Water Cooling

By Paul Williams.
Article posted 2nd March 2008.

Waste heat generated in the motor and speed controller increases proportionally with motor power consumption. Water cooling is the most effective way of keeping your motor and ESC cool enough to avoid power losses and potential damage. Use water pressure on the underside of a planing hull or sponson, or from a water pickup located in the propeller backwash, to circulate mildly pressurised water through the speed controller and then into the motor (in that order - you want the most cooling for the ESC, the motor is not so critical) through a system of aluminium and silicone tube.

Water pickup in the thrust cone of a hydroplane propeller. The pickup is simply a short length of 3/16" diameter brass tube silver soldered to a length of thin brass strip.

Water pickups set through the hull bottom of a mono or catamaran. On some models, you may find it easier to route the pickup back out of the boat through the transom, if space is tight, or you have solid sponsons made from foam/ply.

Most dedicated boat controllers like the YGE Navy and MGM controllers have built-in water cooling - you simply need to connect them to the water system. If your controller doesn't have cooling built-in, you can add a length of brass tube that is thermally coupled to the FETs on the controller board with either solder or heat sink compound like Servisol.

Some models of motor, specifically those from Lehner and Plettenberg, can be ordered with a dedicated cooling jacket that again simply needs to be connected to the water system in your boat. For other motors you can buy or make an aluminium coil. Yet another option is a cooling jacket made from a length of acrylic tube and two rubber 'O' rings. These are not recommended. Firstly, fitting can be very difficult. Secondly, because fitting is so difficult, you run a real risk of cracking the brittle acrylic. Thirdly, they leak - a lot. Frankly, they're not worth the bother. Stick with a simple aluminium coil - they work well and are a proven and reliable method of cooling a motor.

Making nice, neat, tightly-wound coils is a doddle if you have access to a lathe. Firstly, a lathe allows you turn a forming mandrel to make a coil, and secondly if your lathe has a back gear (and hence a very low bottom speed), you can get the lathe to do the hard work for you.

The forming mandrel should be turned slightly undersized to allow for the spring in the coil. For example, a 700BB Turbo motor has an OD of 45mm, requiring a mandrel of about 42mm OD. The 3/16" aluminium tube is fixed to the mandrel with a length of stainless 4mm rod bent in "U" shape such that one end is located in a drilled hole in the mandrel and the other lies tangentially to the OD of the mandrel. A split collar with an M3 set screw holds the tube in place. Gripping the tube in one hand, I can start the lathe in back gear at at 36rpm (backwards) pulling down and to the right to force the coils tightly together. Using this method you can make tightly wound coils that are an exact fit for you motor. 3/16" is an ideal size because the tube flattens out as the coil is wound, giving better surface contact with the motor.

Coil winding using a lathe. The mandrel is a piece of mild steel bar turned to a diameter a few mm less than the motor, in this case a Mega 22/30/1E.

The almost complete coil.

Finished coil.

Coil fitted to a Mega 22/30/1E.

This method of making coils is not without potential hazard. It's only possible on a lathe with a low (less than 50rpm) bottom speed, and you must make doubly sure that you have selected the lowest rpm and the correct direction of rotation. If you own a lathe, you are probably already aware of basic machine safety; if you're not, then you shouldn't be operating a lathe!

Using heat sink compound to increase cooling efficiency. This diagram shows a section through a motor cooling coil. Filling the air gaps between the coil and the motor can with heat sink compound dramatically increases the cooling effect of the coil.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

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Last modified: 08th July 2010 @ 09:05