortcut circulation to cooler. If this is the case with your engine, you don´t need an external. Also make shure that the exhaust cooling (if applied) is bypassed through a bleeding hole during warm-up.

AND STAY AWAY FROM COPPER IN YOUR ALU BOAT!!!!

 

I've been gone for a week. Thanks for the advice.

 

Hi Fast Fred.

there is another thread discussing this issue now and I raised the question about over cooling the return water as well. The general consensus seems to be that the engine mounted thermostat will prevent the over-cooling of the engine.

The by-pass thermostat (that you mention) should be standard equipment on marine engines so over-cooling should not be a problem.

 

And by the way...to the original poster. Keel cooling ? Last week I was in the engine shop to pick up some parts. In this engine shop were dozens..maybe hundreds of second hand, scanvenged heat exchangers, oil coolers, keel coolers, bronze seawater pumps, exhaust water jackets, aqua lifts ...you name it. A whole room full of the stuff on shelves. The gear all looked good, Cheap to buy and only in need of a service. Not a bad way to marinize an engine. Have a swing by your local shop.

 

Another point to mention,

most publications recommend insufficient cooling area. The Manufacturers recommend 2m² per 100hp when the plate or tubing is Al., and 6m² when done in steel. That is on the safe side, even with several layers of paint on the plating and in tropical conditions, or at anchor in warm water.

Regards
Richard

 

The general consensus seems to be that the engine mounted thermostat will prevent the over-cooling of the engine.

When the return water is over 20deg cooler than the engine out water , the slugs of cold water allow back into the engine (as the block thermostat simply releases water) can cause problems from the massive temperature drop.

This is less noticible with heat exchangers as their cooling ability is minor compared to keel cooling , and cost,, size matches them to the engine.

FF

 

Fast Fred,

Just the reply that I was looking for.

On the other thread I asked about installing a heat exchanger cooled engine in combination with the keel cooler, with the argument that the heat exchanger would be better matched to the engine (since it is delivered with the engine) and would therefore prevent overcooling...

There were some good reasons not to run them in the same circuit but if the intend is to run the engine at the optimum temperature it might the best solution, with the benefit that the engine would not be experiencing thermal problems.

What is your take on this?

 

Thermostats don't work so fast it would cause such problems. If there's too much cooling it's means the keel cooler area is multiple times too large and water you are sailing in close to freezing and engine running idle...
To compare this with car's (more common to most) the outside temp got to be -15C before the engine won't warm up running idle. Might take a lot longer time thou.. Driving normal speeds (engine loads round 30%) most car engines achieve normal running temp down to -30C out temps...