Hi Riley,
Watercooling jackets for Ø28 mm motors are easy to find on eBay:
http://www.ebay.de/itm/DE-Leopard-Boat-Motor-Water-Cooling-Jacket-LB28WCJ-40MM-/271100127773?pt=RC_Modellbau&hash=item3f1ed3aa1dAlthough the motors you ordered are four pole motors which run relatively cool, compared to two pole motors, an aluminium coil won't cut it, due to the small contactsurface of the pipe on the can, you really need a jacket where the water runs directly on the can.
Some pictures of your build would be nice to get some idea of the available space inside the hulls.
Will you go for fully submerged props, or surface piercing drive (= roostertail)?
Keep in mind, that with a submerged drive, the angle of the propshaft needs to be as shallow as possible, to avoid pushing the bow down, reeking havoc with the riding attitude and handling.
A shallow Vee at the bow will help getting the cat on the plane, assuming you're not building a scale catamaran with a tunnel section on which the boat will 'float' when at speed (hence me asking for pictures...).
Regarding running partial throttle; a brushless ESC doesn't lower the Voltage to the motor, it chops the full Voltage up in small pulses, the slower you go the smaller the pulses.
This 'chopping' is done by the Fets which do the switching, on and off, the more switching occurs, the more heat builds up.
That's the reason you can't run a brushless setup in partial throttle for longer periods of time, unless you take ESC's that are way too large for the motors and won't heat up, because the load is just too low.
I tried running a brushless setup without watercooling some time ago; a 40-50A outrunner, turning a 60 mm two blade prop on 4S Lipo, controlled by a 150A ESC.
It was a very hot day and the sun warmed up the interior (and the motor and ESC) to 40°C, so I was curious how long things would run before the overheating protection of the ESC would shut the propulsion down.
You can read all about it here:
http://www.modelboatmayhem.co.uk/forum/index.php/topic,34033.0.htmlI just returned from a ten day vacation on the river Weser in Germany and I ran that setup for over an hour every day without any trouble, no overheating (after watercooling was added) of the components, nothing warmed up beyond 45°C.
As you can read in the topic I linked to, I went slightly overboard on the power installed; over a 1000W in a 110 cm long stepped hull with an ESC the can handle three times the max Ampdraw of the motor, but that's what it takes apparently to enable me to run partial throttle for such a long time...
The shallow Vee of my build is something you could copy on yours.
Regards, Jan.